Growing scattered broadleaved tree species in Europe in a changing climate: a review of risks and...

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Growing scattered broadleaved tree species in Europe in a changing climate: a review of risks and opportunities G. E. HEMERY 1 * , J. R. CLARK 2 , E. ALDINGER 3 , H. CLAESSENS 4 , M. E. MALVOLTI 5 , E. O’CONNOR 6 , Y. RAFTOYANNIS 7 , P.S. SAVILL 8 and R. BRUS 9

1 Forestry Horizons, Sylva Foundation, Manor House, Little Wittenham, Oxon OX14 4RA, England 2 Northmoor Trust, Little Wittenham, Abingdon, Oxon OX14 4QZ, England 3 Forest Research Institut Baden-Württemberg, Wonnhaldestr 4, 79100 Freiburg, Germany 4 Unit of Forest and Nature Management, Gembloux Agricultural University, 5030 Gembloux, Belgium 5 Italian National Research Council, Institute of Agroenvironmental and Forest Biology, 05018 Porano (TR), Italy 6 School of Biology and Environmental Science, University College Dublin, Belfi eld, Dublin 4 , Ireland 7 Department of Forestry and Environmental Management, TEI Lamias, Karpenisi 36100, Greece 8 Department of Plant Sciences, Oxford Forestry Institute, University of Oxford, South Parks Road, Oxford OX1 3RB, England 9 University of Ljubljana, Biotechnical Faculty, Vecna pot 83, 1000, Ljubljana, Slovenia * Corresponding author. E-mail: [email protected]

Summary

Scattered broadleaved tree species such as ashes ( Fraxinus excelsior L. and Fraxinus angustifolia Vahl.), black alder ( Alnus glutinosa (L.) Gaertn.), birches ( Betula pendula Roth. and Betula pubescens Ehrh.), elms ( Ulmus glabra Huds., Ulmus laevis Pall. and Ulmus minor Mill.), limes ( Tilia cordata Mill. and Tilia platyphyllos Scop.), maples ( Acer campestre L., Acer platanoides L. and Acer pseudoplatanus L.), wild service tree ( Sorbus domestica L. and Sorbus torminalis L. Crantz), walnuts ( Juglans regia L., Juglans nigra L. and hybrids) and wild cherry ( Prunus avium L.) are important components of European forests. Many species have high economic, environmental and social values. Their scattered distributions, exacerbated in many cases by human activity, may make them more vulnerable to climate change. They are likely to have less ability to reproduce or adapt to shifting climate space than more widespread species. The general impacts of climate change on these scattered species are reviewed. Some specifi c risks and opportunities are highlighted for each species, although there is considerable uncertainty and therefore, diffi culty in quantifying many specifi c risks and/or impacts on scattered broadleaved tree species.

Introduction

The context for broadleaves in the 21st Century

European forests are the single largest natural ecosystem supporting biodiversity in Europe

( UNECE-FAO, 2006 ). As the effects of climate change impact biodiversity and society, forests are likely to be increasingly important in provid-ing ecosystem services (e.g. landscape connectiv-ity, soil and water conservation and habitats for wildlife) and as a source of low-carbon products

Forestry Advance Access published December 24, 2009

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(e.g. wood for construction and provision of ma-terial for bioenergy).

Scattered broadleaves are an important com-ponent of a diverse forest resource, not least as these species often produce high value timber. Long-term forecasts of world timber trends indicate a continuing decrease in hardwood roundwood exports from tropical forests and in-creasing consumption of timber in industrializing countries such as China and India ( Lawson and Hemery, 2007 ). There are therefore good reasons for increasing domestic timber supplies, which in turn will bring multiple benefi ts to society.

Scattered broadleaves in Europe

The current distribution and composition of Eu-ropean forests are relics of glacial and post-glacial history, subsequently infl uenced by human activ-ity. They are relatively species poor compared with American and Asian forests, due to the east-west orientation of mountain ranges in Eu-rope that limited the ability of species to migrate northwards from refugia during the last glacial period.

Scattered broadleaves are considered to be a distinct group of species by a network of the European Forest Genetics Programme (EU-FORGEN), although no formal defi nition ex-ists. According to EUFORGEN (2009) , they are characterized as having scattered distributions in mixed forests and specifi c site requirements and habitats. Management of genetic resources in such species often requires different approaches as compared with more commonly occurring or stand-forming tree species such as oak ( Quer-cus spp.) and beech ( Fagus sylvatica L.). A chief concern regarding scattered broadleaves is the theoretical increased susceptibility to genetic erosion due to their often scattered distributions. In many cases, genetic resources of these spe-cies have suffered from forest management that reduces genetic variation, habitat deterioration and introgression (i.e. movement of genes from plantations or amenity plantings to natural pop-ulations through hybridization) ( EUFORGEN, 2009 ).

Historically scattered broadleaves have been selected against in large parts of Europe, even though they often have high monetary value.

Conifer species and large planting schemes with stand-forming hardwood species have dominated forest practices. By increasing forest diversity, scattered broadleaves may play an important role in insuring against risks, such as pests and patho-gens, fi re and habitat loss.

Projected climate change impacts

Global greenhouse gas emissions have grown since pre-industrial times, with an increase of 70 per cent between 1970 and 2004 ( IPCC, 2007b ). Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, wide-spread melting of snow and ice and rising average sea level ( IPCC, 2007c ). Observational evidence from all continents and most oceans shows that many natural systems are being affected by re-gional climate changes, particularly temperature increases ( IPCC, 2007a ). The risks and oppor-tunities reported in this paper are related to the IPCC scenarios based on widely accepted climate change models.

Projected future impacts of climate change on forests – due to rising temperature and CO 2 (posi-tive for tree growth in short to medium terms), to large-scale stochastic events such as increased in-cidences of fi re, drought (frequency and severity) and increases (distribution and impact) of pests and pathogens – are wide ranging. European for-ests occur across several diverse biomes, each of which is defi ned by specifi c current plant com-munities associated with the particular climate regimes. The impact of projected climate change is likely vary greatly across these with some ex-pected to become more productive (e.g. northern boreal) and others less so (e.g. Mediterranean) ( Table 1 ; Kellomäki and Leinonen, 2005 ).

Many factors will individually, or in varying combinations, affect the growth and health of scattered tree species ( Table 2 ). In reality, there will be many factors at play often with complex interactions, making the identifi cation of a domi-nant factor diffi cult. For instance, in a changing climate, tree susceptibility to disease may in-crease with drought-induced stress, at the same time that new diseases or pests may impact trees and existing pests and diseases may become more destructive.

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Evidence for recent climate change in European forests

The effects and consequences of temperature in-creases have been documented (with ‘ medium confi dence ’ ) on agricultural and forestry manage-ment at Northern Hemisphere higher latitudes. They include earlier spring planting of crops and alterations in disturbance regimes of for-ests by fi res and pests ( IPCC, 2007a ). Climate change is already affecting living systems globally ( Parmesan and Yohe, 2003 ), including the range and abundance of animals and plants ( Parmesan, 2006 ).

Wide-ranging evidence exists for climate change affecting fl oral distributions and abun-dance changes (e.g. Lavergne et al. , 2006 ) and for biome shifts (e.g. Peñuelas and Marti, 2003 ) across Europe. In relation to forests, Lapenis et al. (2005) demonstrated acclimation of trees to warming and changing precipitation patterns. The advance of woody species into higher alti-tudes, probably as a result of global warming, has already been detected ( Sanz-Elorza et al. , 2003 ). Truong et al. (2007) reported change in Boreal forests in northern Sweden, where an up-

ward shift in the tree-line species mountain birch, Betula pubescens ssp. tortuosa Ledeb. (Nyman), is attributed to climate warming. A temperate for-est example is provided by Loacker et al. (2007) , where the spread of walnut ( Juglans regia ) in Alpine valleys in Austria is attributed to milder winters having favoured seed germination and seedling survival.

We review the main factors that may affect Europe’s main scattered broadleaved tree species. Under ‘ range change ’ , we consider the current distribution of the species and how this may af-fect their ability to adapt to change. We review ‘ pests and pathogens ’ and consider possible fu-ture biotic threats for each species. We consider the impact of ‘ drought ’ on each species as this can often have a compounding effect on tree health. We review the role of ‘ reproductive biology and genetics ’ for each species, as these factors may signifi cantly affect the ability of a species to adapt to a changing climate, for example by exploiting colonization opportunities or being prone to in-creased genetic erosion. Finally, under ‘ other fac-tors ’ , we review species-specifi c issues that may have a bearing on their future.

Table 1: Potential impacts of climate change on forest productivity in European forest biomes (based on Kellomäki and Leinonen, 2005 )

Biome Potential impacts on forest productivity

Northern boreal Production is mainly limited by low temperature and often by nutrient availability. Precipitation is currently normally not limiting. Higher air temperatures predicted by scenarios for the future climate will prolong the growing season and thereby is expected to increase production.

Southern boreal Production is more limited by water and less by temperature and nutrients, resulting in a higher production than in the north. Reduced precipitation is likely to cause increased stress.

Temperate maritime Production is higher than in the boreal zone; this is the result of higher air temperature and less water limitation. A warming climate and elevated CO 2 is likely to increase productivity for many species in the short-medium term.

Temperate continental Production is generally more constrained by water than in the temperate maritime zone. Enlarged leaf area index may increase tree’s water requirements, while precipitation may not increase or even reduce. Stress due to drought is likely to increase.

Alpine Production is water limited at low altitudes but not at higher altitudes where precipitation is signifi cantly higher. The future climate at low altitudes is likely to lead to a loss in production if not counteracted by an increase in production as a result of elevated CO 2 . Production at higher altitudes is likely to increase in the future climate, mainly because of a prolonged growing season.

Mediterranean Production is limited by low air humidity and soil water because of high evaporative demand. Production in the future is likely to be less compared with that of today.

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Acer pseudoplatanus , Acer platanoides and Acer campestre – maples

This account deals primarily with sycamore ( Acer pseudoplatanus ) and only to a limited extent

with Norway maple ( Acer platanoides ) and fi eld maple ( Acer campestre ). Like most of the species dealt within this paper, sycamore is never com-mon, rarely exceeding 3 per cent of forest cover ( Spiecker et al. , 2009 ). It seldom grows in pure

Table 2: Summary of climate change impacts on European forests.

Factor Impacts

Temperature Photosynthesis and respiration, soil organic matter decomposition and mineralization, phenology and frost hardiness, species distributional changes and adaptation and evolution are affected by temperature ( Saxe et al. , 2001 ).

CO 2 Increases in CO 2 not only affect the global climate but directly impact plant photosynthesis and respiration. Research has indicated increased growth rates but with impacts on water use, carbon allocation, nutrient allocation and timber quality ( Broadmeadow and Randle, 2002 ).

Wildfi res The European forest resource, in the most part, is neither adapted (e.g. serotinous) nor dependent on wildfi res; therefore, native species are likely to be poorly adapted to wildfi res. Changes to forest ecology are diffi cult to predict, although it is likely that fast colonizers and non-native invasive species may alter existing communities. Some modelling has been undertaken (e.g. Carvalho et al. , 2006 ; Groisman et al. , 2007 ).

Drought Generally, drought will impact by negatively affecting ecosystem productivity and increasing mortality ( Archaux and Wolters, 2006 ). Competitive species, those adapted to cold and wet conditions as well as species with low reproduction rates and/or limited mobility, seem to be the most affected. Fuhrer et al. (2006) cite evidence for intraspecifi c variation in response to drought conditions. Relictual taxa appear more drought tolerant than extinct taxa ( Svenning, 2003 ). Therefore in the long run, a change in the frequency of hot and dry years could affect tree species composition and diversity.

Wind Windthrow damage in Europe increased in the twentieth Century but loss of timber was typically smaller than annual timber harvests ( Schelhaas et al. , 2003 ). Windthrow can also have positive ecological effects but where damage levels are excessive, harvesting or salvage harvesting costs can be very high, particularly in mountainous terrain. Dorland et al. (1999) estimate that annual mean insured damage could increase by 80% in 25 years (i.e. to the year 2015) due to a 2% increase in highest wind speed in The Netherlands.

Precipitation Heavy precipitation can be associated with high costs, in terms of both economies and human life, and can impact the environment particularly through loss of fertile topsoil by soil erosion. Simulations suggest that a climate warming could be associated with a substantial increase in atmospheric moisture content of about 7% per degree of warming ( Frei et al. , 2000 ). Changes to forest cover, tree health and the rainfall climate will also impact water fl ow.

Chilling For species with a large chilling requirement, milder winters might result in inadequate chilling and hence delayed and erratic bud burst in spring ( Cannell and Smith, 1986 ). For example, climatic warming has been linked to premature bud burst of trees in Finnish conditions during mild spells in mid-winter, resulting in heavy frost damage during subsequent periods of frost ( Hänninen, 1996 ).

Pests and pathogens

Future pest and pathogen trends will relate to relationships between pest and pathogen, the health of the host tree species and any natural defence mechanisms/pest predators. However, stressed trees are more susceptible to insect pests and diseases, and many insect pests are likely to benefi t from climate change as a result of increased breeding activity and reduced winter mortality. Climate change has been linked with range expansion, northward and upward, of several insect species of northern temperate forests (e.g. Battisti, 2004 ). Impacts on broadleaved species are uncertain as much of current scientifi c work in this area has focussed on coniferous species. The impact of facultative pathogens such as sooty bark disease of sycamore may worsen, while some insect pests that are present at low levels, or currently not considered important, may become more prevalent ( Broadmeadow, 2000 ).


stands naturally but is one of the few European species which has not only become naturalized in many regions outside its native range but is still spreading ( Savill, 1991 ).

Range change

All three species are likely to become suited to more northerly regions and higher elevations than they are at present, although, in highly exposed locations, Norway and fi eld maples will not. Sycamore, by contrast, grows at higher elevations than most broadleaved trees, and its upper limit is determined more by the presence of suitable soil than by climate ( Jones, 1945 ). Given that syca-more does not thrive in drought-prone regions ( Pinto and Gégout, 2005 ), the southern part of its range is likely to move northwards as the cli-mate becomes hotter and drier, possibly north of Spain, Italy and the Balkans (latitude 44° N).

Pests and pathogens

The only disease likely to affect sycamore due to climate change is ‘ sooty bark disease ’ caused by the fungus Cryptostroma corticale (Ellis & Everh.) ( Gregory and Redfern, 1998 ). It is usually fatal, and attacks are triggered by high summer temperatures and drought, so that predicted climate change is likely to in-crease the incidence of this disease in the south-ern, more continental and lower elevation parts of its distribution.

In Great Britain, damage to sycamore caused by the grey squirrel ( Sciurus carolinensis Gmelin) may increase as a result of reduced winter mortal-ity and increased seed availability ( Broadmeadow and Ray, 2005 ).

Drought

According to Röhrig and Ulrich (1991 , p. 400) of the three maples, A. campestre and A. platanoides are relatively drought tolerant, while A. pseudo-platanus is not. In dry conditions, sycamore tends to exhibit top dieback and premature mortal-ity and also becomes much more susceptible to the usually fatal sooty bark disease ( Strouts and Winter, 1994 , p. 237).

Sycamore is more sensitive to dry conditions than ash and oak, its main competitors on many sites. There is evidence that in some British woods, it is growing more slowly than ash ( Morecroft et al. , 2008 ). In view of projections for more fre-quent summer droughts, this suggests that syca-more is likely to decline in future in many places. This is consistent with evidence from a modelling-based study by Broadmeadow et al. (2005) of tree growth responses to climate change in the UK and the observed European distribution of sycamore that is principally damp montane conditions.

Reproductive biology and genetics

Knowledge about the genetic structure of the maples is uncertain as there has been very little research on these species (as with other European insect-pollinated trees). Nothing is known about this in the context of climate change.

Alnus glutinosa – black alder

Range change

A warming climate in Europe could extend the natural range of black alder to Scandinavia and Russia in the North, where its distribution would be limited by the intensity and length of the pe-riod when frosts occur ( MacVean, 1953 ). To the east and the south, if summer drought fre-quency increases, black alder distribution could be restricted. The alder life cycle may be inhibited because pollen germination requires high atmo-spheric humidity during the summer ( Bensimon, 1985 ). However, linear stands of alder could be maintained outside the main range by the distri-bution of seeds along watercourses draining from these areas, as observed under current conditions in eastern Europe along the rivers Volga, Don or Dnieper ( Jalas and Suominen, 1976 ).

Pests and pathogens

The most damaging pathogen of alder is the fun-gus Phytophthora alni ( Brasier et al. , 1995 ), which has been present in all watercourses across Europe since the 1990s ( Streito, 2003 ). The vector of the fungus is the zoospore that is produced only in

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river water and is especially spread during fl ooding ( Gibbs, 1994 ). Development of the zoospores and the infection of trees are only possible when water temperature is higher then 5°C, mostly limiting susceptibility to the summer months ( Chandelier et al. , 2006 ). An increase in temperature could lengthen this susceptible period and therefore in-crease the impact of the disease in the future.

Drought

Black alder has a limited ability to control its transpiration under a range of conditions ( Braun, 1974 ; Eschenbach, 1995 ) and therefore has a high water demand. This is not likely to be problematical in humid soils with a high water-table, for example in valleys and around marshes ( Claessens et al. , in press ), where water supply is not affected by climate change ( Gaudin, 2007 ). However, on some plateaux sites where it is cur-rently only present due to high summer rainfall (Atlantic Europe: MacVean, 1953 ; Lhote, 1985 ), the species could become susceptible to drought. Across its range, black alder will be restricted to suitable sites with the most humid soils.


Black alder appears to have a large genetic vari-ability and different ecotypes are present in rela-tion to the river basins (Black, Baltic and North Seas) ( Glavac, 1972 ; Franke, 1994 ). The current and future distributions of alder forests will fol-low the river network; therefore, the colonization behaviour of the species may limit its susceptibil-ity to genetic erosion.

Other factors

With projected increases in fl ooding due to higher winter rainfall or extreme summer rainfall, black alder could play an essential role in the protec-tion of riverbanks from erosion ( Köstler, 1968 ; Claessens, 2005 ). The restoration of alluvial alder forest where fl ood water can be tolerated could also decrease the impact of fl ooding on agriculture, road networks and various habitats ( Claessens, 2005 ). Indeed, black alder is one of the species best adapted to fl ooding ( Gill, 1970 ; Gill, 1975 ;

Leipe, 1990 ; Crawford, 1992 ). Such management may go hand in hand with biodiversity objectives ( Schäfer and Joosten, 2005 ), although these activi-ties may only concern marginal land areas.

In the humid soils of the main range of black alder (central and western Europe from the Dan-ube plains to southern Finland), an increase in CO 2 , a lengthening of the growing season and an increase in temperature should increase the pro-ductivity of alder stands. For example, in south-ern Finland, adaptation to longer days appears to favour black alder productivity in a warmer climate ( Glavac, 1972 ).

Betula pendula and Betula pubescens – birches

Range change

Overall, a shift northwards in the ranges of both Betula species is expected. However, this will be affected by local conditions; for example, on strongly wind-exposed terrain in northern Sweden, a century of substantial warming has not offset a decline of birch ( Kullman, 2005 ). An altitudinal shift upwards, in regions where soil type and pre-cipitation allows, is also expected. In addition, the composition of forests may be affected; Betula pendula growth is inhibited by lower temperatures to a greater extent than conifers, so higher temper-atures may lead to a change of structure of boreal forests in Scandinavia ( Aphalo et al. , 2006 ). In Scotland, birch may expand into existing wood-lands via colonization of areas opened up by in-creasing storm damage ( Ray, 2008 ). In Spain, the climate is expected to become more oceanic and the extent of deciduous forests with B. pubescens is likely to increase ( Río et al. , 2005 ).

Pests and pathogens

The extent and severity of geometrid moth out-breaks is expected to increase ( Hagen et al. , 2007 ). In addition to these outbreaks, increased background insect damage could make the spe-cies less competitive as northern forests are in-creasingly exposed to southern insect pests ( Wolf et al. , 2008 ). Annual physiological rhythms affect the digestibility and palatability of birch and, in


northern regions, increased moose browsing could become an issue due to changed local climate or the use of regeneration material from more south-ern latitudes ( Viherä-Aarnio and Heikkilä, 2006 ). One study has, however, indicated that changes in the internal chemistry of trees may prevent in-creased browsing damage of silver birch by the mountain hare and the fi eld vole ( Kuokkanen et al. , 2004 ). Changes in fungal pathogen behaviour and range may result in increased inoculation pressure. Birch trees under stress from drought or exposure will be more susceptible to infection.

Drought

Betula pubescens is more suitable than B. pen-dula for cooler climates and wetter soils, whereas B. pendula is more tolerant of warmer tempera-ture. The range of B. pendula is restricted by soil moisture ( Atkinson, 1992 ) but the species has the widest physiological amplitude of all mid-European broadleaved tree species ( Ellenberg, 1996 ). Areas that are forecast to become drier, such as the southeast of England, are likely to become unsuitable for birch ( Ray, 2002 ). Peterken and Mountford (1996) report evidence for signifi cant drought-related mortality in extreme years. The modelling work of Broadmeadow et al. (2005) suggests that sycamore, birch ( B. pendula ) and beech ( F. sylvatica ) are likely to be the most sensitive species to climate change because of their sensitivity to drought.


Both species of Betula are wind pollinated with wind-dispersed seed capable of exploiting wood-land gaps and clearings, which may ensure gene-fl ow between local populations ( Truong et al. , 2007 ). High levels of variation within populations means that there is a high capacity within birch populations to adapt to climate change ( Kelley et al. , 2003 ; Oksanen et al. , 2005 ) and an op-portunity exists to exploit this through breeding programmes. However, long-distance transfer of reproductive material has not been successful due to local adaptation: birch phenology is governed by temperature sum and photoperiod ( Myking and Heide, 1995 ).

Other factors

Projected climate change could benefi t birch growth in northern Europe ( Kellomäki et al. , 1996 ). However, interactions between tempera-ture, ambient CO 2 , nutrient availability, water restrictions and ozone exposure make it diffi cult to predict increase in growth rates. There is some risk that increasing the diameter of birch may in-crease the probability of windthrow ( Ilisson et al. , 2005 ). Stronger winds on exposed sites will lead to crown and leader damage, while areas that may be at risk from excessive fl ooding are unlikely to produce high-quality trees ( Ray, 2002 ).

Fraxinus excelsior L. and Fraxinus angustifolia Vahl. – ashes

Range change

The distribution of Fraxinus excelsior is de-fi ned by its lack of tolerance to winter cold, late spring frosts and hot summers. New shoots are especially frost sensitive ( Wardle, 1961 ; Rameau et al. , 1989 ; Savill, 1991 ). In the UK, modelling predicts that in the early stages of climate change, ash may replace beech as the most suitable com-mercial broadleaved species across much of southern England; in turn, ash may be replaced by pedunculate oak ( Broadmeadow et al. , 2005 ). The same models predicted that sites for grow-ing quality ash will become more abundant at the northern limit of its range.

Fraxinus angustifolia is found throughout southern and eastern Europe and grows best on rich soils at low altitudes and will withstand tem-porary fl ooding. It requires a mild climate and precipitation between 400 and 800 mm ( FRAXI-GEN, 2005 ); therefore, with the reduced rainfall predicted for Southern Europe, it is likely that its southern limit will be further north.

Pests and pathogens

Since the 1990s, large-scale dieback of ash has been observed around the Baltic countries and more recently spreading to other regions of Europe ( EPPO/OEPP, 2007 ) and arriving in Denmark in 2003 ( Barklund and Rimvis, 2008 ). The fungus Chalara fraxinea (CHAAFR) was

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isolated as the causal factor in 2006 ( Kowalski, 2006 ). Climatic stress factors are likely to exacer-bate susceptibility to this fungus.

The ash bud moth ( Prays fraxinella Bjerk.) is a minor pest on ash trees at present where the larvae burrow into the terminal bud, resulting in the loss of the leader ( Foggo, 1996 ). The pest is likely to benefi t from climate change as a result of increased activity and reduced winter mortality ( Broadmeadow and Ray, 2005 ).

Drought

Ash grows under a wide range of climatic condi-tions providing the soil is suitable ( Kerr and Evans, 1993 ). Ash has a tolerance strategy to water stress ( Carlier et al. , 1992 ) where osmotic changes and elastic adjustment of the cell walls contribute to a drought resistance mechanism. However, where water is severely limited, growth is limited and ash takes on a shrub-like form ( Wardle, 1961 ). Savill (1991) reported that ash requires moist soil conditions and so is not found on drought-prone sites. With summer rainfall likely to be lower under climate change scenarios, suitable sites for growing high-quality ash are likely to decrease.


With the range of F. angustifolia shifting north-wards, it is likely that it will overlap with F. excel-sior more than at present, resulting in increasing hybridization.

Most ash species are deeply dormant with a high chilling requirement to break dormancy ( Bonner, 1974 ; Villiers, 1975 ). At time of seed fall, the seed is not fully formed and a warm period is required for the embryo to elongate, followed by a further cold period to break dormancy. A series of re-ciprocal transplant experiments with F. excelsior , ranging from Inverness in Scotland (latitude 58° N) to the Pyrenees (latitude 43°N), indicate that those populations from more northerly latitudes require more than twice the chilling requirement than those at the southern limit (J.R. Clark, un-published data ). With milder winter tempera-tures, it is possible that chilling requirements to break seed dormancy will not be met, even at the northern limit of the species ’ range, making natu-ral regeneration less likely. Additionally, high

temperatures at time of germination can cause secondary dormancy which negates the effect of any previous cold treatment ( Piotto, 1994 ).

Other factors

F. excelsior is not frost tolerant, but its avoidance strategy is to fl ush late in the spring. However, late frosts can cause forking ( Kerr and Boswell, 2001 ). Dormant trees are very cold hardy, but young shoots are frost sensitive. Although the climate is predicted to warm ( IPCC, 2007c ), late spring frosts are likely to still cause damage to ash species.

Juglans regia L. and Juglans nigra L. – walnuts

The genus Juglans contains ~ 20 species growing from Southern Europe to eastern Asia as well as in North and Central America and the Andes ( Manning, 1978 ). The most common naturally occurring species in Europe is common walnut ( J. regia ). Black walnut ( Juglans nigra ), native to North America, and the hybrid between these two species ( Juglans × intermedia ), are occasion-ally planted.

Range change

Walnuts are suited to mild-warm climates ( Ehring, 2005 ) and generally those governed by a conti-nental climate ( Steven, 1927 ). They are moder-ately site demanding and grow exceptionally well on deep, rich and loamy soils, while extremely dry nutrient-poor soils are not suitable ( Becquey, 1997 ). Extreme temperatures, as well as drought, limit the distribution of walnut ( Schaarschmidt, 1999 ). In the juvenile stage, walnuts are sensitive to frost, particularly if early fl ushing ( Bernetti, 1995 ). Winter hardiness is high, with older trees capable of tolerating winter temperatures as low as − 30°C, although there can be signifi cant varia-tion between and within species ( Poirier et al. , 2006 ). Juglans × intermedia seems to be less sus-ceptible not only to late frost but also to fl ooding ( Mapelli et al. , 1997 ).

A close correlation between mild winters and germination was demonstrated for J. regia in Al-pine valleys ( Loacker et al. , 2007 ). Consequently,


an increase of mean annual temperatures and milder winters may improve the growth and es-tablishment conditions for walnut ( Loacker et al. , 2007 ).

Pests and pathogens

In general, walnuts are relatively disease free al-though under plantation conditions vermin and pathogens can lead to local losses. Armillaria mellea (Vahl) P. Kumm. occurs mainly in contam-inated woods and on cultivated land ( Masson, 2005 ). It especially affects trees after previous damage (e.g. following frost or drought) causing loss of wood quality and ultimately plant death. J. nigra and J. × intermedia are less susceptible than J. regia . Increased temperatures (>20°C) and rain favour the spread of the foliar disease an-thracnose (caused by Gnomonia leptostyla (Fr.) Ces. & De Not.). Increased drought, especially during the growing season, may possibly reduce susceptibility to anthracnose.

Bacterial infections of Xanthomonas camp-estris pv. Juglandis (Pierce) Dye., commonly re-ferred to as ‘ blight ’ , can kill saplings ( Becquey, 1997 ). The bacterium is dispersed by raindrops over short distances. Rain in late spring and summer increases the risk of blight infection. In marginal soils where stress from drought, suffocation or fl ooding is greater, pathogens will become more damaging to tree health and productivity.

Recently, a shallow-bark bacterial canker caused by Brenneria nigrifl uens ( Hauben et al. , 1998 ) has been indentifi ed on J. regia that may be linked to abiotic stress factors ( Loreti et al. , 2006 ).

Drought

Juglans regia and J. nigra are usually inured to drought being deeply rooted species. However, very high summer temperatures may lead to early defoliation, while drought resistance var-ies among varieties or provenances ( Mapelli et al. , 1999 ). Permanently, dry locations should be avoided in the future, with hilly areas and moun-tainous valleys probably being more suitable for walnut cultivation than low altitude plains.


Juglans spp. are among the few tree species that are heterodichogamous ( Luza and Polito, 1988 ), where the division between male and female fl ow-ering is under genetic and climatic control. In com-bination with other environmental factors, and with high human pressure, these factors infl uence both the present and the future potential distribu-tion of walnut. Walnut has always been strongly affected by human selection that has both decreased allelic richness and favoured habitat fragmentation ( Malvolti et al. , 1997 ). Therefore, impact from human disturbance on walnuts across Europe may be less serious than for other forestry species, as it evidently thrives despite a scattered distribution.

Other factors

Walnut can play an important role in water management and soil protection. Juglans regia is capable of growing on unstable slopes where its strong widespread and deep roots protect the upper soil layers ( Ma and Xi, 1990 ).

Prunus avium – wild cherry

Range change

The range of wild cherry may shift north to north-eastwards as forecast for temperate broadleaves in general ( Harrison et al. , 2006 ; Thuiller et al. , 2006 ). There is likely to be increased competition from other species and probably fewer available suitable sites. However, wild cherry is a pioneer species, capable of quickly colonizing clearings by seed and suckers, and possesses high levels of genetic variation ( Russell, 2003 ). It is therefore likely that cherry will migrate successfully with any shift in climate space.

Pests and pathogens

Different impacts from insects are predicted ac-centuated by drought stress which may benefi t sap sucking and leaf-chewing insects ( Lieutier et al. , 2004 ).

Bacterial canker ( Pseudomonas syringae Van Hall, 1904) is a major cause of dieback in wild

FORESTRY10 of 21

cherry plantations ( Santi et al. , 2004 ) and in some European countries, cherry leaf spot ( Blumeriella jaapii (Rehm) Arx.) has been recognized as the most serious sanitary problem of wild cherry ( Santi et al. , 1998 ). Both diseases thrive in moist and cool conditions ( Eisensmith and Jones, 1981 ; Hirano and Upper, 2000 ), so climate change may generally reduce incidence. However, the pro-jected increase of atmospheric moisture content in northern latitudes ( Frei et al. , 2000 ) may ben-efi t the diseases in these areas.

Drought

Wild cherry cannot withstand frequent or severe droughts. In the south of its range, it could suffer from drought-related stress or mortality and in-creased susceptibility to pathogens. Elevated CO 2 does not improve drought tolerance of cherry seedlings under experimental conditions ( Centritto et al. , 1999 ).


Wild cherry has a high chilling requirement, and for germination, low temperatures are required. Sig-nifi cant exposure to higher temperatures induces secondary dormancy that negates the effects of any previous cold treatment ( Stephen et al. , 2004 ). Warmer winters could thus infl uence the success of sexual reproduction. Studies have found that for Prunus avium , fertilization can be ensured even at high temperatures, enabling a broader capacity to respond to environmental changes ( Hedhly et al. , 2003 ; Hedhly et al. , 2005 ). Wild cherry’s genetic diversity may be under threat due to its scattered distribution and rare occurrence, which may be exacerbated by climate change ( Russell, 2003 ).

Other factors

The timing of cherry fl owering has advanced signifi cantly in the last three decades ( Dose and Menzel, 2004 ), especially in central Europe ( Chmielewski and Rötzer, 2001 ). A prolonged growing season may increase tree growth.

Most Prunus species have a dual dormancy in-duction control system, securing timely growth cessation and dormancy induction in autumn.

This would make these species well adapted to avoiding the negative effects of global warming on winter bud development. An exception to this is P. avium , which maintains growth in short days at intermediate temperatures and stops growing only at low temperatures ( Heide, 2008 ). This behav-iour could affect normal winter bud development or cause delayed or erratic bud burst in spring.

Elevated CO 2 has been found to increase growth and water use effi ciency of cherry seedlings ( Centritto et al. , 1999 ) but did not change seed-ling response to water stress ( Centritto, 2005 ). In an experiment in Switzerland, the hypothesis that mature wild cherry trees would accumulate wood biomass faster in a CO 2 -enriched atmosphere was not supported; no increase in stem basal area in-crement was evident ( Asshoff et al. , 2006 ). Novak et al. (2003) reported that wild cherry was among the most severely injured species, as assessed by foliar damage, during peak high ozone episodes.

Sorbus torminalis and Sorbus domestica – service trees

Range change

As forecast for temperate broadleaves ( Harrison et al. , 2006 ; Thuiller et al. , 2006 ), the ranges of ser-vice trees ( Sorbus torminalis and Sorbus domestica ) may shift north to north-eastwards, but the process is likely to be limited or very slow. Both species may persist relatively well at higher altitudes in the south since they are drought tolerant. The northward spread is likely to be slow due to the present scat-tered distribution, poor sexual reproduction and/or seed dispersal ( Rasmussen and Kollmann, 2004b ), as well as low competitive ability in comparison with other temperate species. Due to the scarcity of both species, if natural dispersion is not assisted by humans, a reduction in range is likely. Local expan-sion on appropriate sites in response to the retreat of less drought-resistant species will result in a more densely populated range than at present.

Pests and pathogens

European canker of apple ( Nectria galligena Bres.) ( Kausch-Blecken von Schmeling, 2000 ) and apple scab ( Venturia inaequalis Cooke (Wint.)) are


important diseases that can cause premature leaf fall. Verticillium wilt cause wilting and dieback of branches of both service species ( Butin, 1989 ). Under changed climate conditions, these diseases may cause even greater damage. Service trees are important hosts of fi reblight ( Erwinia amylovora (Burrill)); in the Mediterranean region, the risks are more serious because of favourable climatic conditions for disease development and the exis-tence of wild hosts ( EPPO/OEPP, 2008 ). Warmer and wetter conditions could stimulate fi reblight.

Drought

Service trees perform best on sites with deep fertile soils with good water supply but can tolerate dry conditions more than many other species ( Kausch-Blecken von Schmeling, 1994 ) and fi nd their niche on dry to extremely dry sites. Droughts caused by climate change could reduce the abundance of highly competitive but less drought-resistant species such as beech ( Paganová, 2007a ). The result could be the formation of more favourable sites for the establishment of service trees. However, low water availability and high temperature reduces diameter growth ( Rasmussen, 2007 ), and severe and repeated droughts, more likely in the Mediterranean region, could lead to serious growth reduction, stress, in-creased susceptibility to pests and mortality.


The availability of new sites for colonization and establishment within the present range could be benefi cial for the preservation of genetic di-versity of service trees. However, they may later be at serious risk through habitat fragmentation which could induce a decrease of genetic diversity through reduction of population size and disrup-tion of gene fl ow ( Rotach, 2003 ; Demesure-Musch and Oddou-Muratorio, 2004 ). This would be the case if severe droughts affect populations re-peatedly, and this is more likely to happen in the southern distribution limit of the species.

Other factors

The seed dispersal of wild service trees at the northern distribution limit may be ineffi cient be-

cause of late ripening of fruits and detrimental effects of seed predators ( Snow and Snow, 1988 ) or because most migrating frugivore passerines leave the country before the fruits ripen ( Rasmussen and Kollmann, 2004a ). The effi ciency of seed dispersal may improve with higher temperatures and longer growing seasons. However, at the start of the growing season, there is a higher risk of negative climate impact (sudden fall of tem-peratures) on development and quality of the seeds ( Paganová, 2007b ).

Tilia cordata Miller and Tilia platyphyllos Scop. – limes

Range change

Tilia cordata is a warmth-demanding forest tree. The warmer climate that prevailed in Europe between 6000 BC and 500 AD favoured Tilia species in northern Europe ( Huntley and Birks, 1983 ). Historically , it is much more common and its decline is probably due to selective removal by man ( Pigott, 1991 ). General climate conditions and human impact have been a serious threat to the distribution of Tilia in most European coun-tries ( Jensen and Canger, 2006 ). In some regions, the forest area was reduced to 2 – 4 per cent by 1800. Another factor is the competitive effect of beech, which gradually invaded Europe after the last glacial period and has become dominant in central Europe over the last 2000 years. With warmer summers, its range may shift northwards. However, Tilia species are likely to become more prolifi c throughout their range due to increased natural regeneration.

Pests and pathogens

Phytophthora species have caused signifi -cant mortality of many trees, including Tilia spp. Infection is via free-swimming zoospores during wet periods, particularly when severe summer droughts are followed by late sum-mer rains. Phytophthora cinnamomi (Rands.) is most pathogenic at temperatures of 25°C and above and does not survive freezing condi-tions in the soil. Its present distribution is prob-ably constrained by both summer and winter

FORESTRY12 of 21

temperatures. Modelling with predicted moder-ate climatic warming to 2050 indicates that P. cinnamomi is likely to increase signifi cantly in the Mediterranean and Atlantic regions, includ-ing western Britain but not in central Europe ( Brasier, 1999 ).

Drought

Tilia cordata is one of the most drought-resistant species. The present day northern European latitude limit correlates with the July tempera-ture of 19 – 20°C. Its southern limit follows the isohyet of 500 – 550 mm mean annual precipita-tion, suggesting that drought rather than high temperatures determines this boundary. This is certainly true in the Mediterranean region where most individual trees occur over 200 m altitude on north facing slopes ( Tüxen and Oberdorfer, 1958 ). In the Pyrenees, T. cordata is common but also restricted to slopes with a northerly aspect.


Tilia species fl ower from June until August, de-pending on latitude, and are outcrossing. Op-timal fertilization temperatures range from 15 to 22°C ( Pigott and Huntley, 1981 ) but mast years are infrequent. Temperatures in north-ern England are commonly too low to permit successful fertilization ( Pigott, 1988 ) due to the pollen tube being unable to fully elongate. With the predicted increase in temperature, it is therefore likely that Tilia regeneration will be-come more common, especially at the present northern limits of its range. Due to the selective removal of Tilia sp. by man, genetic erosion is highly likely.

Other factors

Dormant shoots of T. cordata withstand winter temperature down to − 34°C and Tilia platyphyl-los down to − 25°C ( Till, 1956 ). Tilia species are not troubled by spring or autumn frosts, as fl ush-ing is late and bud set early. In Russia, T. cordata tolerates a short frost-free period of only 105 days ( Pigott, 1991 ).

Ulmus glabra , Ulmus laevis and Ulmus minor – elms

Range change

With climate change, elms will probably ex-tend the northern range of their distribution. In southern Sweden, for the fi rst time since the early Holocene, Ulmus glabra has recently be-come established in subalpine birch forests, pos-sibly in response to climate warming of slightly less than 1°C over the past century and, in par-ticular, warmer winters since 1988 ( Kullman, 2003 ). Within the southern boreal zone in Cen-tral Russia, in gaps created by natural distur-bances, tall saplings of U. glabra have grown more quickly than those of spruce suggesting a decrease in canopy spruce and an increase in de-ciduous species in the near future ( Drobyshev, 2001 ).

Pests and pathogens

It seems likely that the future of European elms will be linked to Dutch elm disease (DED) ( Brasier, 2000 ). About 5000 years ago, a wide-spread and sudden elm decline was remarkably synchronous over wide regions of Europe. DED is widely regarded as the main cause of that elm decline ( Parker et al. , 2002 ). Climate change ( Cayless and Tipping, 2002 ), logging and for-est fi res ( Moe and Rackham, 1992 ; Magyari et al. , 2001 ) have also contributed to elm decline. Scolytus bark beetles are the main agents of the long-distance transmission of DED, intro-ducing the pathogen into healthy trees during adult feeding and breeding. A warming climate will probably extend the northern range of Scolytus spp. and infect elm trees in the cooler parts of Northern Europe that have to date es-caped DED ( Caulton et al. , 1998 ). Increasing air temperature and sunshine exposure will favour DED, possibly due to stomatal closure leading to restricted internal water movement, whereas downward spread of the pathogen in the xylem is still facilitated ( Sutherland et al. , 1997 ). However, high summer temperatures may inhibit fungal sporulation in the pupal chambers ( Faccoli and Battisti, 1997 ). Further-more, water stress after infection with DED will

GROWING SCATTERED BROADLEAVES IN A CHANGING CLIMATE: A REVIEW 13 of 21 T

able

3: S

umm

ary

of p

ossi

ble

clim

ate

chan

ge-r

elat

ed im

pact

s on

fac

tors

tha

t m

ay a

ffec

t sc

atte

red

broa

dlea

ved

tree

s in

Eur

ope

Spec

ies

Ran

ge c

hang

ePe

sts

and

path

ogen

sD

roug

ht

Rep

rodu

ctiv

e bi

olog

y an

d ge

neti

csC

omm

ents

Ace

r sp

p.

m

aple

s R

ange

incr

ease

like

ly

in

nor

th

Alt

itud

inal

ran

ge

sh

ift

upw

ards

R

ange

dec

reas

e lik

ely

in

so

uth

and

east

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ptos

trom

a co

rtic

ale

in

cide

nce

may

incr

ease

Incr

ease

d

susc

epti

bilit

y to

path

ogen

s w

ith

st

ress

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r ca

mpe

stri

s

and

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r pl

atan

oide

s

rela

tive

ly d

roug

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to

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nt

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eudo

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ough

t in

tole

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biti

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ding

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)

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len

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ease

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ty

pr

edic

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to u

nkno

wn

de

gree

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l ada

pted

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ula

spp.

birc

hes

Ran

ge in

crea

se li

kely

in

no

rth

for

both

spp

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ltit

udin

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dam

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and

fl ood

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tree

qua

lity

Frax

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spp

.

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ange

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nor

th

Pra

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us a

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lia

in

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ther

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se

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may

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et

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to f

rost

Jugl

ans

spp.

wal

nuts

Ran

ge in

crea

se

lik

ely

in n

orth

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crea

sed

stre

ss w

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in

crea

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ility

to A

nthr

acno

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p.,

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pest

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may

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usce

ptib

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to A

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nd

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anth

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as

Hig

h re

gene

rati

on

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ility

In

crea

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ucti

vity

Fr

ost

sens

itiv

ity

(w

hen

juve

nile

)H

igh

tem

pera

ture

may

lead

to

defo

liati

on

Q56

FORESTRY14 of 21

Spec

ies

Ran

ge c

hang

ePe

sts

and

path

ogen

sD

roug

ht

Rep

rodu

ctiv

e bi

olog

y an

d ge

neti

csC

omm

ents

Pru

nus

spp.

wild

che

rry

Ran

ge in

crea

se li

kely

in n

orth

and

eas

t Ps

eudo

mon

as s

yrin

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an

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lum

erie

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apii

may

dec

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here

war

mer

and

dri

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gae

and

B. j

aapi

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may

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ease

in t

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no

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Dro

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str

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will

incr

ease

impa

ct o

f sa

p su

ckin

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and

leaf

-che

win

g in

sect

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Into

lera

nt o

f fr

eque

nt

or

sev

ere

drou

ght

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iliza

tion

is

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cces

sful

eve

n at

high

tem

pera

ture

s H

igh

rege

nera

tion

abi

lity

Hig

h ch

illin

g re

quir

emen

t

to b

reak

see

d do

rman

cy

m

ay n

ot b

e m

etD

orm

ancy

cont

rol a

ffec

ted

lead

ing

to

del

ayed

win

ter

bud

se

t an

d sp

ring

bud

bur

st

Incr

ease

d pr

oduc

tivi

ty

Flow

erin

g is

pro

ne t

o

fros

t da

mag

e, w

hich

coul

d ne

gati

vely

aff

ect

bi

odiv

ersi

ty

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hly

susc

epti

ble

to

oz

one

dam

age

Sorb

us s

pp.

se

rvic

e tr

ees

Ran

ge in

crea

se li

kely

in n

orth

and

eas

tM

ay p

ersi

st a

t hi

gh

el

evat

ions

in s

outh

Incr

ease

s im

pact

fro

m

N

ectr

ia g

allig

ena ,

Ven

turi

a in

aequ

lais

and

Ver

tici

llium

sp.

wilt

Incr

ease

of

Erw

inia

amyl

ovor

a es

peci

ally

in

M

edit

erra

nean

reg

ion

Ver

y dr

ough

t

tole

rant

N

ew s

ite

avai

labi

lity

w

here

it m

ay

ou

tcom

pete

less

drou

ght-

tole

rant

spec

ies

Smal

l pop

ulat

ions

,

poor

sex

ual

re

prod

ucti

on a

nd

se

ed d

ispe

rsal

and

low

com

peti

tive

abi

lity

Thr

eats

fro

m h

abit

at

fr

agm

enta

tion

and

lack

of s

ite

avai

labi

lity

Tili

a sp

p. li

mes

Ran

ge in

crea

se

lik

ely

in n

orth

P

hyto

phth

era

cinn

amom

i

infe

ctio

n m

ay in

crea

se D

roug

ht r

esis

tant

In

crea

sed

tem

pera

ture

s

will

pro

mot

e hi

gher

fe

rtili

zati

on s

ucce

ss

Hig

hly

tole

rant

of

ch

ange

s to

fro

st t

imin

g G

reat

er a

bund

ance

wit

h

incr

ease

d te

mpe

ratu

res

Ulm

us s

pp. e

lms

Ran

ge in

crea

se

lik

ely

in n

orth

R

ange

of

mai

n ve

ctor

of D

utch

elm

dis

ease

(DE

D)

may

ext

end

no

rthw

ards

Tre

e st

ress

will

incr

ease

sus

cept

ibili

ty

to

DE

D P

olyg

onia

c-a

lbum

rang

e an

d ho

st e

xpan

sion

Dro

ught

may

incr

ease

mor

talit

y in

co

mbi

nati

on w

ith

DE

D

Ulm

us m

inor

is li

kely

to b

e re

stri

cted

to

suck

ers

and

sapl

ings

for

th

e in

defi n

ite

futu

re

Incr

ease

d te

mpe

ratu

re

fa

vour

s el

m o

ver

coni

fers

in

sou

ther

n bo

real

for

ests

Bol

d te

xt d

enot

es o

ppor

tuni

ties

and

pla

in t

ext

deno

tes

risk

s.

Tab

le 3

: Con

tinu

ed


increase disease symptoms on U. minor ( Solla and Gil, 2002 ).

In a changing environment, new diseases or pests may affect elms, while known ones may be-come more aggressive. For instance, the polypha-gous butterfl y Polygonia c-album L., which has the greatest observed range expansion of any butterfl y in Britain during current climate warm-ing, has altered its host plant preference from hop ( Humulus lupulus L.) to include other hosts, par-ticularly U. glabra ( Braschler and Hill, 2007 ).

Drought

No species-specifi c risk or opportunity has been identifi ed. The effect of the severe drought in the UK in 1976 was observed by Coultherd (1978) on established and recently planted elm trees; mortality increased in those trees already suffer-ing from DED.


The adaptive traits of U. glabra display large variation both among populations along climatic gradients and within populations. This may be an insurance against loss of genetic variation if U. glabra populations are increasingly destroyed by DED in the future ( Myking and Skroppa, 2007 ). A drought stress study demonstrated that there is substantial additive genetic variation within pop-ulations of Ulmus laevis which could serve as a genetic basis for adaptation to future changes in water availability ( Black-Samuelsson et al. , 2003 ). Drought, along with land drainage, reclamation schemes and the conversion of fl ood plains to agri-cultural systems, represents an immediate threat to U. laevis in the riparian forests of eastern Europe ( Collin et al. , 2000 ). Ulmus minor is well able to multiply by means of seed and root suckers and it is not threatened with extinction but it will proba-bly be restricted to saplings and shrub-like growth for the indefi nite future ( Gibbs et al. , 1994 ).

Conclusions

This review has revealed considerable uncertainty and therefore diffi culty in quantifying many spe-cifi c risks and/or impacts on scattered broadleaved

tree species. However, where specifi c evidence or modelling does exist, as reviewed in the species sections above, these are summarized in Table 3 . Caution should be exercised in attributing too much weight to the summary. It is extremely dif-fi cult to say whether an impact is high or low and to quantify these in a realistic sense. For any given species and factor, there is a high degree of un-certainty as, in most cases, no research has been undertaken to answer these questions. However, this review may provide a foundation for discus-sion among scientists and policy makers, and in so doing, it may provide an indication of future research priorities and policy directions.

The recent completion of an European Union-funded COST Action (E42) that considered the growing of valuable (scattered) broadleaved tree species, involving some 100 scientists and practi-tioners from 25 European countries ( Hemery et al. , 2008 ), concluded that there is a lack of knowl-edge about provenances and genetics of scattered broadleaves, especially concerning their reaction to climate change. Hemery et al. (2008) recom-mended that the knowledge base of existing seed orchards and clonal reproductive material should be communicated more widely. Scattered broad-leaved trees often are important characteristics in the landscape and valuable elements of biologi-cal diversity. When these are well managed, they may provide additional benefi ts to society and the environment. Scattered broadleaved tree species may enhance ecological, economic and social val-ues of our forests and thereby contribute to the sustainability of forestry .

Acknowledgements

This review was initiated within COST Action E42: growing valuable broadleaved tree species.

Confl ict of Interest Statement

None declared.

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Received 30 September 2009

Growing scattered broadleaved tree species in Europe in a changing climate: a review of risks and...

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