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Fire Synchrony and the Influence of Pacific ClimateVariability on Wildfires in the Florida Keys, UnitedStatesGrant L. Harley a , Henri D. Grissino-Mayer b , Sally P. Horn b & Chris Bergh ca Department of Geography and Geology , The University of Southern Mississippib Department of Geography , The University of Tennesseec The Nature Conservancy , Big Pine Key , FloridaPublished online: 06 Nov 2013.

To cite this article: Grant L. Harley , Henri D. Grissino-Mayer , Sally P. Horn & Chris Bergh , Annals of the Association ofAmerican Geographers (2013): Fire Synchrony and the Influence of Pacific Climate Variability on Wildfires in the Florida Keys,United States, Annals of the Association of American Geographers, DOI: 10.1080/00045608.2013.843432

To link to this article: http://dx.doi.org/10.1080/00045608.2013.843432

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Fire Synchrony and the Influence of Pacific ClimateVariability on Wildfires in the Florida Keys,

United StatesGrant L. Harley,∗ Henri D. Grissino-Mayer,† Sally P. Horn,† and Chris Bergh‡

∗Department of Geography and Geology, The University of Southern Mississippi†Department of Geography, The University of Tennessee

‡The Nature Conservancy, Big Pine Key, Florida

We investigated relationships between climate variability and wildfires in endangered pine rockland communitiesin the Florida Keys, United States, using fire-scarred samples from the canopy dominant Pinus elliottii var. densa.To test broad-scale, spatiotemporal relationships between wildfires and climate, we compared cross-dated fire-scarchronologies from two islands in the lower Florida Keys, Big Pine Key (BPK) and No Name Key (NNK), tomeasured values of the Atlantic Multidecadal Oscillation (AMO), North Atlantic Oscillation (NAO), El Nino-Southern Oscillation (ENSO; NINO3.4), Pacific Decadal Oscillation (PDO), Interdecadal Pacific Oscillation(IPO), and divisional temperature and precipitation over the period from 1856 to 1956. Large-scale climateanomalies captured by ENSO (NINO3.4) and IPO indexes had combined effects on widespread fires. Superposedepoch analysis revealed that widespread fires on BPK occurred during years that were drier than average andwhen constructive phases (years of combined warm [positive] or cool [negative] phases) of ENSO and the IPOoccurred three years and one year prior to fires. Positive phases of the PDO were also significantly associated withwidespread fires three years prior to events, but the PDO was not influential one year prior to fires. Although fireyears were temporally synchronous between the two islands during the period between 1818 and 1924 (n = 10),we did not find significant relationships between climate and fire on the smaller NNK, which suggests that islandsize influences the ability to detect broad-scale climate forcing of wildfires in the Florida Keys. Key Words: ElNino-Southern Oscillation modulation, Interdecadal Pacific Oscillation, pine rockland, superposed epoch analysis, slashpine.

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Para este artıculo, investigamos las relaciones entre la variabilidad del clima y los incendios que ocurren demanera natural en las comunidades de pino de pedregal que se hallan en peligro en los Cayos de la Florida,Estados Unidos, con base en muestreo de las cicatrices dejadas por el fuego en el dosel donde el Pinus elliottii var.densa es la especie predominante. Para probar a escala amplia las relaciones espacio-temporales entre los incendiossilvestres y el clima tomamos las cronologıas determinadas para las cicatrices de fuego en dos de las islas masbajas de los Cayos de la Florida, el Cayo Big Pine (BPK) y el Cayo Sin Nombre (NNK), para compararlas con losvalores medidos de la Oscilacion Multidecadal Atlantica (AMO), la Oscilacion del Atlantico Norte (NAO), laOscilacion Meridional de El Nino (ENSO; NINO3.4), la Oscilacion Decadal del Pacıfico (PDO), la OscilacionInterdecadal del Pacıfico (IPO), y los datos de temperatura y precipitacion a lo largo del perıodo 1856-1956.

Annals of the Association of American Geographers, 10X(XX) 2013, pp. 1–19 C© 2013 by Association of American GeographersInitial submission, February 2013; revised submission, July 2013; final acceptance, July 2013

Published by Taylor & Francis, LLC.

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2 Harley et al.

Las anomalıas climaticas a gran escala captadas por los ındices de ENSO (NINO3.4) e IPO tuvieron efectoscombinados sobre los incendios ocurridos por doquier. El analisis de epoca superpuesto revelo que los incendiosen BPK ocurrieron por todo lado en los anos mas secos que el promedio; y tambien cuando las fases constructivas(anos en los que se combinaron fases calidas [positivas] o frescas [negativas]) de ENSO e IPO se presentaron,respectivamente, tres anos y un ano antes de los incendios. Las fases positivas de la PDO tambien estuvieronsignificativamente asociadas con incendios dispersos por todo lado, tres anos con antelacion a los eventos, aunquela PDO no influyo para nada un ano antes de los incendios. Aunque los anos de incendios fueron temporalmentesimultaneos en las dos islas durante el perıodo que se extendio de 1818 a 1924 (n = 10), no hallamos ningunasrelaciones significativas entre clima e incendios en la mas pequena isla NNK, lo que sugiere que el tamano de laisla influye en la capacidad para detectar en escala amplia que el clima promueva incendios silvestres en los cayosde la Florida. Palabras clave: modulacion de El Nino-Oscilacion Meridional, Oscilacion Interdecadal del Pacıfico, pinode pedregales, analisis de epoca superpuesto, pino slash.

Fire is an important disturbance in many ecologicalcommunities. Plant communities from the trop-ics to the high latitudes are shaped by varied fire

regimes (i.e., fire return interval, fire season, and relativespatial extent) operating at different temporal and spa-tial scales (Taylor and Skinner 1998, 2003; Kipfmuellerand Baker 2000; Heyerdahl, Brubaker, and Agee 2001;Whitlock, Shafer, and Marlon 2003; Shlisky et al.2009). Variations in fire regimes are often linked withalternating patterns or cycles of global-scale climate-forcing mechanisms, such as the El Nino-SouthernOscillation (ENSO) and Pacific Decadal Oscillation(PDO; Mantua et al. 1997), as well as with regionalpatterns of temperature and precipitation (Swetnamand Betancourt 1990, 1998; Grissino-Mayer and Swet-nam 2000; Kitzberger, Swetnam, and Veblen 2001;Heyerdahl, Brubaker, and Agee 2002; Schoennagelet al. 2005; Taylor and Beaty 2005; Kitzberger et al.2007). For example, ENSO is known to affect globaltemperature and rainfall patterns (Ropelewski andHalpert 1986, 1987; Allan, Lindesay, and Parker 1996)and has been shown to affect fire occurrence in westernNorth America from regional scales (Swetnam 1990;Heyerdahl and Alvarado 2003; Brown and Wu 2005;Fule, Villanueva-Diaz, and Ramos-Gomez 2005; Skin-ner et al. 2008) to subcontinental scales (Swetnam andBetancourt 1990; Brenner 1991; Veblen and Kitzberger2002; Kitzberger et al. 2007; Trouet et al. 2010).

In recent decades, most research focused on in-vestigating linkages between climate and historicalwildfire was conducted in western North America, withprevious studies of fire–climate relationships revealinginteractions between fire and ENSO and PDO cycles(Skinner et al. 2008; Taylor, Trouet, and Skinner 2008;Trouet et al. 2010). Climate effects of PDO are oftendefined similarly to those of ENSO because the twomechanisms often operate in tandem (Gershunov andBarnett 1998; Kaplan et al. 1998; Rodgers, Friedrichs,and Latif 2004). Respective climate conditions are

intensified (constructive) when both mechanisms arein phase and weakened (destructive) when out of phase(Biondi, Gershunov, and Cayan 2001; Mote et al.2003). Thus, the PDO has been shown to exert a mod-ulating effect on ENSO teleconnections (Gershunovand Barnett 1998; Kerr 1999; Biondi, Gershunov, andCayan 2001; Cleaveland et al. 2003), and amplifiedfire weather is often linked with interactions betweenphasing of ENSO and PDO in western North America(Norman and Taylor 2003; Schoennagel et al. 2005;Taylor and Beaty 2005). Moreover, both positive andnegative phases of the Interdecadal Pacific Oscillation(IPO) have been shown to promote fire weather. TheIPO is defined by low frequency (fifteen- to thirty-year)variance in Pacific-wide sea surface temperatures(SSTs), much like the PDO (Mantua et al. 1997), butis considered to be a more spatially extensive accountof anomalous warming and cooling of SSTs over theNorth and South Pacific Ocean (Folland et al. 1999;Power et al. 1999; Allan 2000). Power et al. (1999)revealed that the IPO modulates ENSO precipitationand temperature variability over Australia. In addition,Verdon, Kiem, and Franks (2004) discovered thatwildfire risk over Australia increased when ENSOevents were coupled with a negative phase of the IPO.

Although most studies of fire–climate relationshipsderive from western North America, research hasshown that ENSO influences wildfires in the south-eastern United States, particularly in mainland Florida(e.g., Brenner 1991; Jones, Shriver, and O’Brien 1999;Harrison and Meindl 2001; Beckage and Platt 2003;Beckage et al. 2003; Goodrick and Hanley 2009;Slocum et al. 2010). These studies based on modern in-strumental records offer limited insight on fire–climaterelationships before about 1950, but they suggest thatEl Nino is associated with increased precipitationin Florida during the winter dry season and resultsin lower wildfire activity. In contrast, decreased dryseason precipitation during La Nina exacerbates the

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 3

severity of winter droughts and is associated with lowersurface water levels, more lightning strikes, heightenedfire weather, and higher wildfire activity. Beckage et al.(2003) discovered that El Nino/La Nina phase cyclesinfluenced fire regimes in the Everglades from 1948 to1999. They proposed that increased winter (dry season)rainfall associated with El Nino increased plant growthand preconditioned fire activity. In contrast, the de-creased winter rainfall associated with La Nina resultedin lowered surface water levels in the Everglades,increased the spatial continuity of fine fuels (i.e.,grasses), and allowed fires to spread over larger areas.

Kurtzman and Scanlon (2007) provided evidencethat phase cycles of PDO can strengthen or weakenEl Nino and La Nina effects in Florida. In southernFlorida, for example, positive anomalies of El Nino arestrengthened during positive phases of PDO, yieldingincreased winter precipitation. Wachnicka et al. (2013)provided further evidence that the PDO influences cli-mate conditions in southern Florida. They showed thatmajor shifts in diatom assemblage in Biscayne Bay overthe past 600 years that coincided with severe droughtperiods developed during the cool phases of ENSO (LaNina), Atlantic Multidecadal Oscillation (AMO), andPDO or during combined phases of warm AMO andnegative PDO. In addition to PDO and ENSO, othercoupled oceanic–atmospheric oscillations are known toinfluence climate in Florida, such as the AMO andNorth Atlantic Oscillation (NAO), but linkages be-tween AMO and NAO and wildfire are less certain.The AMO is a multidecadal pattern of surface temper-ature variability centered on the North Atlantic Ocean(0–70◦N; Kerr 2000; Knight et al. 2005). In southernFlorida, summer rainfall is below average during theAMO cool (negative) phase but above average duringthe warm (positive) phase (Enfield, Mestad-Nunez, andTrimble 2001). Based on an oscillation of atmosphericmass between the subtropics and the high latitudesover the North Atlantic, the NAO affects both tem-perature and precipitation in the southeastern UnitedStates. The positive phase of the NAO is marked bywarmer and wetter than average conditions during win-ter (Rogers 1984; Visbeck et al. 2001; Katz, Parlange,and Tebaldi 2003). Although it is generally understoodhow these oscillations modulate climate in Florida, re-lationships between these oscillations and historicalwildfire activity are not yet documented. Here, we usenew fire-scar and tree-ring records to improve under-standing of the spatiotemporal relationships betweenclimatic variability and wildfire in the lower FloridaKeys. An understanding of the relationships between

historical wildfire and climate is critically important forland managers seeking to make best use of limited firemanagement resources on islands in the Florida Keys.

Pine rockland is an endangered, fire-dependent plantcommunity restricted in the United States to small areasin mainland south Florida and the lower Florida Keys(Noss, LaRoe, and Scott 1995). Fire is an importantdisturbance in this community type because it encour-ages the regeneration and recruitment of the foundationspecies, South Florida slash pine (Pinus elliottii Engelm.var. densa Little & Dorman, hereinafter “slash pine”)and discourages invasion by tropical hardwood species.In the absence of fire, open-canopy pine forests withdiverse herbaceous understories succeed to dense trop-ical hardwood hammock with depauperate herbaceousunderstories.

In a previous study, Harley, Grissino-Mayer, andHorn (2013) compared the influence of historical wild-fires and recent fire management practices on foreststructure on two adjacent islands, Big Pine Key (BPK)and No Name Key (NNK). Their fire reconstructionon BPK spanned from 1707 to 2010 but included firehistory data from only one study site on BPK, BoneyardRidge (BYR), which they compared to a fire recon-struction on NNK that spanned from 1779 to 2010. AtBYR, the mean fire return interval (MFI) was 6.5 years,but the MFI for the settlement period (1840–1956;4.6 years) was significantly lower than that of the firemanagement period (1957–2010; 7.3 years). On NNK,they reported an MFI of 10 years, and MFIs betweenthe settlement period (9.73) and fire management pe-riod (7.67) were not significantly different.

Here, we build on these previous analyses to quantifythe potential influence of climatic variability on thefire regime for several reasons. The islands of BPKand NNK contain the largest contiguous areas ofpine rockland habitat in the Florida Keys (ca. 675ha). Whereas previous investigations of the effects ofclimate on fires in Florida focused on the past sixtyyears, the abundance of old, fire-scarred slash pines withdemonstrated annual ring formation (Harley, Grissino-Mayer, and Horn 2011; Harley et al. 2012) offered theopportunity to study fire–climate relationships overa longer interval. Moreover, the strategic geographiclocation of the islands, situated along the Florida Straitsbetween the Gulf of Mexico and Atlantic Ocean, sug-gested potential for identifying the effects of multipleoceanic–atmospheric climate-forcing mechanisms onfire occurrence in pine rockland communities.

We developed fire reconstructions from three addi-tional study sites on BPK and then used this new, larger

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sample set, along with fire data from NNK (Harley,Grissino-Mayer, and Horn 2013), to investigate po-tential broad-scale climatic drivers of wildfire on theislands. The objectives of this study were to (1) charac-terize the historical fire regime in pine rocklands on BPKacross the largest area possible, including return inter-val, fire season, and relative spatial extent, using a morespatially expansive data set than previously analyzed;(2) compare the MFI between the BYR site reportedby Harley, Grissino-Mayer, and Horn (2013) and ournew spatially expansive fire data set developed for BPK;(3) determine whether fire years were temporally syn-chronous between islands; and (4) examine potentialrelationships between climatic variability and histori-cal wildfire occurrence on BPK and NNK.

We formally address five questions:

1. What were the temporal and spatial characteris-tics of the fire regime on BPK, and how does theaddition of fire-scarred samples and study sites in-fluence the fire history statistics on BPK comparedto those reported by Harley, Grissino-Mayer, andHorn (2013)?

2. Were historical fires temporally synchronous be-tween the islands of BPK and NNK?

3. Does the BPK fire data set reveal any changesin the historical fire regime during the twentiethcentury?

4. Does a comparison of fire years between islandsoffer any insights into the potential influence ofisland size and historical fire occurrence?

5. What are the relationships between the tempo-ral patterns of fire occurrence on the islands andclimatic variability (particularly drought, ENSO,IPO, PDO, AMO, and NAO), and can these link-ages be used to assess future wildfire risk?

Data and Methods

Study Area

As part of the arcuate chain of islands extending fromthe southern tip of mainland Florida to Key West, com-monly referred to as the Florida Keys, BPK (24.70◦N,81.37◦W) and NNK (24.69◦N, 81.32◦W) are low-lyingislands composed of Pleistocene limestone (Figure 1).Pine rocklands on both islands are characterized bya monospecific pine overstory, a diverse subcanopy ofWest Indian shrubs and palms, and a diverse assemblageof graminoid and dicotyledonous herbaceous species,several of which are endemic (Sah et al. 2004). The

density of live trees is 395 ha−1 on BPK and 195 ha−1

on NNK, and the basal area of live trees is 8 m2 ha−1 onBPK and 6 m2 ha−1 on NNK (Harley, Grissino-Mayer,and Horn 2013). Pine rocklands provide an importanthabitat for state and federally listed endangered speciessuch as the Key deer (Odocoileus virginianus claviumBarbour & Allen), the lower Keys marsh rabbit (Sylvila-gus palustris hefneri Lazell), Kirtland’s warbler (Dendroicakirtlandii Baird), the Florida leafwing butterfly (Anaeatroglodyta floridalis Johnson & Comstock), and the BigPine partridge pea (Chamaecrista lineata var. keyensis(Pennell) H.S. Irwin & Barneby; Snyder, Herndon, andRobertson 1990).

Climate in the lower Florida Keys is defined astropical savanna, with hot summers (mean maximumAugust temperature < 32◦C), cool winters (meanminimum January temperature > 19◦C), and a consis-tent summer-wet, winter-dry season. During the periodbetween 1895 and 2011, mean annual precipitation inthe region was 980 mm, with the majority of rainfalloccurring from May to November (National ClimaticData Center [NCDC] 2013). On the rocklands, thePleistocene-aged Miami limestone is exposed atthe surface and soil is thin to nonexistent. Elevation onthe keys ranges from sea level to 2.4 m, with old-growthslash pines occupying the highest elevations (Figure 1).Old-growth pines remain on the Keys because landdevelopment was minimal in the early twentiethcentury and logging activity was limited because of therugged terrain of the rocklands, low commercial valueof the timber, and difficulty of transporting timber fromthe Keys to the mainland. Protection of pine rocklandhabitat began in 1957 with the establishment of theNational Key Deer Refuge (NKDR).

Archaeological evidence from the lower Florida Keysis limited, and the initial timing of human arrivaland patterns of movement between and around islandsremains largely unknown (Worth 1995). WidespreadEuropean settlement of the Florida Keys did not occuruntil the early nineteenth century, with Key West (lo-cated about 40 km to the south) being the most denselypopulated location from the 1820s (initial settlement)to the early 1900s (Snyder, Herndon, and Robertson1990). In 1938, U.S. Highway 1 was constructed frommainland Florida to the middle Florida Keys but endedabout 65 km from BPK (Clark 2002). In the 1950s,Highway 1 was extended to Key West and a network ofroads began to develop on BPK and NNK, spurringslow population growth. The current population ofBPK and NNK is approximately 5,000 (Harveson et al.2007).

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 5

Figure 1. Study site locations (hatched areas) and LiDAR-derived elevation in the National Key Deer Refuge, Big Pine Key and No NameKey, Florida. Note: NBP = North Big Pine; BYR = Boneyard Ridge; BHS = Blue Hole South; TPS = Terrestris Preserve; NNK = No NameKey; LiDAR = light detection and ranging. (Color figure available online.)

Fire History

Cross-dated fire scars identified in living trees andremnant woody material (standing dead trees and logs)were used to reconstruct the fire regime in slash pinestands at three new sites on BPK: North Big Pine(NBP), Blue Hole South (BHS), and Terrestris Pre-serve (TPS; Table 1). As at the BYR and NNK sites(described in Harley, Grissino-Mayer, and Horn 2013),we used a targeted sampling design to collect spec-imens from trees that contained the greatest num-ber of well-preserved fire scars distributed as broadlyas possible over the site (Guyette and Stambaugh2004; Van Horne and Fule 2006). We used a chain-saw to remove partial cross sections from living treesand full cross sections from remnant material (Arnoand Sneck 1977; Figure 2). A Global Positioning Sys-tem (GPS) was used to record the location of eachsample.

In the laboratory, standard dendrochronologicalmethods were used to sand each fire-scarred sample to ahigh polish (Orvis and Grissino-Mayer 2002) and thencross-date the annual growth rings of each fire-scarred

Table 1. Site characteristics on Big Pine Key and No NameKey, Florida Keys

First Ending Samples AreaStudy site year year collected (n) (ha)

North Big Pine (NBP) 1860 2010 37 20Boneyard Ridge (BYR)a 1707 2010 50 40Blue Hole South (BHS) 1854 2002 10 8Terrestris Preserve (TPS) 1842 2009 13 20Big Pine Key (BPK) 1707 2010 110 88No Name Key (NNK)a 1779 2010 32 75

Note: BPK represents the composite of all four sites on the island: NBP,BYR, BHS, and TPS.aFire-scarred samples collected and reported by Harley, Grissino-Mayer, andHorn (2013).

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6 Harley et al.

Figure 2. Partial cross section of slash pine sample collected onBig Pine Key showing annual growth rings and fire scars (black ar-rows). White scale bar represents 1 cm. The outermost (last-formed)growth ring is at the top.

sample against reference chronologies developed ateach site (Stokes and Smiley 1968). Accuracy ofcross-dating was verified using the computer programCOFECHA (Holmes 1983; Grissino-Mayer 2001a).The calendar year of each ring that contained a firescar was recorded as the fire date and the season of eachfire event was estimated by examining the intraringposition of each scar. The seasonal intra-annual growthdynamics of slash pine in the study area are differentthan defined for the pine species first used to classifyfire-scar seasonality in the southwestern United States(e.g., Baisan and Swetnam 1990). Therefore, fire scarpositions were classified as (1) early (earlywood);(2) transition (earlywood/latewood transition zone;after Huffman et al. 2004); (3) latewood (in latewood);and (4) dormant (ring boundary). We based this clas-sification on cambial phenology research that indicatesthat slash pines form earlywood from February throughJune and latewood from July through November andare dormant during December and January (Langdon1963; Harley et al. 2012).

The relative spatial extent of each fire was estimatedusing an index based on the percentage of specimensthat recorded the fire. Fire-year chronologies developedat each site were then combined to create a compositefire-scar chronology for BPK. The composite BPK andNNK fire-scar chronologies were used to calculate firereturn intervals for fire years recorded by (1) any spec-imen, (2) ≥25 percent, and (3) ≥50 percent of speci-mens. We used FHX2 software to generate the Weibullmedian probability interval (WMPI) and MFI statistics(Grissino-Mayer 2001b) and perform Student’s t teststo compare the MFI statistics between the settlementand fire-management periods. The WMPI offers a moreflexible frequency distribution and more accurately de-scribes skewed data and, when considered with theMFI statistic, provides a more comprehensive under-standing of the distribution of fire frequency intervals.We considered the settlement period to extend fromEuropean American settlement of the lower Keys about1840 (Williams 1991) until the establishment of theNKDR in 1956. The fire management period, charac-terized by prescribed burning by NKDR personnel andwildfire suppression by the Florida Division of Forestryand Monroe County (Florida Keys) Fire Department,extended from 1957 to 2010 (Harley, Grissino-Mayer,and Horn 2013).

To facilitate a visual comparison of fire betweenislands, we constructed burn maps using the GPSlocations of each fire-scarred sample collected from thestudy sites on BPK and NNK. Because of our targeted

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 7

sampling design for collecting fire scars, our ability toinfer spatial patterns of fire spread between study siteson BPK is limited, and these maps should be interpretedwith caution. Nonetheless, the intention of the burnmaps is to facilitate a qualitative analysis of the spatialpatterns of fires on BPK that were synchronous withNNK.

Fire–Climate Relationships

To evaluate climate conditions related to historical(premanagement period) fire occurrence in our studyarea, we used superposed epoch analysis (SEA; Baisanand Swetnam 1990; Swetnam 1993; Grissino-Mayer2001b) to compare indexes of climate (described later)with fire years identified on the islands of BPK andNNK. SEA identifies nonlinear relationships betweenclimate indexes and fire dates by superimposing win-dows of concurrent and lagged climate conditions oneach fire event (year). To test for possible precondi-tioning effects of climate on fire activity, we chosea five-year window (three years before the fire, thefire year, and one year after the fire) in our SEA andused Monte Carlo simulations (N = 1,000) to developbootstrapped 95 and 99 percent confidence intervalsto determine whether climate was significantly differ-ent from average in years before, during, and after fireevents (Grissino-Mayer and Swetnam 2000; Swetnamand Baisan 2003). To represent years of more exten-sive and widespread fire activity, we conducted SEAusing years in which ≥50 percent of sampled trees werescarred.

To assess potential climate forcing of wildfires,we compared our fire chronologies with global- andregional-scale climate variables: (1) SST anoma-lies monitored in the equatorial Pacific Oceanfrom the Nino 3.4 region (1856–2010; 5◦N–5◦S,120◦W–170◦W); (2) PDO values in the form of SSTanomalies in the North Pacific Ocean (1900–2010;poleward of 20◦N; Mantua et al. 1997; Mantua andHare 2002); (3) unfiltered IPO index values in theform of Pacific basin-wide SST variations (1871–2008;Folland et al. 1999); (3) the Azores-Iceland NAO in-dex (1865–2010; Jones, Jonsson, and Wheeler 1997);(4) an AMO index defined as SSTs over 25–60◦N,7–75◦W minus the regression on global mean tempera-ture (1880–2010; van Oldenborgh et al. 2009); (5) totalannual precipitation; and (6) mean annual temperature,both from Florida Climatic Division 7 (1895–2010;NCDC 2013).

Results

Fire History on BPK

Our new, larger sample set from the four study siteson BPK consists of 110 fire-scarred samples, of which71 (65 percent) were successfully cross-dated. Weidentified 373 fire scars in the annual growth rings thatrepresented fifty-seven unique fire events from 1707 to2010 (Figure 3). We considered the period of reliabilityto begin when the first synchronous fire occurred be-tween at least two study sites. The period of reliabilityfor our fire reconstruction therefore spans 1852 to2010. We were unable to cross-date thirty-nine samplesbecause of aberrant growth ring patterns or because thesample contained an insufficient number of growth ringsfor accurate visual or statistical cross-dating. The num-ber of well-preserved fire scars found on individual treesdemonstrated the prevalence of historical fire in thesecommunities. On average, individual trees containedfive scars, and several trees experienced and survived upto twelve fires. The earliest fire recorded at our four sitesoccurred in 1721 and the latest in 2009, both at BYR.The first synchronous fire events recorded between atleast two sites occurred in 1852 and 1854, with firesthat burned between BYR and TPS. Of the twenty-fourfires that occurred during the period from 1852 to 1957,twenty-two (92 percent) were synchronous betweenat least two study sites on BPK. However, four of thenineteen fires that occurred after 1957 (20 percent)were synchronous between at least two study sites.

The interval distributions for all fires (BPK com-posite record 1852–2010) were positively skewed, withmore short intervals (two to six years) between fires.The WMPI was shorter (four years) than the MFI (fiveyears; Table 2). Years between fire events ranged fromone to twenty-six. We were able to classify fire seasonal-ity for 95 percent (n = 355) of the fire scars; 25 percentof fires occurred at the transition between earlywoodand latewood, and 75 percent were positioned in thelatewood. We found no evidence of early season or dor-mant season fires (e.g., fire scars positioned within theearlywood zone or at an annual growth ring boundary,respectively). Differences between the MFI during thesettlement and fire management periods were signifi-cant (p < 0.05, t test; Table 3). Moreover, the percent-age of scarred trees during the settlement period wassignificantly higher than during the fire-managementperiod (p < 0.05, t test).

Ten widespread fires (≥50 percent of trees scarred)occurred between 1707 and 1851, but sample depth was

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Figure 3. Fire history chart (1707–2010) for the four study sites on Big Pine Key and one site on No Name Key in the National Key DeerRefuge, Florida. Top: Solid black line is the number of samples (N) plotted with the percentage of trees that recorded fire in each fire year,represented by black bars. Bottom: Each horizontal line represents a tree and vertical tick marks represent fires. The composite record belowthe chart includes all fires.

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 9

Table 2. Composite fire interval statistics for Big Pine Key,Florida Keys

Composite fire recordCharacteristics (1852–2010)

Samples (n) 71Total fires 57Mean fire return interval

All fires 5.14≥25 percent scarred 8.23≥50 percent scarred 12.52

Weibull median probability intervalAll fires 4.39≥25 percent scarred 7.54≥50 percent scarred 11.92

Range 1–26SD 4.33

low during this period. During the period of reliability,our reconstruction revealed thirteen widespread fireson BPK. Of these thirteen fires, ten occurred before theestablishment of the NKDR in 1957. After the estab-lishment of the NKDR, the three widespread fires thatwere recorded were prescribed burns conducted by firemanagement personnel (Bergh and Wisby 1996).

Fire Synchrony Between Islands

Fires occurred between at least one study site onBPK and NNK during the years 1818, 1841, 1852,1854, 1876, 1889, 1899, 1905, 1914, and 1924 (Fig-ure 4). Fire years during the period between 1818 and1924 were highly synchronous between islands. Dur-ing this period, eleven fires were recorded at NNK, tenof which were synchronous fire years with BPK. Firescars from these events were all positioned within thelatewood, indicating that they all occurred during the

Table 3. Composite fire statistics for settlement(1840–1956) and fire-management (1957–2010) periods on

Big Pine Key, Florida Keys

Period n M SE SD

Mean fire interval1852–1956 25 4.40∗ 0.48 2.401957–2010 18 2.78∗ 0.29 1.22

Percent scarred1852–1956 26 38.74∗ 3.41 17.371957–2010 19 26.58∗ 6.22 27.10

Note: The year 1852 is the beginning of the period of reliability in thefire reconstruction (see text). Mean values with an asterisk are statisticallydifferent (p < 0.05, t test).

summer months. Most of the fires on NNK that weresynchronous with BPK were widespread, with fires scar-ring ≥50 percent of samples collected. However, onlythe 1852, 1854, 1899, and 1905 fires were widespreadon BPK (Figures 3 and 4). Moreover, during the periodbetween 1818 and 1924, five widespread fires occurredon BPK (1829, 1838, 1871, 1894, 1911) that were notrecorded in the NNK fire reconstruction. We found nosynchronous fires between islands after 1924. A qualita-tive analysis of the burn maps revealed a pattern of spa-tial heterogeneity of fires on BPK that were synchronouswith NNK (Figure 4). For example, most synchronousfire years on BPK were recorded at BYR (except 1914),many of which also burned at NBP (1889, 1899, 1905,1924). Other noteworthy patterns include the fires dur-ing 1852, 1854, 1889, 1899, and 1905, which burnedat both BYR and TPS but not BHS, and the 1914 fire,which was only recorded at the NBP site.

Fire–Climate Relationships

SEA indicated that widespread fires on BPK beforethe establishment of the NKDR were likely to occurduring years of low precipitation and one year aftercombined effects of strong La Nina events and nega-tive IPO phases (values were significantly below aver-age for precipitation [p < 0.05], NINO3.4 [p < 0.0],and IPO [p < 0.01]; Figure 5). Although PDO valueswere predominately negative one year prior to fires, de-partures from the mean were not significant (p > 0.05).Moreover, antecedent climate conditions forced by Pa-cific climate variability were important for widespreadfire years. Three years prior to fire events, values ofNINO3.4 (p < 0.05), PDO (p < 0.05), and IPO (p< 0.01) were significantly above average (wetter thanaverage conditions). Although temperature was aboveaverage during fire years, results were not statisticallysignificant. We expected precipitation to follow climateconditions forced by ENSO and PDO–IPO variability.Precipitation was above average three years prior tofires, but departures were not significant. One year priorto fires (during strong La Nina and negative IPO phases;drier than average conditions), there was no indicationthat precipitation departed from the mean on BPK (Fig-ure 5). SEA results on BPK did not suggest connectionsbetween Atlantic climate mechanisms (AMO, NAO)and wildfires (results not shown). On NNK, SEA re-vealed no significant relationships between widespreadhistorical fires and climate variables (Figure 6).

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Figure 4. Historical fire maps showing study sites that recorded fires during synchronous years between Big Pine Key and No Name Key inthe National Key Deer Refuge, Florida. The year of fire occurrence is shown in the upper left corner of each map and sites are highlighted todenote fire occurrence during that year. Note: NBP = North Big Pine; BYR = Boneyard Ridge; BHS = Blue Hole South; TPS = TerrestrisPreserve; NNK = No Name Key. (Color figure available online.)

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 11

Figure 5. Results from superposed epoch analysis showing departures from mean annual (A) ENSO, (B) IPO, (C) PDO, (D) precipitation(mm), and (E) temperature (◦C) during years in which a widespread (≥50 percent of recorder trees scarred from all four sites) fire occurredon Big Pine Key. Horizontal lines are the 95 percent (dashed) and 99 percent (solid) confidence intervals derived from 1,000 Monte Carlosimulations performed on the entire data sets. Solid black bars represent years with departures that exceeded confidence limits. Please notedifferences in y-axes and that departures from mean annual Atlantic Multidecadal Oscillation and North Atlantic Oscillation are not shown.Note: ENSO = El Nino-Southern Oscillation; IPO = Interdecadal Pacific Oscillation; PDO = Pacific Decadal Oscillation.

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Figure 6. Results from superposed epoch analysis showing departures from mean annual (A) ENSO, (B) IPO, (C) PDO, (D) precipitation(mm), and (E) temperature (◦C) during years in which a widespread (≥50 percent of recorder trees scarred from all four sites) fire occurredon No Name Key. Horizontal lines are the 95 percent (dashed) and 99 percent (solid) confidence intervals derived from 1,000 Monte Carlosimulations performed on the entire data sets. Solid black bars represent years with departures that exceeded confidence limits. Please notedifferences in y-axes and that departures from mean annual Atlantic Multidecadal Oscillation and North Atlantic Oscillation are not shown.Note: ENSO = El Nino-Southern Oscillation; IPO = Interdecadal Pacific Oscillation; PDO = Pacific Decadal Oscillation.

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 13

Discussion

The Fire Regime on Big Pine Key

Our reconstruction of fire history shows that fre-quent surface fires from the 1850s to the 1950s de-fined the fire regime on BPK. Low sample depth andscarcity of fire scars and recorder years preclude char-acterization of the fire regime before 1850, but the fewfire-scarred samples that extend prior to 1850 reveal atleast fourteen separate fire events in the early 1800s.The intra-annual positions of fire scars were most com-monly in the latewood zones of annual growth ringsor at the transition between earlywood and latewoodzones. Harley et al. (2012) found that slash pines formearlywood tracheids from February through June andlatewood tracheids from July through November andhave a dormant vascular cambium during Decemberand January. These data, combined with our results, in-dicate that several fire events coincided with the onsetof the summer-wet season (transition scars), but mostfires on BPK burned during the later part of the growingseason (July–November).

In southern Florida, most thunderstorms and light-ning strikes occur from May to October, and previousstudies found that the largest lightning-caused fires onthe mainland occur during this period (Taylor 1981;Beckage et al. 2003; Slocum, Platt, and Cooley 2003).Our fire seasonality results are similar to those reportedin two areas of northern Florida: a slash pine (Pinus el-liottii Engelm. var. elliottii) savanna located on Little St.George Island, Apalachicola Bay (Huffman et al. 2004),and a longleaf pine (Pinus palustris Mill.) savanna lo-cated near Eglin Air Force Base (Henderson 2006). TheMFI on BPK (five years) was similar to a four-year fire in-terval reported by Huffman et al. (2004) and a six-yearfire interval reported by Henderson (2006). Duncan,Weishampel, and Peterson (2011) used a fire simula-tion model to estimate the pre-European fire regimeand obtained different results for a slash pine habitat onan Atlantic coast barrier island in east-central Florida.They suggested that during the past 500 years, the nat-ural fire return interval on the island ranged from sevento twenty-one years, with an MFI of fourteen years. Al-though the MFI reported by Duncan, Weishampel, andPeterson (2011) was considerably longer that the MFIwe reported for BPK, the fire interval range was similar.

A comparison of MFIs between the Harley, Grissino-Mayer, and Horn (2013) study and the composite BPKfire record presented in this article (6.5 years and 5 years,respectively) revealed that increasing the spatial and

temporal implications of the fire reconstruction pro-vided a more comprehensive database with which toanalyze fire activity on BPK and is likely a more accu-rate representation of the historical fire regime that ex-isted during the settlement period. Van Horne and Fule(2006) conducted a test focused on highlighting thestatistical relationships between sample size and subse-quent reconstructed MFI estimates in a frequent sur-face fire regime in the southwestern United States anddiscovered that as sample size increased to about fiftyfire-scarred specimens, the MFI decreased toward anasymptote and remained constant. Although our studydesign precludes a detailed assessment of the thresholdat which little new information of the MFI is gainedwith additional samples, we suggest that the thresh-old in our study area is between 50 and 110 specimens(Table 1).

Twentieth-Century Changes in the HistoricalFire Regime

During the twentieth century, our fire history showsthe influences of increasing human population and theestablishment of the NKDR on fire regimes. Duringthe settlement period, fires on BPK burned aboutonce every four years and were spatially extensive,possibly burning large areas of the island. As the humanpopulation on BPK slowly increased from the earlyto middle 1900s, fires became more frequent and lesswidespread. After the establishment of the NKDR in1957, fires occurred nearly twice as often as during thesettlement period and were spatially restricted, likelybecause of increased roads, active fire managementpolicies, and more incendiary fires due to increasedpopulation. Although we found an increase in firefrequency during the twentieth century, we did notnotice a change in fire season between the settlementand fire management periods; mid- to late-season firescharacterized the entire study period (1707–2010).

Combining the fire records from the four study sitesprovides a better understanding of the spatial extentof fires across BPK. Our data suggest that before the1950s, wildfires burned large areas of the key. Begin-ning in the 1960s, however, fires became patchier, scar-ring fewer trees. Several site-specific, widespread firesdid occur during the fire management period, but thesewere prescribed burns conducted by NKDR personnelin 1977, 1990, and 2004. Habitat fragmentation and ac-tive fire management were likely causes of the changein spatial extent of fires between the settlement and fire

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management periods as documented by the differencebetween percentages of trees scarred during these twotime periods.

Island Fire Synchrony and Pacific Climate Forcingof Wildfires

During the period between 1818 and 1924, the levelof fire-year synchronicity between BPK and NNK washigh. This period of fire synchrony between keys ischaracterized by the following: (1) most fires that weresynchronous between BPK and NNK were widespreadon NNK but appeared to be localized on BPK, and(2) five widespread fires that occurred on BPK duringthis period were not recorded as fire years on NNK.An explanation of synchronized wildfire between is-lands is difficult given the historical nature of our dataand the lack of written records before the 1950s. Dur-ing the period between 1961 and 1996, which coversmost of the fire management period, Bergh and Wisby(1996) used written records (i.e., newspaper articles,NKDR fire records, personal communications) to doc-ument fires that occurred across the entire islands ofBPK and NNK. In their compilation of historical fireevents, they found evidence that a wildfire during 1961was spotted over a short distance (1 km) between twoareas on BPK. The characteristics of fire synchrony be-tween keys (widespread on NNK vs. localized on BPK)and the geography of the two islands support an expla-nation that widespread NNK fires spotted over to BPK,especially given that a westward (easterly) wind prevailsduring the year (Pitts 1994; Lee and Williams 1999).However, the closest distance between any of the fourBPK study sites and NNK is about 5 km, making spot-ting between islands unlikely over this distance (e.g.,Albini 1983; Albini, Alexander, and Cruz 2012).

Another possible explanation of fire synchrony isthat thunderstorms resulted in multiple fire ignitions(either multiple ignitions from one thunderstorm orseparate lightning ignitions but during the same sea-son of a given fire year), which is a well-documentedoccurrence (e.g., Rorig et al. 2007; McCarthy, Poss-ingham, and Gill 2012). Thunderstorms and lightningstrikes in south Florida are most common during thesummer months, and this is consistent with the season-ality of fires on BPK and NNK based on the intra-annualgrowth-ring position of scars. Beckage et al. (2003) em-phasized a linkage between widespread fires, La Nina,and lightning in Everglades National Park of southernmainland Florida. They suggested that weather con-ditions brought on by La Nina result in fewer clouds,

decreased dry-season rainfall, and low surface water lev-els. These conditions could cause rapid surface warm-ing, create more thunderstorms and lightning strikes,and increase likelihood of ignition of dry fuels overlarge areas, which would in turn contribute to more fre-quent fires. They also found that increased numbers oflightning strikes coincided with La Nina drought con-ditions, which possibly leads to widespread fires. Duringthe fire management period, Bergh and Wisby (1996)found a total of fifty-eight documented fires on BPKand NNK, of which twenty-seven were wildfires andthirty-one were prescribed burns. Of the documentedwildfires, at least six were reportedly caused by lightningand occurred during summer; hence, there is evidencethat summer wildfires ignited by lightning occur on thekeys. Although our historical burn maps have limita-tions and we interpret them with caution (Figure 4),the spatial patterns of fire occurrence between islandssupport the argument for multiple fire ignitions, partic-ularly in the pattern of fire occurrence during the year1914. During 1914, fires occurred at NNK and NBP.If fires were spotting over from NNK, one would ex-pect fire to burn at BYR (closest site to NNK) beforespreading to the NBP site. The other spatial pattern offire that was noted (i.e., no synchronous fire recordedat BHS) is likely because the fire reconstruction fromBHS is not robust and only includes seven fire-scarredspecimens, thus precluding further details of historicalfire patterns over the island.

As the largest island in the lower Florida Keys, BPKwas historically the most fire-prone island (Bergh andWisby 1996). Our fire data support this hypothesisand a similar island-size effect has been suggested byseveral other studies (e.g., Bergeron 1991; Niklasson,Drobyshev, and Zielonka 2010). Compared to NNK(ca. 450 ha), the larger size of BPK (ca. 2,350 ha) likelyresulted in a higher frequency of lightning strikes perthunderstorm during the summer-wet season. Addition-ally, we did not find a significant relationship betweenclimatic variability and fire at NNK, which suggeststhat island size might influence the ability to detectbroad-scale climate forcing of wildfires.

We did not find evidence to suggest linkages be-tween Atlantic climate forcing mechanisms and wild-fire on BPK or NNK. Although the AMO and NAO areknown to influence the climate of Florida, a connectionbetween these oscillations and fire in Florida is not yetestablished. Our analyses of fire and climate revealedthat the historical fire regime on BPK was influenced byyears of low precipitation in combination with climatevariability in the Pacific Ocean. Our ENSO results are

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Fire Synchrony and Pacific Climate Variability on Wildfires in the Florida Keys 15

related to the findings of Beckage et al. (2003), and wesuggest that the same interacting effects of El Nino andLa Nina influence fire occurrence on BPK. In additionto the interacting effects of ENSO on fires, we suggestthat modulation of El Nino and La Nina phases by PDOand IPO (i.e., positive anomalies of El Nino strength-ened by positive PDO and IPO phases and negativeanomalies of La Nina strengthened by negative phasesof IPO) could influence fires on BPK. We found thatEl Nino conditions, strengthened by positive PDO andIPO phases, occur three years prior to fire events andcould produce more biomass and increase the growth offine fuels (e.g., grasses, shrubs, palms; Figure 5). We sug-gest that strong La Nina conditions, in congruence withdry years, bolstered by negative phases of the IPO, oneyear prior to fire events could result in drier fuels causedby rapid heating of the surface, and increased lightningstrikes. These conditions seem to promote fire weatherthat supports the frequent and widespread wildfires onwhich the pyrogenic pine rocklands depended prior tothe advent of intentional human ignitions that likelyplagued the fire management period.

Although we discovered that constructive phases ofENSO and IPO likely promote fire occurrence on BPK,we did not find convincing evidence that La Nina con-ditions were modulated by negative phases of the PDO.Values of PDO one year prior to fire events were gen-erally negative, yet departures from the mean were notsignificant (Figure 5). Kurtzman and Scanlon (2007)suggested that PDO modulation on La Nina winterprecipitation was negligible, which might explain ourinsignificant finding of PDO modulation during LaNina. However, Wachnicka et al. (2013) suggested thatdrought, forced by La Nina and negative phases of thePDO, promoted major diatom mortality events over thepast 600 years in southern Florida. The most reasonableexplanation of our varied PDO and IPO results likelyderives from the differences in the PDO and IPO in-dexes that were included in this study. Although thePDO presented by Mantua et al. (1997) includes SSTvariance poleward of 20◦N in the North Pacific, theIPO includes variance to at least 55◦S in the SouthPacific; hence the IPO is more spatially extensive thanthe PDO. The ENSO and IPO results returned by SEAare very similar, and although patterns of ENSO and theIPO are alike, they differ in several ways. The IPO ischaracterized by marked basin-wide equatorial symme-try, less SST variance in the eastern Pacific, and morevariance outside the tropics (Folland et al. 2002).

We suggest that the association between the PDOand IPO and fire occurrence be viewed with cau-

tion. First, the period during which we investigatefire–climate relationships (1856–1956) is only longenough to include one full cycle of the PDO, and al-though the IPO cycles at fifteen- and thirty-year fre-quencies, only three phases of the IPO have been iden-tified during the twentieth century as defined fromMeteorological Office HadISST data (Rayner et al.1999): a positive phase (1922–1944), a negative phase(1946–1977), and another positive phase (1978–1998;Salinger, Renwick, and Mullan 2001). Hence, our studyperiod only covers one full cycle of the IPO as well.Second, the physical processes of the IPO are still be-ing investigated. Thus, it is not clear how independentthe IPO is from ENSO red noise (Folland et al. 2002).Although our results between fire and PDO and IPOare statistically significant, additional research on thestrength and spatiotemporal dynamics of phase shifts ofPDO and IPO over time, their influence on Florida’s cli-mate, and fire–climate relationships in southern Floridaare necessary to infer a more confident causal link be-tween PDO or IPO and fire in the region.

Conclusions

Pine rockland is an endangered ecosystem that, inthe Florida Keys, is restricted to a few small islands.Although pine rockland currently covers only a smallland area, this ecosystem preserves endemic species thatdepend on fire for persistence. In summary, we providemultiple lines of evidence that detail a more completesuggestion of the relationships between climatic vari-ability and historical wildfires on the two largest islandsin the lower Florida Keys. We suggest that in the FloridaKeys, historical wildfires occurred more frequently onlarger islands than on smaller islands and that islandsize could influence the ability to detect relationshipsbetween climatic variability and wildfire. Given therelationship between climatic variability and wildfireon BPK and the abundance of mid- to late-season firescars (contemporaneous with summer thunderstorms),we suggest that lightning was a primary igniter of histor-ical fires. Based on the fire–climate evidence, we em-phasize that preconditioning of fine fuels by ENSO,IPO, and PDO occurred before widespread historicalfires on BPK. El Nino conditions, strengthened by pos-itive phases of the IPO and PDO, three years prior tofire events could produce more biomass and increasethe growth of fine fuels. In contrast, La Nina condi-tions, strengthened by negative phases of the IPO, oneyear prior to fire events could promote fire weather by

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drying out fuels and increasing the frequency of light-ning strikes.

The wildland–urban interface on BPK and NNK isvirtually nonexistent, with many homes containing lit-tle or no defensible space against wildfire. The U.S. Fishand Wildlife Service (USFWS) operates with limitedpersonnel and with a limited budget and thus any infor-mation related to broad-scale climatic drivers of wildfireoccurrence can be used in fire management plans toassess future wildfire risk on the islands. Although theUSFWS actively suppresses wildfires, they maintain asuccessful prescribed fire-management program (Sahet al. 2010; Harley, Grissino-Mayer, and Horn 2013).Our results suggest that the coupled oscillatory interac-tions of ENSO, PDO, and IPO should be monitored bythe USFWS and be considered in their fire suppressionand prescribed fire efforts. The addition of historicalfire data from other ecosystems in southern Florida, theBahamas, and Cuba might reveal the fire–climaterelationships that we show here across a much broaderspatial scale.

Acknowledgments

This research was supported by the National Sci-ence Foundation under grant numbers 1002479 and0538420 and by the U.S. Fish and Wildlife Ser-vice. We thank Anne Morkill, Chad Anderson, DanaCohen, and Philip Hughes for access to the NationalKey Deer Refuge; Douglas Heruska, Kohy Honeyman,Niki Garland, Desiree Kocis, Ann McGhee, Alex Pi-lote, John Sakulich, and Rebecca Stratton for field as-sistance; Kody Honeyman, Joshua Turner, and Christo-pher Petruccelli for laboratory assistance; and threeanonymous reviewers for comments that improved thisarticle.

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Correspondence: Department of Geography and Geology, The University of Southern Mississippi, Hattiesburg, MS 39406, e-mail:Grant.Harley@usm.edu (Harley); Department of Geography, The University of Tennessee, Knoxville, TN 37996, e-mail: grissino@utk.edu(Grissino-Mayer); shorn@utk.edu (Horn); The Nature Conservancy, Big Pine Key, FL 33043, e-mail: cbergh@tnc.org (Bergh).

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