Black Oak and Tanoak Woodlands Habitat Description

Climate change vulnerability assessment for the Northern California Climate Adaptation Project Copyright EcoAdapt 2019

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Black Oak and Tanoak Woodlands

Northern California Climate Change Vulnerability Assessment Synthesis

An Important Note About this Document: This document represents an initial evaluation of vulnerability for black oak and tanoak woodlands in northern California based on expert input and existing information. Specifically, the information presented below comprises vulnerability factors selected and scored by regional experts, relevant references from the scientific literature, and peer-review comments and revisions (see end of document for a glossary of terms and brief overview of study methods). The aim of this document is to expand understanding of habitat vulnerability to changing climate conditions, and to provide a foundation for developing appropriate adaptation responses.

Peer reviewers for this document included Mathew Cocking (Natural Resources Conservation Service), Earl Crosby (Karuk Tribe), Frank Lake (U.S. Forest Service), and Kari Norgaard (University of Oregon). Vulnerability scores were provided by Eureka and Redding workshop participants.

Table of Contents

Habitat Description ............................................................................................................................1

Executive Summary ............................................................................................................................4

Sensitivity and Exposure ....................................................................................................................5 Sensitivity and future exposure to climate and climate-driven factors..................................................... 6 Sensitivity and future exposure to changes in natural disturbance regimes .......................................... 11 Sensitivity and current exposure to non-climate stressors ...................................................................... 20

Adaptive Capacity ............................................................................................................................ 22 Habitat extent, integrity, continuity, and permeability .......................................................................... 22 Habitat diversity ...................................................................................................................................... 23 Resistance and recovery .......................................................................................................................... 24 Management potential ........................................................................................................................... 25

Public and societal value ..................................................................................................................... 25 Management capacity and ability to alleviate impacts ...................................................................... 26 Ecosystem services .............................................................................................................................. 29

Recommended Citation .................................................................................................................... 29

Literature Cited ................................................................................................................................ 29

Vulnerability Assessment Methods and Application .......................................................................... 42

Habitat Description Black oak (Quercus kelloggii) and tanoak (Notholithocarpus densiflorus) occur as component species within various forest types (e.g., mixed evergreen, mixed conifer) at all elevations, typically in combination with conifers and other hardwoods (McDonald 1990; Long et al. 2016; Norgaard et al. 2016; Calflora 2019; CNPS 2019). Within these broader forested landscapes, both species can occur as stand dominants or in pure stands (i.e., orchards) where frequent fire prevents them from being outcompeted for water and light by other species (see Box 1; McDonald 1969, 1990; Tappeiner et al. 1990; Taylor 2010; Cocking et al. 2015). This assessment


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focuses on these stands, where black oak and tanoak occur as dominant species within a woodland habitat structure.1

Box 1. Relationship with tribal management

Routine cultural burning and other orchard stewardship activities have been used by northwestern California tribes for thousands of years to favor the development of oak-dominated stands in areas where they would not be created or maintained by natural factors (e.g., soil, precipitation, elevation, lightning ignitions; Anderson 2005, 2007; Bowcutt 2013; Halpern 2016; Long et al. 2016; Norgaard et al. 2016; Karuk Tribe 2019). Tribal management practices facilitate the development of oak stands dominated by older, large-diameter trees with full crowns and abundant acorn crops. For instance, cultural burning removes surface and ladder fuels at frequent intervals (every 3–5 years for black oak), preventing higher-intensity fires that kill mature trees and result in the loss of acorn crops for the several decades it takes for sprouting stems to regrow. In the absence of tribal management, both black oak and tanoak can be maintained as stand dominants on sites where lightning ignitions are particularly common (e.g., Taylor 2010). However, under these conditions they typically occur as smaller trees with narrow crowns within more mixed stands (Tappeiner et al. 1990; Taylor 2010). By contrast, the occurrence of large-diameter trees with full crowns in pure or almost-pure stands requires low- to moderate-intensity fire at a frequency that natural ignitions are unlikely to create or maintain (Anderson 2005; Bowcutt 2013; Long et al. 2016).

Because the persistence of this habitat type on the landscape is strongly associated with tribal management, this assessment focuses more heavily on cultural aspects of this ecosystem compared

to other assessments.

Black oak woodlands are usually found from sea level up to 1,800 m (1,000-6,000 ft; Pavlik et al. 1991; Gaman & Firman 2006; Skinner et al. 2006), and are most heavily concentrated in northern California and the western Sierra Nevada (McDonald 1990; Calflora 2019). They are intolerant of shade and are most often found in well-drained soil on relatively mesic montane sites (McDonald 1990; Jimerson & Carothers 2002; CNPS 2019). Other tree species commonly associated with black oak woodlands include ponderosa pine (Pinus ponderosa), Oregon white oak, tanoak, Pacific madrone (Arubutus menziesii), and Douglas-fir (Pseudotsuga menziesii), among others (McDonald 1990; Long et al. 2016; CNPS 2019). Understory species can include California hazel (Corylus cornuta californica), bush chinquapin (Castanopsis sempervirens), California gooseberry (Ribes californicum), Ceanothus spp., greenleaf manzanita (Arctostaphylos manzanita), chamise (Adenostoma spp.), buckthorn (Rhamnus spp.), serviceberry (Amelanchier spp.), snowberry (Symphoricarpos spp.), willow (Salix spp.), Rubus spp., Rosa spp., Prunus spp., western poison oak (Rhus diversiloba), oceanspray (Holodiscus discolor), and bearclover (Chamaebatia foliolosa; Bolsinger 1988; McDonald & Tappeiner 2002).

1 For the purposes of this assessment, woodland structures are being modeled after the descriptions of oak-dominated habitats in Altman and Stephens (2012), which are primarily distinguished based on canopy cover. Black oak and tanoak primarily fall within the open canopy (25–50% cover) or closed canopy (50–75% cover) categories. Degraded black oak and tanoak woodlands can occur with over 75% cover due to tree/shrub encroachment, but intact systems would not typically fall into this category.


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Tanoak woodlands are concentrated at lower elevations along the North Coast and North Coast Range (Tappeiner et al. 1990; Bowcutt 2013). Tanoak woodlands primarily occur from 150-900 m (500-3,000 ft) in elevation (Niemiec et al. 1995), although the species can be found between 0-2,000 m (0-6,500 ft) across its entire range (McDonald & Tappeiner 2002). Disjunct populations of tanoak also occur in the Klamath Mountains and Sierra Nevada (Tappeiner et al. 1990). Unlike black oak, tanoak is shade-tolerant and primarily occurs as a component of the sub-canopy or shrub layer within the Douglas-fir and coast redwood (Sequoia sempervirens) zone (Tappeiner et al. 1990; Fryer 2008). However, large-diameter trees with full crowns can dominate the canopy within woodlands maintained through frequent low-intensity fire (Anderson 2005; Bowcutt 2013; Norgaard et al. 2016). Tanoak is most commonly found on productive sites with abundant moisture and deep, well-drained soils (Tappeiner et al. 1990). Common associates in addition to Douglas-fir and coast redwood include Pacific madrone, black oak, California bay (Umbellularia californica), Port-Orford-cedar (Chamaecyparis lawsoniana), white fir (Abies concolor), western hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis), Pacific yew (Taxus brevifolia), and incense cedar (Calocedrus decurrens; Tappeiner et al. 1990; Niemiec et al. 1995; Norgaard et al. 2016; CNPS 2019). On less productive sites and in warmer, drier areas on the periphery of its geographic and elevational range, it can be found with canyon live oak, interior live oak, Oregon white oak, ponderosa pine, sugar pine (P. lambertiana), and giant golden chinquapin (Chrysolepis chrysophylla; Tappeiner et al. 1990; Niemiec et al. 1995; Vuln. Assessment Reviewer, pers. comm., 2018). Understory species composition can include evergreen huckleberry (Vaccinium ovatum), California hazel, salal (Gaultheria shallon), Pacific rhododendron (Rhododendron macrophyllum), western poison oak, flowering current (Ribes sanguineum), thimbleberry (Rubus parviflorus), and tall Oregon grape (Mahonia aquifolium), among others (Tappeiner et al. 1990; Niemiec et al. 1995). Both black oak and tanoak are considered ecological and cultural keystone species (Anderson 2005; Bowcutt 2013; Halpern 2016; Long et al. 2016; Norgaard et al. 2016; Karuk Tribe 2019). They play a critical role in the vertebrate food web (Raphael 1987; McDonald 1990; Long et al. 2016) and acorns from these species were historically the preferred food source for northwestern California tribes (Anderson 2005, 2007; Norgaard 2005; Lynn et al. 2014; Halpern 2016; Long et al. 2016, 2017; Norgaard et al. 2016). Many plants and fungi associated with black oak and tanoak woodlands are also highly valued by northern California tribes for food, basketry, medicine, and other purposes (Anderson 1993, 2005; Shebitz 2005; Anderson 2007; Ortiz 2008a; Anderson & Lake 2013; Lake & Long 2014; Long et al. 2016; Norgaard et al. 2016; Long et al. 2017; Karuk Tribe 2019).


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Executive Summary The relative vulnerability of black oak and tanoak oak woodlands in northern California was evaluated as moderate by regional experts due to moderate-high sensitivity to climate and non-climate stressors, moderate exposure to projected future climate changes, and moderate adaptive capacity.

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Climate and climate-driven factors:

• Climatic water deficit, soil moisture, precipitation amount and timing, drought, heat waves, timing of snowmelt/runoff

Disturbance regimes:

• Disease (including sudden oak death), wildfire, introduced insects Non-climate stressors:

• Fire exclusion, roads/highways/trails, timber harvest, livestock grazing, residential and commercial development, agriculture

Black oak and tanoak woodlands are primarily sensitive to changes in climate stressors that alter water availability, including reduced soil moisture, changes in precipitation amount and timing, increased drought, more heat waves, and earlier timing of snowmelt and runoff. Increased moisture stress impacts acorn germination and seedling/sapling growth and survival, increases vulnerability to disturbance-related mortality, alters fuel dynamics and wildfire behavior, and impacts the prevalence of insect pests and disease. Black oak and tanoak are well-adapted to low- and moderate-intensity fire, which maintains woodland structure and persistence on the landscape by limiting competition from other conifers and hardwoods. However, changes in the frequency, intensity, and/or severity of fires may increase tree mortality, prevent successful sapling recruitment, and potentially shift stand structure toward shrubby, multi-stemmed trees. While mortality from insects and disease are historically low in black oak and tanoak, the spread of diseases such as sudden oak death is likely to cause significant shifts in woodland structure and composition as dominant overstory trees are lost. Black oak may also be impacted by the goldspotted oak borer (Agrilus auroguttatus) if it becomes established in the region due to the movement of infested firewood. Non-climate stressors have significantly contributed to the loss and degradation of black oak and tanoak woodlands across the region, and are likely to exacerbate the negative impacts of climate change. Fire suppression and exclusion of prescribed fire and tribal stewardship has resulted in conifer encroachment, lack of oak regeneration, and subsequent declines in black oak woodlands. Historic extraction of tanoak in the 1800s and early 1900s also had a major effect on the age distribution and structure of tanoak woodlands, and chemical controls were historically used to keep tanoak (and sometimes black oak) at low densities in timberlands.

Black Oak and

Tanoak Woodlands Rank Confidence

Sensitivity Moderate-High High

Future Exposure Moderate Low

Adaptive Capacity Moderate High

Vulnerability Moderate High


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Other non-climate stressors that impact black oak and tanoak woodlands include residential and commercial development, agriculture, transportation infrastructure (e.g., roads, highways, and trails), and livestock grazing. These stressors can degrade black oak and tanoak woodlands by fragmenting habitats, spreading invasive plants and pathogens, and altering ecosystem processes.

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Factors that enhance adaptive capacity:

+ Adapted to rapid recovery following wildfire (depending on intensity)

+ Historically high understory species diversity, which supports a broad array of wildlife

+ Holds significant cultural value for northern California tribes Factors that undermine adaptive capacity:

− Significant loss of black oak and tanoak woodlands due to fire exclusion and associated encroachment of conifers and fire-intolerant hardwoods

− Low tree vigor and increased fuel availability and continuity in dense stands has decreased resistance to fire and other disturbances

The extent and integrity of black oak ecosystems has been significantly reduced by fire exclusion and associated conifer encroachment. High topographic diversity within the region has contributed to the wide distribution and heterogeneous composition of black oak and tanoak woodlands, which support very high biodiversity. Because they can sprout following complete stem mortality, black oak and tanoak are resistant to disturbances and exhibit rapid post-fire recovery. However, reduced vigor due to increased density of conifers and fire-intolerant hardwoods caused direct black oak mortality and enhanced vulnerability to disturbances such as intense fire. Both black oak and tanoak are uniquely valued by tribal communities in the region, and the further loss of black oak and tanoak woodlands would continue to erode the many ecological and cultural services traditionally provided by these habitats. The scientific literature suggests several management strategies that can result in the successful restoration of black oak and tanoak woodlands and may increase the resilience of this habitat to climate impacts. These include removing encroaching conifers from black oak woodlands, reintroducing frequent, low- to moderate-intensity fire, and enhancing collaborative partnerships between tribes, public land managers, and private landowners that focus on ecocultural restoration.

Sensitivity and Exposure Black oak and tanoak woodlands were evaluated by regional experts as having moderate-high overall sensitivity (high confidence in evaluation) and moderate overall future exposure (low confidence) to climate and climate-driven factors, changes in disturbance regimes, and non-climate stressors. Climate changes are projected to alter the distribution of black oak and tanoak woodlands across the state by the end of the century, primarily due to warmer temperatures, decreased moisture availability, and increased wildfire activity (Lenihan et al. 2008; Monleon & Lintz 2015; Davis et al. 2016a; Serra-Diaz et al. 2016b). In general, oaks are projected to have an advantage


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in a hotter, drier climate compared to pines and other conifers (McIntyre et al. 2015; Anderegg et al. 2015), and Lenihan et al. (2008) projected expansion of hardwoods into areas currently dominated by conifers. Some studies have already observed shifts in patterns of regeneration for tanoak and black oak, with regeneration in both becoming more concentrated at higher elevations (Monleon & Lintz 2015; Serra-Diaz et al. 2016b). Tanoak regeneration is also occurring in a smaller geographical area, suggesting that future range contractions may occur (Serra-Diaz et al. 2016b). For black oak, high interannual variability in climate conditions produces significant year-to-year expansion and contraction of areas suitable for black oak establishment (Davis et al. 2016a). Overall, the frequency with which windows of seedling establishment occur is likely to decline over the coming century (Davis et al. 2016a; Serra-Diaz et al. 2016a). However, exposure to climate change may be buffered by topographic features on some sites, including higher-elevation sites and on steep, north-facing slopes (Dobrowski 2011; Olson et al. 2012; Flint et al. 2013). It is important to note that studies examining species distributions (e.g., Monleon & Lintz 2015; Serra-Diaz et al. 2016b) and those looking at very broad categories of forest types (e.g., Lenihan et al. 2008) do not provide information about the climate conditions and disturbance regimes that support the occurrence of black oak and tanoak within a woodland habitat structure, as opposed to their existence as a component species within other forest types (e.g., mixed conifer, mixed evergreen). Additionally, black oak is less tolerant of warm, dry conditions than other common Quercus species (e.g., blue oak, valley oak, interior live oak), so studies discussing general trends in the genus may not adequately represent black oak.

Sensitivity and future exposure to climate and climate-driven factors

Regional experts evaluated black oak and tanoak woodlands as having moderate sensitivity to climate and climate-driven factors (moderate confidence in evaluation), with an overall moderate-high future exposure to these factors within the study region (low confidence). Key climatic factors that affect black oak and tanoak woodlands include climatic water deficit, soil moisture, precipitation amount and timing, drought, heat waves, and timing of snowmelt/runoff.2

2 Climate and climate-driven factors presented are those ranked as having a moderate or higher impact on this habitat type; additional climate and climate-driven factors that may influence the habitat to a lesser degree include storms. Black oak and tanoak woodland vegetation may also be impacted by warmer winter temperatures and fewer days below freezing (Vuln. Assessment Reviewer, pers. comm., 2018), particularly for species such as black oak that requires chilling prior to acorn germination (McDonald 1990).

Potential Changes in Habitat Distribution

• Likely upslope shifts in distribution (no latitudinal shifts observed)

• Overall range contractions likely for tanoak

• Declining frequency of windows of opportunity for black oak seedling establishment

• Possible refugia include higher-elevation sites and steep, north-facing slopes

Source(s): Dobrowski 2011; Olson et al. 2012; Flint et al. 2013; Monleon & Lintz 2015; Davis et al. 2016a; Serra-Diaz et al. 2016b


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Climatic water deficit, soil moisture, precipitation amount and timing, and drought Water availability and corresponding moisture stress impacts growth, seedling establishment, and acorn production, and is strongly associated with woodland species composition and habitat distribution (Tappeiner et al. 1986, 1990; Jimerson & Carothers 2002; McDonald & Tappeiner 2002; Davis et al. 2016b, 2016a). Climatic water deficit (CWD) provides a “plant-relevant” metric to account for the interaction between water (precipitation) and energy (temperature; Stephenson 1998), and is considered a useful way to measure moisture stress for oaks because it is strongly correlated with species distribution (Tappeiner et al. 1990; Lutz et al. 2010; Davis et al. 2016b) and seedling recruitment (McDonald & Tappeiner 2002; Davis et al. 2016a).3 In California, where winter rain provides the majority of annual moisture between December and March, the balance between plant water supply and water demand shifts over the course of the year, with CWD increasing as soil moisture from winter rains is depleted and evapotranspiration increases in warmer months (Stephenson 1998; Davis et al. 2016b). Site-level CWD is highly influenced by soil water-holding capacity (Lutz et al. 2010), as well as topographic variability (e.g., slope, aspect) that alters evaporative demand (Davis et al. 2016a, 2016b). In general, oak seedling survival and growth are higher on sites that maintain higher moisture levels due to patterns of precipitation, topography, and access to surface water sources and groundwater (Davis et al. 2016a, 2016b). Moisture stress is one of the primary limiting factors for seedling survival (Tappeiner et al. 1986; McDonald & Tappeiner 2002; Davis et al. 2016a), although both black oak and tanoak seedlings have adaptations that increase the potential for survival (McDonald & Tappeiner 2002). For instance, above-ground shoot emergence in both species is delayed because energy is first concentrated into the development of a long taproot that increases access to soil moisture (McDonald & Tappeiner 2002). Additionally, seedlings can resprout following dieback under stressful environmental conditions such as drought, and black oak seedlings maintain very low growth rates in the first year while their root systems develop (McDonald & Tappeiner 2002). Tanoaks have a deep root system and sclerophyllous leaves (i.e., thick, shiny leaves), which provide further resistance to drought (Niemiec et al. 1995). They also have some ability to close their stomata in response to decreasing soil moisture, minimizing water loss (Harrington et al. 1994). Oaks (and hardwoods more generally) are likely to have an advantage over conifers as climate conditions become warmer and drier (McIntyre et al. 2015; Anderegg et al. 2015). Oaks, in particular, are well-adapted to periods of summer drought, though black oak is less tolerant of dry conditions than other oaks (McDonald & Tappeiner 2002). CO2 fertilization may also offset some of the impacts of reduced moisture (Vuln. Assessment Workshop, pers. comm., 2017). However, drier conditions are likely to limit black oak seedling establishment by reducing the frequency of years with cool, moist conditions that allow acorn germination and seedling

3 CWD, calculated as potential evapotranspiration (PET) minus actual evapotranspiration (AET), measures the degree to which the impact of local atmospheric conditions (particularly air temperature and relative humidity) on plants and soil exceeds available moisture (Stephenson 1998). Due to increased evaporative demand as air temperatures rise, even areas where precipitation may increase are expected to see a rise in CWD under future climate conditions (Thorne et al. 2015; Micheli et al. 2018).


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survival, particularly at lower elevations (Davis et al. 2016a). For tanoak, drier conditions and increases in drought are likely to reduce acorn production, which has been observed to decrease in dry years (Tappeiner et al. 1990; McDonald & Tappeiner 2002). Increases in CWD are associated with higher tree mortality rates (van Mantgem et al. 2009), particularly for drought-sensitive seedlings and saplings (Young et al. 2017) and larger, older trees near the limits of their range (Lutz et al. 2010). During periods of severe and/or prolonged drought, mortality is most likely to occur on drier sites and in dense stands due to greater competition for soil moisture resources (Young et al. 2017). Reduced vigor in drought-stressed trees may also increase their vulnerability to wildfire, outbreaks of insect pests, and disease (Kliejunas 2011; Bowcutt 2013; van Mantgem et al. 2013; Millar & Stephenson 2015). Severe drying may also impact mycorrhizal networks such as those associated with tanoaks (Classen et al. 2015; Vuln. Assessment Workshop, pers. comm., 2017), which are crucial to ecosystem functioning and support many species valued by northern California tribes, including tanoak mushrooms (Anderson & Lake 2013). Projected increases in winter rainfall (Pierce et al. 2018; Swain et al. 2018) are likely to increase spore production and transmission of the Phytophthora ramorum, the pathogen that causes sudden oak death (Davidson et al. 2005; Kliejunas 2011; Meentemeyer et al. 2011; DiLeo et al. 2014). By contrast, dry conditions and low soil moisture have a strong limiting effect on sudden oak death (Venette & Cohen 2006).

Regional Precipitation, Climatic Water Deficit (CWD), Soil Moisture, and Drought Trends4

Historical & current trends:

• 7.2–9.4 cm (2.8–3.7 in) increase in mean annual precipitation and 1.1 cm (0.4 in) decrease to 0.4 cm (0.2 in) increase in average annual CWD between 1900 and 2009 for the Northwestern California and Southern Cascade ecoregions (Rapacciuolo et al. 2014)

• No trends available for soil moisture

• Drought years have occurred twice as often over the last two decades compared to the previous century (Diffenbaugh et al. 2015)

• 2012–2014 drought set records for lowest

Projected future trends:

• 20% decrease to 34% increase in mean annual precipitation by 2100 (compared to 1951–1980) for the North Coast, Northern Coast Range, Northern Interior Coast Range, Klamath Mountain, and Southern Cascade ecoregions

(Flint et al. 2013; Flint & Flint 2014)5

• Increases in average annual CWD by 2100 (compared to 1951–1980; Flint et al. 2013; Flint & Flint 2014) o 9–29% increase on the North Coast o 7–24% increase in the Northern Coast

4 Trends in climate factors and natural disturbance regimes presented in this and subsequent summary tables are not habitat-specific; rather, they represent broad trends and future projections for the study region. The precipitation, temperature, climatic water deficit, and snowpack projections for this project are derived from the Basin Characterization Model, which uses modified Jepson ecoregions (Flint et al. 2013; Flint & Flint 2014). Projections for all other factors are based on a review of relevant studies in the scientific literature. For this project, exposure was evaluated by calculating the magnitude and direction of projected change within the modified Jepson ecoregions that include habitat distribution within the study geography. 5 Projections for changes in annual and seasonal precipitation by ecoregion can be found in the full climate impacts table (https://bit.ly/2LHgZaG).

https://bit.ly/2LHgZaG


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Regional Precipitation, Climatic Water Deficit (CWD), Soil Moisture, and Drought Trends4

precipitation, highest temperatures, and most extreme drought indicators on record (Griffin & Anchukaitis 2014; Diffenbaugh et al. 2015)

Range o 5–16% increase in the Northern Interior

Coast Range o 10–32% increase in the Klamath Mountains o 16–43% increase in the Southern Cascades

• Seasonal changes are projected to be more significant as the wet season becomes wetter and shorter (i.e., later onset of fall rains and earlier onset of summer drought) and the dry season becomes drier and longer (Pierce et al. 2018; Swain et al. 2018)

• Overall, interannual variability is expected to increase (Pierce et al. 2018; Swain et al. 2018)

• Increased CWD and decreased top-level soil moisture is likely even if precipitation increases due to temperature-related changes in evaporative demand (Thorne et al. 2015; Micheli et al. 2018; Pierce et al. 2018)

• Drought years are twice as likely to occur over the next several decades due to increased co-occurrence of dry years with very warm years (Cook et al. 2015)

• 80% chance of multi-decadal drought by 2100 under a high-emissions scenario (Cook et al. 2015)

• Severe droughts that now occur once every 20 years will occur once every 10 years by 2100 and once-in-a-century drought will occur once every 20 years (Pierce et al. 2018)

Summary of Potential Impacts on Habitat (see text for citations)

• Changes in oak woodland species composition and habitat distribution based on patterns of water availability

• Reduced seedling establishment and tree growth due to drier conditions

• Increased mortality of large trees and those near the limits of their tolerance for water stress, particularly in dense stands where competition for soil moisture is high

• Increased vulnerability of drought-stressed trees to mortality from wildfire, insect pests, and disease

• Possible increased dominance and/or expansion of oaks into areas currently occupied by conifers that are less tolerant of water stress

• Likely increases in spore production and transmission of P. ramorum based on projected increases in winter rainfall


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Heat waves In northern California, late summer and fall heat waves are frequently associated with the persistent high pressure systems that produces prolonged periods of high temperatures and low humidity (Schroeder et al. 1964). Wildfire risk increases dramatically during heat waves (Schroeder et al. 1964), and high temperatures are correlated with increases in fire size, rate of spread, and severity (Sharples 2009; Estes et al. 2017). Extreme fire conditions can occur when the high pressure system also produces warm, dry east winds (i.e. foehn winds; Schroeder et al. 1964). It is likely that heat waves also impact oaks and associated vegetation directly, though little information is available. However, tanoak is relatively sensitive to heat, preferring at least partial shade for growth (Niemiec et al. 1995). Sudden exposure to bright sunlight can damage foliage, causing rapid dieback in exposed parts of the crown (McDonald & Tappeiner 2002).

Regional Heat Wave Trends


• Increase in the frequency of humid nighttime events over the past several decades (Gershunov & Guirguis 2012)

• High interannual and interdecadal variability in heat waves (Gershunov & Guirguis 2012)


• Increased heat waves, with the greatest increase in humid nighttime heat waves and in coastal areas (Gershunov & Guirguis 2012)

• 2–6°C (3.6–10.8°F) increase in the temperature of the hottest day of the year by 2100 (Pierce et al. 2018)


• Increased wildfire risk and greater likelihood of extreme fire behavior, particularly when heat waves are accompanied by foehn winds

Timing of snowmelt and runoff Snowmelt is an important source of soil moisture (Bales et al. 2011) in montane areas that support black oak woodlands (Vuln. Assessment Reviewer, pers comm., 2019). Increasing air temperatures and shifts toward more precipitation falling as rain rather than snow are expected to drive earlier timing of snowmelt and runoff (Knowles et al. 2006; Rauscher et al. 2008; Reba et al. 2011), reducing soil moisture availability during the growing season and leading to longer periods of summer drought (Bales et al. 2011; Reba et al. 2011). Earlier snowmelt has also been correlated with increased wildfire activity, including fire size and length of the fire season (Westerling 2016). Forests that receive a significant portion of their annual precipitation from snow are particularly sensitive to the impacts of earlier snowmelt on wildfire (Westerling 2016).

Regional Snowmelt Trends


• 15–40-day shift towards earlier date of 90% snowmelt across the western U.S. since 1915


• Likely 5–15-day shift towards earlier timing of snowmelt-driven runoff in northern California


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Regional Snowmelt Trends

(Hamlet et al. 2005)

• 10–30-day shift towards earlier timing of snowmelt-driven runoff across the western U.S. since 1948 (Stewart et al. 2005)

by 2100 (up to 60-day shift across the western U.S.; Stewart et al. 2004; Rauscher et al. 2008)


• Reduced growing season soil moisture availability and longer periods of summer drought

• Increase wildfire activity (e.g., fire size, length of fire season) in mid- to high-elevation forests

Sensitivity and future exposure to changes in natural disturbance regimes

Regional experts evaluated black oak and tanoak woodlands as having moderate-high sensitivity to changes in natural disturbance regimes (high confidence in evaluation), with an overall moderate future exposure to these stressors within the study region (moderate confidence). Key natural disturbance regimes that affect black oak and tanoak woodlands include disease (including sudden oak death), wildfire, and introduced insects.6 Disease Historically, most native pathogens have not caused extensive mortality in black oak (Haavik et al. 2015), although the species can be impacted by oak mistletoe (Phoradendron villosum) and a number of fungal diseases that cause root rot (e.g., Armillaria mellea) and leaf or twig blight (e.g., Apiognomonia errabunda, Septoria quercicola; Swiecki & Bernhardt 2006). Canker rot fungi (e.g., Inonotus andersoni, I. dryophilus) can also cause trunk and branch cankers in black oak, resulting in large cavities and potentially causing complete mortality over the course of several years (Swiecki & Bernhardt 2006). Tanoak is most commonly impacted by fungi that cause decay after entering a tree injured by fire (Niemiec et al. 1995). Fire was historically utilized to control diseases that impacted oaks and other important cultural resources (Bowcutt 2013; Halpern 2016). Changes in climate can impact the prevalence and severity of disease by directly influencing pathogen production, transmission, and survival, or indirectly by altering tree defenses, host susceptibility, and community interactions (Kliejunas 2011; Sturrock et al. 2011; Weed et al. 2013; Kolb et al. 2016). The spread and establishment of exotic pathogens, in particular, are likely to be associated with changes in climate such as warmer temperatures and altered patterns of precipitation (Kliejunas 2011; Meentemeyer et al. 2011) in addition to a shifts in forest structure and composition associated with anthropogenic factors (e.g., fire exclusion; Haavik et al. 2015). Warmer temperatures may cause the distribution of many diseases to expand, exposing new areas of forest and woodlands to novel pathogens (Kliejunas 2011). In general, diseased trees are more vulnerable to drought and other stressors (Kolb et al. 2016). Armillaria and other root rot pathogens are also more likely to colonize drought-stressed trees,

6 Disturbance regimes presented are those ranked as having a moderate or higher impact on this habitat type; additional changes in disturbance regimes that may influence the habitat to a lesser degree include wind.


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suggesting that these could become more severe and/or widespread as drought increases (Sturrock et al. 2011; Kolb et al. 2016). Sudden oak death, caused by the introduced pathogen Phytophthora ramorum, is the most significant threat to black oak and tanoak woodlands and has caused extensive tree injury and mortality in coastal forests since it was discovered in the 1990s (Davidson et al. 2003; Rizzo & Garbelotto 2003; Meentemeyer et al. 2004; McPherson et al. 2010), including the loss of over a million tanoak trees in less than twenty years (Bowcutt 2014). The disease is currently distributed from coastal central California northwards into Humboldt County, with an additional infected site occurring in Curry County, Oregon (Meentemeyer et al. 2011; Filipe et al. 2012). Sudden oak death causes very high rates of mortality in tanoak, which is more susceptible to infection than most other species (Rizzo et al. 2005; McPherson et al. 2010; Cobb et al. 2012; Metz et al. 2012); mortality also occurs in black oak and several other true oaks (McPherson et al. 2010; Kliejunas 2011). Sudden oak death can affect over 130 plant species (Kliejunas 2011), many of which are utilized by tribes for food, medicine, and other purposes (Ortiz 2008b, 2008a; Voggesser et al. 2013). These include California bay, Pacific madrone, California hazel, evergreen huckleberry, manzanita, and salmonberry (Rubus spectibilis; Ortiz 2008a, 2008b). Transmission of sudden oak death Spread of P. ramorum is facilitated by host plants that support spore production, including California bay and tanoak (Rizzo & Garbelotto 2003; Cobb et al. 2010). California bay has the highest spore production and does not experience mortality (Rizzo & Garbelotto 2003; Davidson et al. 2005), and the presence of this species is a major driver in transmission of P. ramorum (Cobb et al. 2010, 2012; McPherson et al. 2010; Haas et al. 2016). While tanoak also supports spore production, high mortality rates in infected tanoak trees result in more limited spread of the disease (Cobb et al. 2010). Moisture is required for P. ramorum spore production and survival, with frequency, timing, and intensity of winter rain and storms playing a large role in inoculum production (Davidson et al. 2005; Venette & Cohen 2006; Kliejunas 2011; DiLeo et al. 2014). Spores are dependent on aerial transmission, which primarily occurs through rainsplash or being carried downstream as well as through human-mediated channels (e.g., movement of contaminated soils; Davidson et al. 2005; Kliejunas 2011). Because plant gathering risks spreading the disease, many tribal members have been forced to restrict visits to the forest (Anderson 2007; Voggesser et al. 2013). Impacts of sudden oak death Sudden oak death can cause leaf blight, stem dieback, and/or complete mortality following the development of trunk cankers (Rizzo & Garbelotto 2003; Kliejunas 2011). Affected trees may remain relatively asymptomatic for several years following initial infection, then die quickly after onset of visible symptoms (Kliejunas 2011). Large trees are more susceptible to infection and have higher mortality rates (McPherson et al. 2010; Metz et al. 2012; Haas et al. 2016), and beetle attacks in infected trees can reduce life expectancy by an additional 65–70% (McPherson et al. 2010). However, open woodland habitats and stands with high woody plant diversity have lower infection risk due to reduced host density (Skinner et al. 2006; Haas et al. 2016). Because the impacts of sudden oak death are species-specific, patterns of mortality can significantly


13

alter woodland structure and species composition (Cobb et al. 2012; Metz et al. 2012; Haas et al. 2016). For instance, higher rates of mortality in large tanoaks and black oaks can allow increased dominance of species with lower susceptibility to infection, such as Douglas-fir (Cobb et al. 2010, 2012; Metz et al. 2012). California bay may also become increasingly dominant within the understory (Haas et al. 2016), potentially increasing transmission and infection risk due to greater host density (Cobb et al. 2012; Metz et al. 2012). In areas impacted by sudden oak death, disease-related mortality and resulting shifts in species composition may also alter fuel composition and availability, potentially impacting fire behavior (Metz et al. 2011, 2013, 2017; Forrestel et al. 2015; Varner et al. 2017). However, the relationship between disease-related mortality, fuel dynamics, and fire severity is complex and depends on multiple factors, including the length of time between infection and fire (Metz et al. 2011). For example, fire severity may increase in stands that burn shortly after infection due to dead leaves and twigs that remain on standing trees (Metz et al. 2011). In the intermediate stages of disease, a mixture of aerial, ladder, and surface fuels in various stages of decomposition can promote intense fires and transmit surfaces fires into the canopy (Metz et al. 2013). As time since infection progresses, fallen trees provide a greater proportion of surface fuels, which can cause fires to smolder for long periods of time and increase soil damage (including a loss of soil nutrients) and post-fire erosion (Metz et al. 2011; Cobb et al. 2016). Shifts in forest structure and species composition can also alter fuel dynamics on longer time scales due to changes in litter composition that affect fuelbed flammability (Varner et al. 2017). Although it is unlikely that tanoak will completely disappear from the forest due to its sprouting ability, the presence of the P. ramorum pathogen may prevent tanoak maturing (Cobb et al. 2012; Metz et al. 2012) by killing sprouts repeatedly before they produce acorns (Ramage et al. 2011; Bowcutt 2014). Thus, sudden oak death has the potential to contribute to functional extirpation of this species across large parts of its range (Dillon et al. 2013), potentially leading to novel ecosystem conditions as other species are unlikely to fulfill the role of tanoak within the region (Ramage et al. 2011; Metz et al. 2012). For instance, the loss of acorns could have cascading effects on ecosystem food webs, reducing habitat quality and food resources for many northwestern California tribes and wildlife (Monahan & Koenig 2006; Fryer 2008; Ramage et al. 2011; Bowcutt 2014). Ecosystem processes (e.g., nutrient cycling) could also be impacted by the loss of the extensive mycorrhizal networks associated with tanoak (Bergemann et al. 2013; Cobb et al. 2013b). Changes in soil nitrogen availability and litterfall dynamics due to the mortality in overstory trees also increases the risk of impacts to ecosystem processes (Rizzo & Garbelotto 2003; Cobb et al. 2013b).

Interactions between climate changes and sudden oak death Changing climate conditions may alter patterns of spore production, disease transmission, and mortality in forests impacted by sudden oak death (Meentemeyer et al. 2004; Venette 2009; Kliejunas 2011; Sturrock et al. 2011). Models projecting P. ramorum dispersal and infection risk based on host availability and weather conditions suggest that coastal areas from Mendocino County through southwestern Oregon are at very high risk of infection over the next several decades (Meentemeyer et al. 2011). Warmer winter temperatures and increased winter and


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spring precipitation would likely enhance spore production and increase infection risk (Kliejunas 2011; Meentemeyer et al. 2011; DiLeo et al. 2014), with favorable weather conditions potentially doubling the rate of spread by 2030 (Meentemeyer et al. 2011). Conversely, drier summer conditions could reduce disease prevalence by limiting P. ramorum (Davidson et al. 2005; Venette & Cohen 2006), although the most significant increases in drought stress are projected to occur in areas already considered climatically unsuitable for the spread of sudden oak death (Venette 2009).

Regional Disease Trends


• Northwards expansion of sudden oak death into Humboldt County since the discovery of the disease in the 1990s (Meentemeyer et al. 2004)


• Range expansion in many diseases as temperatures increase, particularly exotic pathogens (Kliejunas 2011)

• Increased spore production and infection risk in the North Coast and North Coast Range regions, resulting in range expansion of sudden oak death northwards and slightly inland (Venette 2009; Meentemeyer et al. 2011)

• Suitable weather conditions (e.g., warmer temperatures, increased winter and spring precipitation) could double the rate of spread by 2030 (Meentemeyer et al. 2011)


• Changes in pathogen production, transmission, and survival, as well as alterations in tree defenses, host susceptibility, and community interactions

• Increased tree mortality in response to introduced pathogens such as P. ramorum and/or during periods of drought when tree vigor is reduced o Possible functional extirpation for tanoak in many parts of the species’ range

• Shifts in species composition and woodland structure due to the loss of overstory trees

• Potential changes in fire behavior due to altered fuel composition and availability

• Reduced food resources and habitat quality for wildlife, with possible impacts to the food web

• Changes in ecosystem processes due to the loss of ectomycorrhizal networks associated with tanoak

• Loss of cultural resources including mature acorn-producing trees, tanoak mushrooms, and other culturally-valuable species susceptible to sudden oak death

Wildfire Fire is a critical disturbance that influences species composition and structure in black oak and tanoak woodlands (Pavlik et al. 1991; Martin & Sapsis 1992; Mensing 2005; Skinner et al. 2006; Stuart & Stephens 2006; Fryer 2008; Cocking et al. 2012b; Bowcutt 2013; Long et al. 2016). The forested landscape in northern California is dominated primarily by mixed-severity fire


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regimes,7 with fire generally becoming more frequent and less severe on drier inland sites (Fryer 2008; Perry et al. 2011). At lower elevations where lightning ignitions are rare, human ignitions historically dominated fire regimes (van Wagtendonk & Cayan 2008; Crotteau et al. 2015; Keeley & Syphard 2017). Prior to Euro-American settlement, black oak stands on mid-elevation sites may have been burned every 5–7 years (Skinner et al. 2006; Anderson 2007; Cocking et al. 2012b; Long et al. 2016; Norgaard et al. 2016), while tanoak was burned as frequently as every year (Schenck & Gifford 1952; Anderson 2007). Following the loss of tribal management and introduction of widespread fire suppression in the early 1900s, fire return intervals lengthened considerably in black oak and tanoak woodlands and surrounding forested areas (Skinner et al. 2006, 2009; Stuart & Stephens 2006; Safford & Van de Water 2014; Steel et al. 2015; Taylor et al. 2016). Ultimately, most black oak woodlands across the region have been lost as a result of increased competition associated with fire exclusion and the removal of tribal oak stewardship that maintained stands dominated by large-diameter trees (Parsons & DeBenedetti 1979; Kauffman & Martin 1987; Anderson 2005; Skinner & Taylor 2006; Stuart & Stephens 2006; Anderson 2007; Fryer 2007; Lake 2007; Cocking et al. 2012b; Lake & Long 2014; Long et al. 2016).8 Impacts of low- and moderate-intensity fire Periodic fire limits encroaching shrub and conifer species and opens up the canopy, allowing shade-intolerant species such as oaks to persist without being overtopped by conifers competing for light and soil moisture (Fryer 2008; Perry et al. 2011; Cocking et al. 2012b, 2014, 2015; Crotteau et al. 2015; Long et al. 2016). For black oaks, fire may enhance acorn germination and growth, likely due to decreased competition from understory vegetation on disturbed sites (Kauffman & Martin 1987). Frequent low- and moderate-intensity fire promotes and maintains more open stands by killing conifer seedlings while protecting large mature acorn-bearing trees (Anderson 2005, 2007; Skinner & Taylor 2006; Bowcutt 2013; Cocking et al. 2014, 2015; Halpern 2016; Long et al. 2016, 2017; Norgaard et al. 2016). Fire also reduces insect pests (e.g., filbert weevil [Curculio occidentis], filbertworm [Cydia latiferreana]) and disease that affect tree health and acorn quality (Anderson 2005, 2007; Halpern 2016; Long et al. 2016). Many tribes implement cultural prescribed burns in the fall to kill overwintering insects and promote the production of acorns and other resources such as tanoak mushrooms and evergreen huckleberry (Anderson 2005, 2007; Anderson & Lake 2013; Bowcutt 2013; Lake 2013; Halpern 2016; Long et al. 2016; Norgaard et al. 2016; Karuk Tribe 2019). Additionally, periodic low-intensity fire also supports greater understory plant and fungi diversity (Martin & Sapsis 1992; Wayman & North 2007; Anderson & Lake 2013), prevents the accumulation of high fuel levels that could lead to more frequent high-severity fires that kill mature acorn-bearing trees (Underwood et al. 2003; Bowcutt 2013; Long et al. 2016), and breaks down organic

7 Definitions of mixed-severity fires vary somewhat, but typically describe irregular patches of low-, moderate-, and high-severity fire. Patch sizes and the proportion of high-severity fire depend on forest type, as well as topography, weather, fuel, disturbance history, and other factors that influence fire behavior (Hessburg et al. 2007, 2016; Halofsky et al. 2011; Perry et al. 2011). Patches are generally described at an intermediate or stand-level spatial scale, as opposed to individual trees or landscape-scale (Halofsky et al. 2011; Perry et al. 2011). 8 Refer to the section on non-climate stressors for a more complete discussion of the impacts of fire suppression on black oak and tanoak woodlands.


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matter, releasing nutrients to the soils where they become available for plant use (Anderson 1993, 2005; Neary et al. 1999). Many wildlife species associated with oak-dominated ecosystems also benefit from fire. For instance, black-tailed deer (Odocoileus hemionus columbianus) and elk (Cervus elaphus) benefit from the resulting high-quality forage, and the Karuk burn black oak stands in the spring to promote high-quality understory forage for these species (Bowcutt 2013; Halpern 2016). Impacts of high-intensity fire The impacts of higher-intensity fire are complex, and the severity of these fires varies across space and time depending on factors such as fire frequency and preexisting woodland condition (Neary et al. 1999; Perry et al. 2011; Cocking et al. 2012b, 2014, 2015; Long et al. 2016; Norgaard et al. 2016; Nemens et al. 2018). High-severity and mixed-severity fires can promote the persistence and restoration of shade-intolerant oaks by killing encroaching conifers and allowing oak regeneration in burned areas through burl sprouts and seedlings (Cocking et al. 2012b, 2014, 2015; Crotteau et al. 2015; Long et al. 2016; Nemens et al. 2018). Multiple studies have shown very high rates of sprouting (90%) in black oaks that experienced topkill following a severe wildfire; sprouts in these areas grow rapidly, increasing their probability of successfully outcompeting conifer seedlings (Cocking et al. 2014; Hammett et al. 2017). Following high-severity fire, sprouting hardwoods such as black oak and tanoak often dominate early-successional vegetation, along with fire-stimulated shrubs such as greenleaf manzanita (Shatford et al. 2007; Odion et al. 2010; Cocking et al. 2014; Crotteau et al. 2015; Welch et al. 2016; Nemens et al. 2018). However, high-intensity fires are more likely to cause tree injuries and/or complete mortality, particularly for seedlings and saplings (Hammett et al. 2017; Nemens et al. 2018). Fires tend to be more severe in dense stands due to greater accumulation of litter, as well as the presence of ladder fuels that can promote more severe crown fires (Skinner et al. 2006; Fryer 2008). High levels of competition and associated decreases in tree vigor, such as occurs in fire-excluded stands, also increase the risk of crown or whole tree mortality (Skinner et al. 2006; Cocking et al. 2014), particularly for old legacy oaks (Vuln. Assessment Workshop, pers. comm., 2017). Although black oak and tanoak woodlands are well-adapted to post-fire recovery, changes in wildfire regimes due to warmer, drier conditions may impact oak mortality and regeneration rates, species distribution, and stand structure, as well as multiple other factors that influence oak regeneration and tree vigor (Jimerson & Carothers 2002; Bowcutt 2013; Norgaard et al. 2016; Hammett et al. 2017; Nemens et al. 2018). More frequent, larger, and/or higher-intensity fires are likely to kill large conifers and may allow oaks to reclaim dominance and/or expand their distribution (Welch et al. 2016; Long et al. 2017; Metlen et al. 2017; Nemens et al. 2018). However, this would likely be accompanied by a loss of mature, acorn-bearing oaks, including those within culturally-vital woodlands historically maintained by northern California tribes (Bowcutt 2013; Long et al. 2016, 2017; Norgaard et al. 2016; Hammett et al. 2017). Repeated high-severity fire can also prevent the development of larger trees with fire-resistant characteristics (Long et al. 2016, 2017; Nemens et al. 2018), changing forest and woodland structure by converting stands of mature, large-diameter trees to shrubby, multi-stemmed growth forms (Norgaard et al. 2016; Hammett et al. 2017; Nemens et al. 2018). Thus, the


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development of mature black oak and tanoak woodlands may be prevented in areas where high fuel accumulations following a century of fire exclusion has increased the likelihood of high-severity fire (Long et al. 2016; Norgaard et al. 2016; Nemens et al. 2018). High-severity fire can also affect belowground processes and structures, such as mycorrhizal networks. This can happen directly through very high soil temperatures during intense fires, as well as indirectly, through changes in the aboveground vegetation that impact fungal and bacterial community composition, soil properties (e.g., chemical and physical structure), and other factors (Neary et al. 1999; Anderson & Lake 2013; Graham et al. 2016; Smith et al. 2017). For instance, the loss of host trees following high-severity fires can reduce tanoak mushroom populations until older trees and shrubs are once again established (Hosford et al. 1997; Anderson & Lake 2013; Norgaard et al. 2016). The loss of mycelium nets associated with tanoak mushrooms also impacts nutrient availability to tanoak and associated species (Anderson & Lake 2013). Changes to fungal and soil microbial communities can affect water cycling, mineralization rates, and plant establishment and composition following fire (Neary et al. 1999; Cowan et al. 2016).

Regional Wildfire Trends


• 85% of U.S. Forest Service lands in northern California are burning less frequently compared to pre-1850 fire return intervals, largely due to fire suppression (Safford & Van de Water 2014)

• Fire size and total area burned increased on U.S. Forest Service lands in northwestern California between 1910-2008, with the highest values occurring after 2000 (Miller et al. 2012)

• Changes in large fires (over 400 ha) in the inland northern California/Sierra Nevada region since the 1970s (Westerling 2016): o 184–274% increase in frequency o 270–492% increase in total area burned o 215% increase in length of the fire season

• Changes in fire size, area burned, and fire frequency over the past several decades remain well below historical tribally-influenced frequency and extent of burning in California (Stephens et al. 2007)

• No significant trends in the average areal proportion of high-severity fire were documented in northwestern CA from 1984–2008 (Miller et al. 2012; Parks et al. 2015; Law & Waring 2015; Keyser & Westerling 2017)


• State-wide, up to 77% increase in mean annual area burned and 50% increase in the frequency of extremely large fires (>10,000 ha) by 2100 (Westerling 2018) o Greatest increases in burned area (up to

400%) occur in montane forested areas in northern California (Westerling et al. 2011; Westerling 2018)

o Less significant increases or possible decrease along the North Coast (Westerling et al. 2011)

• Little projected change in fire severity in northwestern California by 2050 in models based solely on historical fire-climate relationships (Parks et al. 2016) o However, human activity and fuel buildup

from decades of fire suppression have altered historical fire-climate relationships (Taylor et al. 2016; Syphard et al. 2017; Wahl et al. 2019), and projections that incorporate these factors suggest that more significant increases in fire severity and size may occur (Mann et al. 2016; Wahl et al. 2019)

• The majority of impacts to natural and human ecosystems come from extreme fire events


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Regional Wildfire Trends

o The relatively short period of record for fire severity data may obscure long-term trends

o To date, there are no peer-reviewed studies on trends in northern California fire severity that include data from the last ten years

(i.e., fires that have a low probability of occurring in any given place and time), which are likely to increase over the coming century (Westerling 2018) o Generally, these patterns are not well-

represented in studies that evaluate indices of mean fire size, intensity/severity, etc.


• Immediate: o Reduced shrub and conifer encroachment following low- to moderate-intensity fire, allowing

oak regeneration o Reduced insect pests that affect tree health and acorn quality o Higher rates of tree injury and mortality following more intense fires, which may cause the loss

of culturally vital acorn-bearing stands

• Short-term (~2-year): o Enhanced reproduction via root crown sprouting (and, to a lesser degree, black oak acorn

germination) o Increased dominance of sprouting oaks in high-severity burned patches where non-sprouting

conifers have been eliminated o Reduced fuel accumulation following low-intensity fires, reducing the future risk of severe fire o Increased biodiversity following low-intensity fires, which creates a mosaic of burned and

unburned habitat that supports many wildlife species

• Long-term: o Enhanced regeneration and persistence of black oak and tanoak woodlands due to loss of

competing conifers o Possible transition to shrubby, multi-stemmed trees following repeated high-severity fires o Loss of acorn-producing trees where high-severity fires are too spatially extensive and frequent

to allow maturation

Insects Although oaks are not affected by insect pests as frequently as conifers (McDonald & Tappeiner 2002), ambrosia beetles (Monarthrum scutellare and M. dentiger) and western oak bark beetles (Pseudopityopthorus pubipennis) may opportunistically attack trees stressed by drought or disease (Davidson et al. 2003; Swiecki & Bernhardt 2006). Mortality of seedlings and sprouts is rare, though insect damage to acorns is common (McDonald & Tappeiner 2002; Swiecki & Bernhardt 2006; Anderson 2007). Late summer and fall prescribed burns, such as those implemented by northern California tribes, are timed to kill these pests during the period of their life cycle when they are present in the duff and soil (Anderson 2005, 2007; Halpern 2016; Long et al. 2016). Black oaks in northern California are potentially at risk from the goldspotted oak borer (GSOB), a previously unknown insect pest within the state (Coleman & Seybold 2008). The GSOB is native to southeastern Arizona, central Mexico, and Guatemala, and was likely introduced into


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California through the movement of infected firewood (Coleman & Seybold 2016). The GSOB has already caused the mortality of over 80,000 oaks in San Diego County (GSOB Steering Committee 2012), and satellite infestations have more recently been discovered in Riverside (2012), Orange (2014, 2017), and Los Angeles (2015) counties as well (GSOB Steering Committee 2018). Where the GSOB has been present for over a decade, infestation rates can be greater than 80% and mortality rates of over 70% have been documented in black oak (Coleman & Seybold 2016). The primary host species for the GSOB in California are black oak, coast live oak (Q. agrifolia), and possibly canyon live oak (Coleman & Seybold 2008, 2016). Because none of these species overlap with the native range of this insect, they lack natural defenses against attack by the GSOB and are highly susceptible to damage and mortality (Coleman & Seybold 2008, 2016). Oaks with visible insect damage (e.g., staining, exit holes on trunk and branches) progressively show signs of premature leaf drop, twig and branch die off, and, eventually, mortality (Coleman & Seybold 2008, 2016; Coleman et al. 2011). Although large, healthy trees can be affected by the GSOB, drought stress appears to increase mortality rates; conversely, injury to the phloem and cambium increases drought stress in infested trees, particularly older trees (Coleman et al. 2011; Coleman & Seybold 2016). Secondary attack from other wood borers and bark beetles is often observed on affected trees in later stages of deterioration (Coleman & Seybold 2016). Spread of the GSOB northwards into central and northern regions of California is possible due to abundant black oak and coast live oak as potential hosts (Venette et al. 2015). Much of northern California is climatically suitable for GSOB survival, particularly north of the Central Valley and in the foothills of the Coast Range (Venette et al. 2015). Based solely on insect dispersal by flight, spread of the GSOB could occur at a rate of 9.3 km (5.8 mi) per year (Venette et al. 2015). The movement of beetle-infested firewood is likely to increase the risk of satellite infections in more distant areas, including northern California, although many public education campaigns are focused on preventing the movement of firewood (Coleman & Seybold 2016).

Regional Insect Trends


• Gold-spotted oak borer (GSOB) discovered in San Diego County in 2004 (Coleman & Seybold 2008)

• Satellite infestations of GSOB discovered in Riverside County in 2012, Orange County in 2014 and 2017, and Los Angeles in 2015 (GSOB Steering Committee 2018)


• Introduction of the GSOB into climatically suitable areas of central and northern California is possible and will likely be associated with the movement of beetle-infested firewood (Venette et al. 2015; Coleman & Seybold 2016)

• In general, more severe insect outbreaks are likely as temperatures increase and periods of drought become more frequent (Raffa et al. 2008; Kolb et al. 2016)


• High rates of mortality in black oak if the GSOB spreads into northern California, particularly in


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Regional Insect Trends

drought-stressed trees

• Increased risk of drought-related stress and secondary attack from other insects (e.g., wood borers, bark beetles) on trees affected by the GSOB

Sensitivity and current exposure to non-climate stressors

Regional experts evaluated black oak and tanoak woodlands as having moderate-high sensitivity to non-climate stressors (high confidence in evaluation), with an overall moderate-high current exposure to these stressors within the study region (high confidence). Key non-climate stressors that affect black oak and tanoak woodlands include fire exclusion, roads/highways/trails, and timber harvest.9 Fire exclusion Fire exclusion over the past century (including the absence of prescribed/cultural burning and active suppression of natural ignitions) has contributed to the decline of black oak woodlands, which are dependent on low- to moderate-intensity fire for persistence (see Table 1; Kauffman & Martin 1987; Stuart & Stephens 2006; Anderson 2007; Fryer 2007; Long et al. 2016; Norgaard et al. 2016). Fire exclusion has allowed the establishment of shade-tolerant species (e.g., Douglas-fir, white fir, incense-cedar, tanoak, California bay) in the understory of black oak woodlands, where they are able to mature and develop fire-resistant bark during long fire-free intervals (Jimerson & Carothers 2002; Skinner et al. 2006; Cocking et al. 2012b, 2015; Long et al. 2016; Norgaard et al. 2016). Encroaching trees compete with black oaks for light, water, nutrients, and space; after overtopping the oak canopy, they cause crown dieback and mortality in existing trees (Cocking et al. 2012b; Bowcutt 2013; Cocking et al. 2015; Crotteau et al. 2015). Legacy trees within encroached stands likely have decreased vigor and may be more vulnerable to mortality from disturbances such as drought and high-severity wildfire (Skinner et al. 2006; Cocking et al. 2012b; Long et al. 2016). Changes in fuel type, structure, and availability associated with fire exclusion and subsequent conifer encroachment can also contribute to shifts in wildfire regimes (Taylor & Skinner 2003; Skinner et al. 2006; Bowcutt 2013; Cocking et al. 2015), promoting less frequent but more severe canopy fires during periods of extreme fire weather (Cocking et al. 2015). Severe fires associated with high fuel consumption significantly increase the rate of complete black oak and tanoak mortality (Kauffman & Martin 1987; Cocking et al. 2012b; Long et al. 2016). In more mesic forests, fire exclusion has not necessarily increased fire severity (Fryer 2008; Steel et al. 2015), likely due to more rapid decomposition of potential fuels (Agee 1996; Odion et al. 2004). In some cases, fire suppression activities (e.g., strategic firing, fire line construction, safety mitigation activities) may impact mature acorn-producing trees and associated understory

9 Climate and climate-driven factors presented are those ranked as having a moderate or higher impact on this habitat type. Additional climate and climate-driven factors that may influence the habitat to a lesser degree include land-use conversion for livestock grazing, residential and commercial development, and agriculture.


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species, including those with high cultural value (Long et al. 2016; Norgaard et al. 2016). For instance, individual trees with existing cavities that pose hazards to firefighters may be felled (Long et al. 2016; Norgaard et al. 2016). Fire suppression activities may also damage the mycelium net associated with tanoak-dominated woodlands (Norgaard et al. 2016). Table 1. Impacts of fire exclusion on the resilience of black oak and tanoak woodlands to climate stressors and climate-driven changes in fire regimes (table adapted from Norgaard et al. 2016).

Prior to Fire During Fire After Fire

• Promotes shade-tolerant species that outcompete black oaks

• Reduced vigor in acorn-producing oaks stressed by drought and competition

• Reduced regeneration of oaks

• High tree density and fuel loads increase the risk of high-severity fire

• High rates of topkill in large, old acorn-producing black oaks during intense fires in dense forests

• Damage or loss of mature acorn-bearing trees and valued understory species during fire suppression activities

• Loss of mature acorn-producing trees and greater proportion of shrubby growth forms following repeated high-severity burns

Source(s): Kauffman & Martin 1987; Cocking et al. 2012b; Bowcutt 2013; Long et al. 2016

Roads, highways, and trails Many black oak and tanoak woodlands are located near roads or trails, likely because many of these are based on historical avenues of travel by indigenous people (Lake 2013; Long et al. 2017). While transportation corridors can facilitate access and the gathering of cultural resources (Long et al. 2017), proximity to roads, highways, and trails also increase habitat degradation and loss (Bolsinger 1988; Holmes 1990). For instance, large oaks are felled within the right-of-way for transportation corridors (Vuln. Assessment Reviewer, pers. comm., 2018). Roads contribute to the spread of invasive plants whose seeds may be carried on vehicles (Coffin 2007). Invasive grasses, in particular, thrive in roadside environments (Coffin 2007), and can contribute to the alteration of fire regimes by providing continuous fine fuels for wildfire (Brooks et al. 2004). Transportation networks can also contribute to the spread of sudden oak death as contaminated soil is moved from infected sites (Davidson et al. 2005; Swiecki et al. 2017). Timber harvest Black oak is the primary hardwood species used for lumber in California (Bolsinger 1988). Timber harvesting of black oak is regulated by the California Department of Forestry and Fire Protection (CAL FIRE) and can occur on private lands (CAL FIRE 2017), and is sold which include about 60% of the total estimated area of black oak timberland (Bolsinger 1988). Tanoak is also designated as a commercial species, but is primarily used for fuel and pulp (Tappeiner et al. 1990).


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Black oak and tanoak have often been removed in timberlands with aerial herbicides in order to promote the growth of commercially valuable conifers (Bolsinger 1988; Bowcutt 2011). In more open woodland areas, the cutting of trees for fuel has resulted in the removal of many large oaks (Bolsinger 1988; Holmes 1990; Huntsinger & Fortmann 1990), particularly as the value of oak woodlands for livestock production declined in the 1980s (Holmes 1990). Historic extraction of tanoak by the tanbark industry in the 1800s and early 1900s also had a major effect on the age distribution and structure of tanoak woodlands today (Bowcutt 2011). However, the promotion of tanoak for tannin extraction ended with the advent of chemical replacements (Vuln. Assessment Reviewer, pers. comm., 2018).

Adaptive Capacity Black oak and tanoak woodlands were evaluated by regional experts as having moderate overall adaptive capacity (high confidence in evaluation).

Habitat extent, integrity, continuity, and permeability

Regional experts evaluated black oak and tanoak woodlands as having a low geographic extent (high confidence in evaluation), low-moderate structural and functional integrity (high confidence), and low continuity (high confidence). Landscape permeability for black oak and tanoak woodlands was evaluated as low (high confidence). Fire exclusion and associated conifer encroachment, land-use conversion, timber harvest and clearcuts, agriculture, and roads/highways/trails were identified as the primary barriers to habitat continuity and dispersal across the study region.10 Despite the widespread occurrence of oaks within many forest types across the region (McDonald 1990; Tappeiner et al. 1990; CNPS 2019), drastic declines have occurred in the extent and functional integrity of woodlands dominated by large-diameter trees with wide crowns have black oak and tanoak woodlands, due largely to the exclusion of tribal management and subsequent fire suppression (Bowcutt 2013; Long et al. 2016, 2017). On privately-owned lands at lower elevations, woodlands may be further fragmented by the clearing of trees for housing developments, vineyards, cannabis cultivation, and transportation corridors (Bolsinger 1988; Holmes 1990; Huntsinger & Fortmann 1990; Heise & Brooks 1998; Jimerson & Carothers 2002). Although black oak and tanoak are able to resprout following fire and other disturbances, reproduction via seed is also vital for species migration and adaptation to changing conditions (McDonald 1990; Long et al. 2016). Seed dispersal is typically assisted by species such as western grey squirrels (Sciurus griseus), California scrub jays (Aphelocoma californica), and acorn woodpeckers (Melanerpes formicivorus; McDonald 1990). Because animal vectors are required to disperse seed away from the tree, changes in the abundance or distribution of these species could also restrict the ability of black oaks to respond to climate change (McDonald 1990; Long et al. 2016).

10 All barriers presented were ranked as having a moderate or higher impact on this habitat type.


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Habitat diversity

Regional experts evaluated black oak and tanoak woodlands as having high physical and topographical diversity (high confidence in evaluation), high component species diversity (high confidence), and high functional diversity (high confidence). The complex biophysical and geologic systems typical of northern California drive high overall diversity in black oak and tanoak woodlands of the region (Vuln. Assessment Reviewer, pers. comm., 2017). All oak-dominated habitat types support a broad array of understory species, as well as wildlife, insects, fungi, and lichens (CalPIF 2002; Davis et al. 2016b). However, species diversity has declined over the past century due to fire exclusion, conifer encroachment, and the establishment of invasive species, among other variables (Jimerson & Carothers 2002; Cocking et al. 2015; Long et al. 2016). Both black oak and tanoak are considered ecological and cultural keystone species (Anderson 2005; Bowcutt 2013; Halpern 2016; Long et al. 2016; Norgaard et al. 2016). They play a critical role in the vertebrate food web by providing food resources for wildlife that cache or immediately consume the large, nutritious acorns (e.g., black bears [Ursus americanus], black-tailed deer, acorn woodpecker (Melanerpes formicivorus), western gray squirrel (Sciurus griseus; Raphael 1987; McDonald 1990; Long et al. 2016), including many culturally-valued species (Anderson 2007; Lake et al. 2010; Risling Baldy 2013; Lake & Long 2014; Long et al. 2016, 2017; Norgaard et al. 2016). Some of these small mammals also provide prey for larger predators such as Pacific fishers (Pekania pennanti pacifica) and northern spotted owls (Strix occidentalis caurina; Raphael 1987; Bowcutt 2014). Cavities in black oak and tanoak trunks or dead/broken branches of large and decadent black oaks are used as resting habitats for Pacific fishers (Zielinski et al. 2004) and northern spotted owl nests (Long et al. 2014). Other wildlife species associated with black oak and tanoak woodlands include ring-tailed cats (Bassariscus astutus), coyotes (Canis latrans), porcupines (Erethizon dorsatum), pileated woodpecker (Dryocopus pileatus), mountain quail (Oreortyx pictus), and band-tailed pigeon (Columba fasciata; Long et al. 2016, 2017; Norgaard et al. 2016). Many plant species associated with black oak and tanoak woodlands also hold cultural value for northern California tribes, including the tanoak mushroom (Tricholoma magnivelare; also called the American matustake; Richards & Creasy 1996; Anderson & Lake 2013), oyster mushroom (Pleurotus cornucopiae), black trumpet (Craterellus cornucopioide), lion’s mane (Hericium erinaceus), prince's pine (Chimaphila umbellata), Oregon grape, mock orange (Philadelphus lewisii), serviceberry, oceanspray, evergreen huckleberry, California hazel, ceanothus, black oak, golden chinquapin, Port Orford-cedar, California bay, canyon live oak, Sadler oak, Pacific madrone, sugar pine, ponderosa pine, and Oregon white oak (Anderson & Lake 2013; Bowcutt 2014; Norgaard et al. 2016). Northern California tribes depend on cultural burning practices to increase the productivity and predictability of valued resources by maintaining a series of resource patches in various successional stages (see Box 2; Anderson 2005; Lake 2013; Voggesser et al. 2013; Long et al. 2016; Norgaard et al. 2016).


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Box 2. Cultural burning as a keystone process

Cultural burning is considered a “keystone process” that contributes to heterogeneity of species and habitats (Hankins 2015). Thus, tribal management activities focused on maintaining black oak and tanoak woodlands on the landscape also enhance overall ecological integrity (Anderson 2005; Long et al. 2016; Norgaard et al. 2016; Karuk Tribe 2019). In addition to supporting high species diversity and increased food security for northern California tribes, cultural burning maintains food sources and cover for wildlife, contributes to the health of riverine and riparian habitats, and reduces surface and ladder fuels on the landscape (Kimmerer & Lake 2001; Anderson 2005, 2007; Norgaard 2005; Mensing 2006; Anderson & Lake 2013; Bowcutt 2013; Lake 2013; Halpern 2016; Long et al. 2016, 2017; Norgaard et al. 2016; Karuk Tribe 2019).

Resistance and recovery

Regional experts evaluated black oak and tanoak woodlands as having moderate-high resistance to climate stressors and natural disturbance regimes (high confidence in evaluation). Recovery potential was evaluated as moderate (high confidence). In general, mature oaks are better able to survive disturbances compared to seedlings and saplings (Cocking et al. 2014; Hammett et al. 2017; Nemens et al. 2018). Mature black oaks have thicker bark than most other hardwoods (Plumb 1980) and are resistant to low- and moderate-intensity fires that kill the seedlings of encroaching conifers (Long et al. 2016). By contrast, tanoak is highly flammable due to resin, oils, and wax in the leaves (Fryer 2008), though mature trees have thick, relatively fire-resistant bark and can usually survive moderate-intensity fires (Tappeiner et al. 1990; Fryer 2008). However, young stems of both species are highly vulnerable to fire due to thin bark (Plumb 1980; Fryer 2008; Hammett et al. 2017), and mature trees are still more sensitive to fire than most associated conifers at maturity (Skinner et al. 2006). Oaks are adapted to recover rapidly following injury or topkill during more severe fires, primarily through prolific sprouting from their root collar (burls), which can result in multi-stemmed trees (Plumb 1980; Kauffman & Martin 1987; Tappeiner et al. 1990; McDonald & Tappeiner 2002; Fryer 2008; Cocking et al. 2012b, 2014; Crotteau et al. 2015; Long et al. 2016; Hammett et al. 2017; Nemens et al. 2018). Tanoak is a particularly strong sprouter, potentially producing hundreds of sprouts from a single large burl that then thin out over time (McDonald & Tappeiner 2002). Oaks that regenerate by sprouting are able to produce acorns more rapidly than trees starting out as seedlings due to their established root system (Long et al. 2016). However, sprouting trees can still take over 10 years to produce acorns again (Crotteau et al. 2015; Long et al. 2016; Hammett et al. 2017), and several decades before heavy acorn production is restored (McDonald 1990). Although black oak and tanoak woodlands have some resistance to drought and low- to moderate-intensity fire, enhanced competition from conifers and other hardwoods following fire exclusion have reduced tree vigor and increased vulnerability disturbance-related mortality (Cocking et al. 2012b, 2014; Long et al. 2016). Warmer, drier conditions associated with climate change may also cause higher rates of mortality in existing oak populations, further limiting oak


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regeneration and resilience to fire and other disturbances (Cocking et al. 2012b; Long et al. 2016). However, recovery in degraded systems can be relatively rapid where it is aided by management approaches that release overtopped oaks and reintroduce frequent low-intensity fire (Biswell 1999; Cocking et al. 2012b; Lake & Long 2014; Hankins 2015; Long et al. 2016; Norgaard et al. 2016; Karuk Tribe 2019).

Management potential

Public and societal value Regional experts evaluated black oak and tanoak woodlands as having moderate-high public and societal value (high confidence in evaluation). Although the general public does not always place high value on black oak and tanoak woodlands, this habitat type is of critical importance to northern California tribes (Anderson 1993, 2005; Anderson & Lake 2013; Lake 2013; Risling Baldy 2013; Long et al. 2016, 2017; Norgaard et al. 2016; Karuk Tribe 2019). This increases tribal vulnerability to climate-driven change that affects this habitat (see Box 3), but also provides opportunities to restore black oak and tanoak woodlands and enhance the cultural services provided by these habitats following the incorporation of tribal values and stewardship practices into habitat management activities. Ecocultural restoration focuses on achieving both ecological and cultural goals, such as increasing forest resilience to high-severity fire as well as protecting the cultural services offered by mature oaks (e.g., abundant production of high-quality acorns; Long et al. 2017). In order to accomplish this, ecocultural restoration explicitly considers the socioeconomic needs and values of contemporary tribal communities and the public (Long et al. 2014, 2017), which includes food security, access to resources and traditional management practices, and knowledge sovereignty (Lake 2013; Voggesser et al. 2013; Long et al. 2014, 2017; Norgaard 2014a, 2014b; Long & Lake 2018; Karuk Tribe 2019).

Box 3. Tribal value and associated vulnerabilities

Many of the cultural services historically provided by black oak and tanoak woodlands have declined since Euro-American settlement, such as the production of abundant, high-quality acorn crops and other food, fiber, and medicinal resources (Bowcutt 2013; Long et al. 2016, 2017; Long & Lake 2018). These declines are due in large part to the loss of tribal stewardship practices that maintained the presence of black oak and tanoak woodlands on the landscape (Anderson 2005, 2007; Bowcutt 2013; Halpern 2016; Long et al. 2016, 2017; Long & Lake 2018). Opportunities for gathering traditional resources associated with this habitat have also been limited directly by tribal loss of access to ancestral lands and the ability to manage the landscape (Bowcutt 2013; Norgaard 2014a; Long & Lake 2018) and fire exclusion policies that result in oak stands either being entirely replaced by conifers or dense brush limiting or preventing access to existing stands (Richards & Creasy 1996; Jewell & Vilsack 2014; Norgaard 2014a, 2014b). The loss of acorn groves and orchards that have been managed and utilized as gathering sites for centuries has resulted in reduced food security for tribal members, and has also impacted cultural and personal identity, health, and traditional ecological knowledge of tribal members no longer able to practice traditional ways of life (Garrett 2010; Jewell & Vilsack 2014; Long et al. 2014; Norgaard 2014a, 2014b; Norgaard et al. 2016).


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Over the past 15-20 years, state legislators have passed measures such as the California Oak Woodlands Conservation Act of 2001 (Cal. Assem. Bill 242), which recognizes the importance of oak woodlands and provides incentives for their protection, and the Oak Woodlands Conservation Act of 2004 (Cal. Sen. Bill 1334), which requires management plans for oak woodlands at the municipal and county levels (Gaman & Firman 2006; Green & Magnuson 2011). Additionally, recent legislation (Cal. Assem. Bill No. 1958) has made it easier for land managers to remove encroaching conifers from black oak woodlands in northern California without being subject to conifer restocking requirements (CAL FIRE 2017). However, existing regulations and perceptions of risk may also complicate efforts to reintroduce frequent fire into black oak and tanoak woodlands (Green & Magnuson 2011). Some forest treatments are restricted where rare species such as the Pacific fisher and northern spotted owl are present (Long et al. 2016, 2017; Metlen et al. 2017), and these often include the dense, multi-layered stands where encroaching conifers are negatively impacting black oak and tanoak woodlands (Long et al. 2017). However, targeted treatments to release encroached black oaks may support rodent populations that serve as prey (Thompson et al. 2015), and studies suggest that fishers in the Sierra Nevada are unaffected following the treatment of up to 2.6% of the landscape each year (Zielinski et al. 2013) and return to more heavily treated areas within just a few years (Sweitzer et al. 2016). High-quality acorn-gathering sites valued by northern California tribes currently comprise a small area of potentially suitable habitat for these species (Long et al. 2017). Thus, it is likely that carefully optimizing the placement of forest restoration efforts on the landscape can minimize impacts to rare species and cultural resources, thus providing the greatest benefits to both human communities and the ecosystem over the long term (Long et al. 2017; Metlen et al. 2017).

Management capacity and ability to alleviate impacts11 Regional experts evaluated the potential for reducing climate impacts on black oak and tanoak woodlands through management as high (high confidence in evaluation). Regional experts identified use conflicts and/or competing interests for black oak and tanoak woodlands as livestock grazing, agriculture, and fuelwood harvest (Vuln. Assessment Workshop, pers. comm., 2017). Successful restoration and management of black oak woodlands through release of overtopped oaks and the reintroduction of frequent fire is well-documented within the scientific literature (Agee 1999; Biswell 1999; Cocking et al. 2012b; Lake & Long 2014; Cocking et al. 2015; Hankins 2015; Halpern 2016; Long et al. 2016; Norgaard et al. 2016), suggesting that there is high potential for effective management to successfully support climate adaptation within these ecosystems (Vuln. Assessment Reviewer, pers. comm., 2017; Karuk Tribe 2019). Active management of black oak and tanoak woodlands benefits humans as well as wildlife that depend on this habitat for food and nesting/denning or resting areas (CalPIF 2002; Long et al. 2014, 2016, 2017; Karuk Tribe 2019). Restoration that focuses on supporting tribal values is also

11 Further information on climate adaptation strategies and actions for northern California can be found on the project page (https://bit.ly/31AUGs5).

https://bit.ly/31AUGs5


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likely to increase social and ecological resilience to changes in climate conditions, including disease, drought, and uncharacteristically severe fires (Long et al. 2014; Long & Lake 2018; Karuk Tribe 2019). Because frequent fire is required for ongoing persistence of black oak within the landscape (Pavlik et al. 1991; Mensing 2005; Long et al. 2016), the reintroduction of frequent fire through a return to cultural burning practices and/or the implementation of other prescribed burns and managed wildfires is widely recognized to be a critical restoration strategy (Martin & Sapsis 1992; Agee 1999; Biswell 1999; Neary et al. 1999; Hessburg & Agee 2003; Shebitz 2005; Senos et al. 2006; Skinner et al. 2006; Lake 2013; Jewell & Vilsack 2014; Lake & Long 2014; Hankins 2015; Halpern 2016; Long et al. 2016, 2017; Norgaard et al. 2016; Taylor et al. 2016; Hammett et al. 2017; Nemens et al. 2018; Karuk Tribe 2019). Where oak canopies have already been pierced and overtopped by conifers, more intensive mechanical treatments may also be necessary to initiate recovery by releasing suppressed oaks from competition and increasing tree vigor (Cocking et al. 2012b, 2014, 2015; Crotteau et al. 2015; Long et al. 2016). However, restoration efforts may not be effective where significant crown dieback and mortality have already occurred (Cocking et al. 2012b, 2015). Installation of native grasses following release can reduce erosion and support more rapid establishment of native understory vegetation (Cocking et al. 2015). Post-treatment control of invasive grasses/forbs and conifer seedlings may also be necessary to allow released oaks to become reestablished and maintain dominance (Stuart & Stephens 2006; Cocking et al. 2012b). Prior to the reintroduction of fire, mechanical treatments may be necessary to remove accumulated fuels, which reduces the risk of high-intensity wildfire until sensitive sprouts and seedlings have had a chance to mature and develop greater resistance to fire (Cocking et al. 2012b; Nemens et al. 2018). After the removal of larger conifers and hardwoods, frequent low- to moderate-intensity fire can be used to limit competition from small encroaching trees while also reducing the risk of more severe fires that may kill mature acorn-producing trees (Cocking et al. 2012b; Crotteau et al. 2015; Long et al. 2016, 2017; Karuk Tribe 2019). Frequent fire also creates openings for seedling regeneration, increases native species diversity, reduces pests and disease, creates cavities for wildlife, and allows the regrowth of fire-dependent plants and fungi (Anderson 2005, 2007; Stuart & Stephens 2006; Hankins 2015; Halpern 2016; Long et al. 2016, 2017; Norgaard et al. 2016; Karuk Tribe 2019). Consequently, reintroducing frequent fire enhances the resilience of black oak and tanoak woodlands to climate and non-climate stressors and maintains the valued cultural and ecological services provided by legacy trees (Lake & Long 2014; Long et al. 2016, 2017; Norgaard et al. 2016; Hammett et al. 2017; Karuk Tribe 2019).


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Table 2. Effects of prescribed burning on black oak and tanoak woodlands across time (table adapted from Norgaard et al. 2016). Cultural burning practices, in particular, have played a role in maintaining these habitats on the landscape over very long time scales.

Immediate 2-Year Long Term

• Limits competition from encroaching conifer seedlings and saplings while protecting acorn-bearing adults

• Releases nutrients to the soil

• Eliminates pests that affect tree health and acorn quality

• Promotes reproduction via root crown sprouting or acorn seeding (for black oaks)

• Increases the diversity of understory shrubs, forbs, grasses, and ferns

• Supports wildlife species that benefit from a mosaic of burned and unburned habitat for hunting and foraging

• Eliminates competition from shade-tolerant saplings, allowing oaks to remain dominant

• Protects stands from high-severity fire by limiting fuel accumulation

• Promotes the production of a series of ecocultural resources in the stand over time

Source(s): Kauffman & Martin 1987; Anderson 2005, 2007; Fryer 2007; Cocking et al. 2012b; Bowcutt 2013; Long et al. 2016; Karuk Tribe 2019

Several efforts have highlighted the potential for collaborative partnerships between tribes, public land managers, and private landowners that focus on ecocultural restoration of black oak and tanoak woodlands (Cocking et al. 2012a; Klamath Bird Observatory & Lomakatsi Restoration Project 2014; Long et al. 2017; Metlen et al. 2017; Hatcher et al. 2017; USDA Forest Service 2018). For instance, the Western Klamath Restoration Partnership has collaborated with the U.S. Forest Service on the Somes Bar Integrated Fire Management Project to implement a range of stewardship treatments within Six Rivers National Forest in accordance with traditional ecological knowledge (TEK) and tribal customs (USDA Forest Service 2018). The goal of this project is to restore black oak and tanoak woodlands, promote fire-adapted communities, and enhance cultural resources (USDA Forest Service 2018). Examples of management actions include thinning in dense areas to reduce ladder fuels and promote the development of larger, fire-resistant trees with high ecological and cultural value (USDA Forest Service 2018). For tanoak, one of the most critical management needs is to address the threat of sudden oak death, which has the potential to cause functional extinction of the species across large parts of northern California (Alexander & Lee 2010; Cobb et al. 2012, 2013a; Bowcutt 2013; Voggesser et al. 2013; Swiecki et al. 2017; Metz et al. 2017). However, there are currently few effective strategies for either eradication or active suppression of the disease, particularly at larger scales (Alexander & Lee 2010; Bowcutt 2013; Swiecki et al. 2017). Currently, slowing the spread of disease by preventing the movement of contaminated soil and infected nursery stock is a significant focus (Swiecki et al. 2017). For instance, management activities designed to reduce the rate of spread along roads and trails could include installing signage, closing roads and trails (especially during the wet season), posting preferred travel direction on trails, altering road surface materials, and providing additional instruction to road crews (Bowcutt 2013; Swiecki et al. 2017).


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Many communities, local governments, and regional/state-wide collaborations have also set up monitoring networks focused on early detection of sudden oak death in order to proactively manage tanoak infection (Alexander & Lee 2010). Tribal involvement in efforts to slow the spread of sudden oak death has helped to communicate the importance of this threat to local landowners, who may not place high value on tanoak (Alexander & Lee 2010). Other efforts have involved surveying the area to determine whether naturally-occurring resistance to sudden oak death can be leveraged to aid conservation benefits (Cobb et al. 2018). The protection of tanoak refuges in areas where environmental conditions are less conducive to the disease may also allow the preservation of uninfected trees (Bueno et al. 2010; Dillon et al. 2013; Vuln. Assessment Reviewer, pers. comm., 2018), particularly those within cultural landscapes (Bowcutt 2013). Seed banks and living collections may also provide a way to preserve genetic diversity and allow for the future reintroduction of tanoak into infected areas (Bowcutt 2014).

Ecosystem services Black oak and tanoak woodlands numerous ecological and cultural ecosystem services, including:

• Provisioning of food, fiber, fuel, and genetic resources;

• Flood/erosion control;

• Support of carbon sequestration, primary production, soil formation/retention, and nutrient cycling; and

• Cultural/tribal uses for spiritual/religious purposes, knowledge systems, educational values, aesthetic values, social relations, sense of place, cultural heritage, inspiration, and recreation (Vuln. Assessment Workshop, pers. comm., 2017).

Mycorrhizae associated with tanoak and other early-successional hardwoods and shrubs also play an important role in the ecosystem by enhancing conifer establishment after disturbances such as fire and clearcutting (Borchers & Perry 1990; Perry et al. 2011).

Recommended Citation Hilberg LE, Reynier WA, Kershner JM. 2019. Black Oak and Tanoak Woodlands: Northern California Climate Change Vulnerability Assessment Synthesis. Version 1.0. EcoAdapt, Bainbridge Island, WA.

Further information on the Northern California Climate Adaptation Project is available on the project website (https://tinyurl.com/NorCalAdaptation).

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Northern California Climate Adaptation Project:

Vulnerability Assessment Methods and Application

Defining Terms

Exposure: A measure of how much of a change in climate or climate-driven factors a resource is likely to experience (Glick et al. 2011).

Sensitivity: A measure of whether and how a resource is likely to be affected by a given change in climate or factors driven by climate (Glick et al. 2011).

Adaptive Capacity: The ability of a resource to accommodate or cope with climate change impacts with minimal disruption (Glick et al. 2011).

Vulnerability: A function of the sensitivity of a particular resource to climate changes, its exposure to those changes, and its capacity to adapt to those changes (IPCC 2007).

Vulnerability Assessment Model

The vulnerability assessment model applied in this process was developed by EcoAdapt (EcoAdapt 2014a; EcoAdapt 2014b; Kershner 2014; Hutto et al. 2015; Gregg 2018),12 and includes evaluations of relative vulnerability by local and regional stakeholders who have detailed knowledge about and/or expertise in the ecology, management, and threats to focal habitats, species groups, individual species, and the ecosystem services that these resources provide. Stakeholders evaluated vulnerability for each resource by discussing and answering a series of questions for sensitivity and adaptive capacity. Exposure was evaluated by EcoAdapt using projected future climate changes from the scientific literature. Each vulnerability component (i.e., sensitivity, adaptive capacity, and exposure) was divided into specific elements. For example, habitats included three elements for assessing sensitivity and six elements for adaptive capacity. Elements for each vulnerability component are described in more detail below. In-person workshops were held in Eureka, Redding, and Upper Lake between May and October 2017. Participants self-selected habitat and species group/species breakout groups and evaluated the vulnerability of each resource. Participants were first asked to describe the habitat and/or to list the species to be considered in the evaluation of an overarching species group. Due to limitations in workshop time and participant expertise, multiple resources were not assessed during these engagements. Evaluations for remaining habitats, species groups, and species were completed by contacting resource experts.13

12 Sensitivity and adaptive capacity elements were informed by Lawler 2010, Glick et al. 2011, and Manomet Center for Conservation Sciences 2012. 13 Resources evaluated by experts included: coastal bluff/scrub habitats, coastal conifer hardwood forest, true fir forest, lakes/ponds, freshwater marshes, vernal pools, seeps/springs, native insect pollinators, native ungulates, salamanders, frogs, native mussels, marbled murrelet, and northwestern pond turtle.


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Stakeholders assigned one of five rankings (High, Moderate-High, Moderate, Low-Moderate, or Low) for sensitivity and adaptive capacity. EcoAdapt assigned rankings for projected future climate exposure. Rankings for each component were then converted into scores (High-5, Moderate-High-4, Moderate-3, Low-Moderate-2, or Low-1), and the scores were averaged (mean) to generate an overall score. For example, scores for each element of habitat sensitivity were averaged to generate an overall habitat sensitivity score. Scores for exposure were weighted less than scores for sensitivity and adaptive capacity because the uncertainty about the magnitude and rate of future change is greater. Sensitivity, adaptive capacity, and exposure scores were combined into an overall vulnerability score calculated as: Vulnerability = [(Climate Exposure*0.5) x Sensitivity] - Adaptive Capacity Elements for each component of vulnerability were also assigned one of three confidence rankings (High, Moderate, or Low). Confidence rankings were converted into scores (High-3, Moderate-2, or Low-1) and the scores averaged (mean) to generate an overall confidence score. These approximate confidence levels were based on the Manomet Center for Conservation Sciences (2012) 3-category scale, which collapsed the 5-category scale developed by Moss and Schneider (2000) for the IPCC Third Assessment Report. The vulnerability assessment model applied here assesses the confidence associated with individual element rankings and, from these rankings, estimates the overall level of confidence for each component of vulnerability and then for overall vulnerability. Stakeholders and decision-makers can consider the rankings and scores presented as measures of relative vulnerability and confidence to compare the level of vulnerability among the focal resources evaluated in this project. Elements that received lower confidence rankings indicate knowledge gaps that applied scientific research could help address.

Vulnerability Assessment Model Elements

Sensitivity & Exposure (Applies to Habitats, Species Groups, Species)

• Climate and Climate-Driven Factors: e.g., air temperature, precipitation, freshwater temperature, soil moisture, snowpack, extreme events: drought, altered streamflows, etc.

• Disturbance Regimes: e.g., wildfire, flooding, drought, insect and disease outbreaks, wind

• Future Climate Exposure: e.g., consideration of projected future climate changes (e.g., temperature and precipitation) as well as climate-driven changes (e.g., altered fire regimes, altered water flow regimes, shifts in vegetation types)

• Stressors Not Related to Climate: e.g., tectonic and volcanic events; residential or commercial development; agriculture and/or aquaculture; roads, highways, trails; dams and water diversions; invasive and other problematic species; livestock grazing; fire suppression; timber harvest; mining; etc.


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Sensitivity & Exposure (Applies to Species Groups and Species)

• Dependencies: e.g., dependencies on sensitive habitats, specific prey or forage species, and the timing of the appearance of these prey and forage species (concern for mismatch)

Sensitivity & Exposure (Applies to Species ONLY)

• Life History: e.g., species reproductive strategy, average length of time to reproductive maturity

Adaptive Capacity (Applies to Habitats, Species Groups, Species)

• Extent, Integrity, and Continuity/Connectivity: e.g., resources that are widespread vs. limited, structural and functional integrity (e.g., degraded or pristine) of a habitat or health and functional integrity of species (e.g., endangered), isolated vs. continuous distribution

• Landscape Permeability: e.g., barriers to dispersal and/or continuity (e.g., land-use conversion, energy production, roads, timber harvest, etc.)

• Resistance and Recovery: e.g., resistance refers to the stasis of a resource in the face of change, recovery refers to the ability to “bounce back” more quickly from the impact of stressors once they occur

• Management Potential: e.g., ability to alter the adaptive capacity and resilience of a resource to climatic and non-climate stressors (societal value, ability to alleviate impacts, capacity to cope with impacts)

• Ecosystem Services: e.g., provisioning, regulating, supporting, and/or cultural services that a resource produces for human well-being

Adaptive Capacity (Applies to Habitats ONLY)

• Habitat Diversity: e.g., diversity of physical/topographical characteristics, component native species and functional groups

•

Adaptive Capacity (Applies to Species Groups, Species)

• Dispersal Ability: i.e., ability of a species to shift its distribution across the landscape as the climate changes

• Intraspecific/Life History Diversity: e.g., life history diversity, genetic diversity, phenotypic and behavioral plasticity

Literature Cited

EcoAdapt. 2014a. A climate change vulnerability assessment for aquatic resources in the Tongass National Forest. EcoAdapt, Bainbridge Island, WA. 124 pp.

EcoAdapt. 2014b. A climate change vulnerability assessment for resources of Nez Perce-Clearwater National Forests. Version 3.0. EcoAdapt, Bainbridge Island, WA. 398 pp.

Glick P, Stein BA, Edelson NA. 2011. Scanning the conservation horizon: A guide to climate change vulnerability assessment. National Wildlife Federation, Washington, D.C.

Gregg RM, editor. 2018. Hawaiian Islands climate vulnerability and adaptation synthesis. EcoAdapt, Bainbridge Island, WA. 284 pp.


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Hutto SV, Higgason KD, Kershner JM, Reynier WA, Gregg DS. 2015. Climate change vulnerability assessment for the north-central California coast and ocean. Marine Sanctuaries Conservation Series ONMS-15-02. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Office of National Marine Sanctuaries, Silver Spring, MD. 473 pp.

Intergovernmental Panel on Climate Change (IPCC). 2007. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Pages 617–652 in M. L. Parry, O. F. Canziani, J. P. Palutikof, P. J. van der Linden, and C. E. Hanson, editors. Cambridge University Press, Cambridge, UK.

Kershner JM, editor. 2014. A climate change vulnerability assessment for focal resources of the Sierra Nevada. Version 1.0. EcoAdapt, Bainbridge Island, WA. 418 pp.

Lawler J. 2010. Pacific Northwest Climate Change Vulnerability Assessment. From http://climatechangesensitivity.org

Manomet Center for Conservation Sciences and National Wildlife Federation. 2012. The vulnerabilities of fish and wildlife habitats in the Northeast to climate change. A report to the Northeastern Association of Fish and Wildlife Agencies and the North Atlantic Landscape Conservation Cooperative. Manomet Center for Conservation Sciences, Plymouth, MA.

Moss R, Schneider S. 2000. Towards consistent assessment and reporting of uncertainties in the IPCC TAR. In R. Pachauri and T. Taniguchi, editors. Cross-Cutting Issues in the IPCC Third Assessment Report. Global Industrial and Social Progress Research Institute (for IPCC), Tokyo.

http://climatechangesensitivity.org/

Black Oak and Tanoak Woodlands Habitat Description

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