Volume 36, Issue 3 e13847
CONSERVATION PRACTICE AND POLICY
Open Access

Climate adaptation of biodiversity conservation in managed forest landscapes

Kristoffer Hylander

Corresponding Author

Kristoffer Hylander

Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden

Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden

Correspondence

Kristoffer Hylander, Department of Ecology, Environment and Plant Sciences, Stockholm University, 106 91 Stockholm, Sweden.

Email: [email protected]

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Caroline Greiser

Caroline Greiser

Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden

Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden

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Ditte M. Christiansen

Ditte M. Christiansen

Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden

Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden

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Irena A. Koelemeijer

Irena A. Koelemeijer

Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden

Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden

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First published: 08 October 2021
Citations: 14

Article impact statement: The resistance and transformation strategies should be used in parallel when implementing climate adaptation for biodiversity conservation.

Abstract

en

Conservation of biodiversity in managed forest landscapes needs to be complemented with new approaches given the threat from rapid climate change. Most frameworks for adaptation of biodiversity conservation to climate change include two major strategies. The first is the resistance strategy, which focuses on actions to increase the capacity of species and communities to resist change. The second is the transformation strategy and includes actions that ease the transformation of communities to a set of species that are well adapted to the novel environmental conditions. We suggest a number of concrete actions policy makers and managers can take. Under the resistance strategy, five tools are introduced, including: identifying and protecting forest climate refugia with cold-favored species; reducing the effects of drought by protecting the hydrological network; and actively removing competitors when they threaten cold-favored species. Under the transformation strategy, we suggest three tools, including: enhancing conditions for forest species favored by the new climate, but currently disfavored by forest management, by planting them at suitable sites outside their main range; and increasing connectivity across the landscape to enhance the expansion of warm-favored species to sites that have become suitable. Finally, we suggest applying a landscape perspective and simultaneously managing for both retreating and expanding species. The two different strategies (resistance and transformation) should be seen as complementary ways to maintain a rich biodiversity in future forest ecosystems.

Abstract

es

Adaptación Climática de la Conservación de la Biodiversidad en Paisajes Forestales Gestionados

Resumen

La conservación de la biodiversidad en los paisajes forestales gestionados necesita complementarse con estrategias nuevas debido a la amenaza del cambio climático acelerado. La mayoría de los marcos de trabajo para la adaptación de la conservación de la biodiversidad ante el cambio climático incluye dos estrategias principales. La primera es la estrategia de resistencia, la cual se enfoca en acciones para incrementar la capacidad de las especies y comunidades para resistir el cambio. La segunda es la estrategia de transformación e incluye acciones que facilitan la transformación de las comunidades a un conjunto de especies que están bien adaptadas a las nuevas condiciones ambientales. Sugerimos un número de acciones concretas que los gestores y los formuladores de políticas pueden tomar. Bajo la estrategia de resistencia, introducimos cinco herramientas, incluyendo: identificación y protección de los refugios climáticos forestales con especies favorecidas por el frío, reducción de los efectos de la sequía mediante la protección de la red hidrológica y extirpación activa de los competidores cuando amenacen a las especies favorecidas por el frío. Bajo la estrategia de transformación, sugerimos tres herramientas, incluyendo: mejorar las condiciones para las especies forestales favorecidas por el nuevo clima pero actualmente desfavorecidas por la gestión forestal, mediante su siembra en sitios adecuados fuera de su distribución principal e incrementando la conectividad en el paisaje para incrementar la expansión de las especies favorecidas por el calor hacia sitios que se han vuelto más adecuados. Finalmente, sugerimos aplicar una perspectiva de paisaje y gestionar simultáneamente tanto para las especies en retirada y en expansión. Las dos estrategias diferentes (resistencia y transformación) deberían considerarse como maneras complementarias para mantener una biodiversidad rica en los ecosistemas forestales del futuro.

INTRODUCTION

It is not straightforward to protect biodiversity in forest landscapes whose purpose is also to produce timber, wood pulp, and energy. Up until now, forest management and policy have been based on ecological frameworks, such as metapopulation ecology and natural disturbance dynamics (Hanski, 1999; Pickett & White, 1985). These frameworks, despite acknowledging fluctuations, assume the return to a state of equilibrium. However, unidirectional effect of climate change cannot be addressed with classic equilibrium thinking. Modern conservation approaches in forests need to incorporate the effects of climate change when developing policy and management recommendations (Mantyka-Pringle et al., 2012; Morelli et al., 2020). We sought, with a focus on hands-on actions, to inform politicians, authorities, policy makers, nongovernmental organizations, and forest managers about new research in climate adaptation of biodiversity conservation.

FOREST BIODIVERSITY IN A CHANGING CLIMATE

Forest species responses to climate change

Research on current and historical responses to climate change stresses that tracking of a suitable climate is necessary for a species' survival in the long run (Corlett & Westcot, 2013), even though evolutionary changes of populations as a response to changing conditions may happen simultaneously (De Kort et al., 2020). Many species shift their distributions toward higher latitudes or elevations when the climate warms. However, species react to the changes in an individualistic manner when tracking their climate niches, and interactions between different changing climate variables might cause nonintuitive responses (Lenoir et al., 2010; Rapacciulo et al., 2014; Weiskopf et al., 2020). Such examples include down-slope shifts to track wetter conditions as a response to drier climates, which means moving in a direction with higher temperatures, and point to the need for consideration of multiple climate variables (Crimmins et al., 2011; Harsch & HilleRisLambers, 2016).

The climate that forest understory species experience is different from the climate above the trees or in a nearby field. The canopy buffers temperature extremes such that temperature variation is lower inside than outside the forest (De Frenne et al., 2019). On top of variation in canopy cover, heterogeneous topography, and differences in soil moisture, forest understory species can experience microclimates that vary over very short distances (Greiser et al., 2018; De Frenne et al., 2021). To understand how species respond to climate change, one needs to understand how the microclimate is changing. Such changes depend on the combined effects of macroclimate change and how forest structure and composition change (Zellweger et al., 2020).

Climate refugia and time lags

At each latitude, some species are favored by climate change, whereas others are disfavored (Figure 1). In continuous landscapes, species often occur more sporadically and in habitats of higher quality toward their range margins (Gaston, 2009; Canham & Thomas, 2010) (Figure 1). For example, in the western United States, trees with a northern distribution occur more frequently on cold north-facing slopes in the southern part of their range (Ackerly et al., 2020), illustrating that species may compensate for the regional climate by shifting across topoclimatic gradients. Sites with surviving populations at the contracting rear edge are often called refugia or microrefugia (Rull, 2009). Such places can be important as nuclei for recolonizations of areas from which a species has been extirpated, as shown, for example, by the recolonization of trees from refugia when much land opened up for colonization in northern Europe after the last glaciation (Rull, 2009). Under current climate change, sites hosting populations of species at their warm-range margin might function as climate refugia if they are protected (Morelli et al., 2020). However, the more the climate changes, the less likely these sites will function as nuclei for recolonization (Hannah et al., 2014; Keppel & Wardell-Johnson, 2015). Also, in heterogeneous landscapes, cold and warm places with their respective species pool are relatively close to each other, which increases the risk of cold-favored species being outcompeted by neighboring and quickly invading warm-favored species (Ackerly et al., 2020). Protecting refugia may limit immediate population decline in response to regional climate warming and provide time for the species to disperse or adapt (Morelli et al., 2020).

Details are in the caption following the image
Simplified framework showing how species with different climate niche optimum are distributed along latitude (left panel) and how that may play out across a particular place (right panel). This is exemplified for the northern hemisphere (north is toward the upper parts of the figure). This framework does not consider that species respond individualistically to multiple climate-change variables or that species are affected by biotic interactions, such as competition

Time lags between changed climatic conditions and visible effects in communities are common and are a function of the speed of climate change and the dispersal and persistence capacity of a species (Loarie et al., 2009; Svenning & Sandel, 2013). Human land use can modify all the abovementioned factors, for example, reducing the speed of range shifts due to lack of connectivity in fragmented landscapes (Mantyka-Pringle et al., 2012; Corlett & Westcott, 2013). If species cannot expand to new areas with suitable conditions, their overall distributions will shrink.

Extreme events and interacting stressors

Although research has been focused on changes of mean temperature values, it is often not changes in mean values that cause the most drastic changes in biodiversity, but rather short- and long-term climatic extremes (Körner & Hiltbrunner, 2018; Maxwell et al., 2019). The exceptionally warm and dry summer of 2018 in Europe illustrates the biological consequences of uncommon weather events (Peters et al., 2020). The drought caused large forest fires and was a primary cause of outbreaks of spruce bark beetle (Ips typographus), which benefitted from both the warm weather (increased population size) and the stressed trees (higher susceptibility). The end result was widespread dieback of spruces in Europe (Obladen et al., 2021). Large-scale changes in tree densities and subsequently in tree species composition significantly affect forest microclimates and biodiversity (Zellweger et al., 2020). It will be the combined effects of forest management, natural disturbances, and all aspects of climate change, including increasing severity and frequency of extreme events and their interactions and feedbacks, that will determine the availability of suitable microclimates that species can survive in or track.

Economic value, climate change, and biodiversity

Forest managers also need to address climate impacts on tree growth and forest productivity and consider carbon storage an economic value in managed forests. Currently, rotation forestry is the dominant management form in European boreal forests (Gauthier et al., 2015). It creates a patchwork of mostly even-aged and single-species stands that are eventually clearcut and then replanted. As an alternative, continuous-cover forestry practices have been suggested, which are often based on selective cutting and can create mixed-aged and mixed-species stands. The additional challenges for forestry arising from climate change produce not only trade-offs (e.g., between natural wildfire disturbances and timber production), but also opportunities (e.g., a shift from rotation forestry to continuous-cover forestry supports carbon storage, sequestration, and biodiversity conservation). The management of trees, deadwood, and soil will be critical to biodiversity in forest ecosystems. Sustainable forest management should be holistic and incorporate the fact that only a functioning forest with a rich biodiversity will be able to produce goods and sustain services in the long run (Bradshaw et al., 2009).

Frameworks for adaptation of biodiversity conservation

There are various ways of categorizing adaptation approaches (e.g., Millar et al., 2007; Fisichelli et al., 2016; Aplet & McKinley, 2017; Schuurman et al., 2020). The most common way is to divide them into two major strategies: manage for resistance of the ecosystem to climate change and transform the landscape so the ecosystem can adapt to the new climate. Through the lens of species conservation, Graae et al. (2018) call these strategies, respectively, “stay” (i.e., resistance) and “go” (i.e., transformation).

Identifying and protecting climate refugia (Morelli et al., 2020) and protecting areas with high biodiversity value are typical tools under the resistance strategy. Much research has gone into models to select representative areas across a managed landscape that, if protected, would cover the climate requirements of many species (Hanson et al., 2020). When protected areas are very large, species may be able to track their climate niche within the protected areas (Stralberg et al., 2020). Otherwise, easing species’ movements by increasing connectivity across managed landscapes is a typical tool of the transformation strategy (Jennings et al., 2020), as well as active intervention to accelerate a shift in forest type by, for example, introducing species beyond their distribution limit (Oliver et al., 2012; Gray et al., 2016). Theoretical frameworks of climate adaptation for biodiversity conservation are well developed, but empirical evidence for them is still scarce and few of the suggested tools have been tested in real landscapes (Prober et al., 2019; Clifford et al., 2020). We devised suggestions on possible actions for biodiversity conservation under climate change in managed and mosaic forest landscapes. Some of our suggestions may best apply to the Fennoscandian boreal forests, but we believe many are generalizable. We did not consider policy development, such as society's adaptive capacity, socioeconomic feedbacks, or stakeholder collaboration (Burch et al., 2014; Petersen et al., 2018). Instead, we developed concepts and suggest direct actions that could sustain forest species and help them adapt to climate change. We focused on managed landscapes, where there are more opportunities for direct conservation actions, such as planting or removing certain tree species.

We considered resistance and transformation strategies (Figure 2) and how to combine the strategies at a landscape scale (Figure 3). The identified tools should be considered alongside conventional conservation tools when modifying environmental goals, legislation, policy, and management recommendations to address climate change. Because the changing climate has unpredictable effects on biodiversity, we envision that our suggested approaches will need to be constantly adapted and modified.

Details are in the caption following the image
Eight climate adaptation tools for biodiversity conservation in the face of climate change grouped under two principal strategies: resistance and transformation. Under each strategy, several different tools can be used, depending on circumstances (see Figure 3 for an application of some of them in a real landscape)
Details are in the caption following the image
An example of the different tools (resistance [blue] and transformation [orange]) for climate adaptation of biodiversity conservation as applied to different parts of the landscape to reduce negative effects of climate change on biodiversity while taking into consideration the inevitable changes. This example shows how the tools could be planned and implemented simultaneously at different scales and in different parts of a managed landscape

RESISTANCE-STRATEGY TOOLS

Identifying current and future climate refugia

Given unidirectional climate change, populations at the warm edge of the species’ range may occur in places that are colder than surrounding patches (Greiser et al., 2020); such places can act as climate refugia. In heterogeneous landscapes and for stable, isolated populations, identifying and protecting climate refugia can be critical to slowing the adverse effects of climate change on biodiversity (Morelli, 2020). It is not always known which places act as climate refugia; therefore, one needs to use proxies. One option is to locate relatively cool sites in the landscape by mapping well-known microclimate forcing factors (e.g., forest density, soil moisture, and topographic solar radiation index) (e.g., Greiser et al., 2020). Such maps together with other information of conservation values could be used to prioritize protection of areas with suitable microclimates (Barrows et al., 2020).

Protecting cold microclimates can also be used in the core distribution of a species, assuming that colder sites will act as climate refugia in the future when the climate is even warmer. Special effort should be put into identifying and protecting climate refugia in forested landscapes with a somewhat rugged terrain because terrain and vegetation in combination have the potential to create particularly cold and stable microclimates (Greiser et al., 2018). Species inhabiting topographically heterogeneous landscapes can also have large genetic variation, which promotes variability in responses and improves chances of adaptation (De Kort et al., 2020). Heterogeneous terrain may also support a wide range of species (e.g., due to microclimatic buffering [Suggitt et al., 2018]) that could enhance the resilience of the forest ecosystem functions due to an interspecific variation in response to climate change (Folke et al., 2004).

Reducing effects of drought and fire

A generally warmer climate also implies an increased frequency of extreme temperatures and thereby increased evapotranspiration. Although predictions differ regarding future precipitation, it is in many regions advisable to plan for scenarios with increasing frequencies of droughts and heatwaves and precipitation events that are less evenly distributed (IPCC, 2021). Soil moisture can buffer declines in air humidity in forests. Moister sites also have higher evapotranspiration rates and can, therefore, cool the understory more efficiently (Davis et al., 2019). Thus, an important preventive activity likely to increase the resistance of forest biodiversity is to protect the hydrological network of the landscape, for example, by restoring ditched wetlands and channelized streams (McLaughlin et al., 2017).

With a warmer and drier climate, the frequency, size, and intensity of forest fires will increase. Identifying and protecting potential fire refugia, as well as actions to limit the size and intensity of fires (Stephens et al., 2013, 2014), could reduce the effects of such events on biodiversity (Krawchuk et al., 2020). Disturbances other than fire, such as insect outbreaks, could increase with drought and heat waves (Gely et al., 2020). However, for insect outbreaks, activities other than those associated with hydrology, such as growing mixed stands instead of monocultures, are needed (Jactel et al., 2017; Krawchuk et al., 2020).

Buffer zones

When valuable microclimates are identified, it is crucial to determine whether there are risks of negative edge effects because sunlight and wind penetrate from edges (e.g., at clear-cuts, Hylander, 2005). Easy but important actions are to maintain dense forest stands as protective shields outside the core protected areas of climate refugia, both at sunny equator-facing edges and at edges facing prevailing winds. The necessary width of these forest buffers depends on several factors. The depth of microclimatic edge influences has been estimated to range from 12 to 100 m, but can potentially penetrate up to 240 m into the forest, depending on biome, edge type, and microclimate factors (temperature and humidity) (Chen et al., 1995; Pohlman et al., 2009).

Alternative management regimes for cold-favored species

Because forest microclimates are buffered from the ambient climate, at fresh clearcuts, there are many species that are sensitive to the drastic change in microclimate. As forest microclimate buffering increases as regional temperatures increase (De Frenne et al., 2019), the environment for many species after clearcutting will become even more hostile in a warmer climate. One way to increase the survival rate of many species through the regeneration phase of a forest management cycle is to use continuous-cover forestry (Atlegrim & Sjöberg, 2004). Yet, there are also forest species that need canopy disturbances and sun exposure. It is not known whether these species are also negatively affected by increasingly hot conditions under climate warming. If they are, this could lead to a trade-off in managing for a species optimal habitat and climate buffering. It would be interesting to know whether, for example, species that rely on exposed dead wood after a forest fire can tolerate a warmer climate differently depending on whether the burnt area is on the poleward (cooler) or equatorward (warmer) side of a mountain.

Active management to combat warm-favored antagonists

Because it can be biotic interactions, such as competition or herbivory, rather than physiological constraints that set species’ warm-range edges, combatting antagonists, such as competitors or herbivores, could be a successful way of implementing the resistance strategy (Ettinger & HilleRisLAmbers, 2013; Greiser et al., 2021, but see Sirén & Morelli, 2020). Even though we do not know of any examples in forests, there are examples from alpine environments, where plants grow at lower elevations than they would if there were no active removal of large competitors by grazing and fire (Johansson et al., 2018). Similarly, there are examples of elevated tree lines after reduction of grazing of alpine grasslands (Speed et al., 2012). Under certain conditions, expanding species can become invasive and threaten local biodiversity. In such situations, rapid action to combat further spreading is important.

TRANSFORMATION-STRATEGY TOOLS

Promoting warm-favored tree species and genotypes

Species at their poleward range margin are likely favored by a warmer climate, which is why they may initially be overlooked in conservation efforts. Although they might be favored by a warming climate, they still might be disfavored by current forest management and even protected-area management. If, for example, naturally regenerated saplings of a warm-favored tree species are removed during stand development to favor the planted tree species, range expansion of the warm-favored species is hindered. To meet the demands of such species, as well as their associated biodiversity, it is important to consider climate change in restoration activities (inside and outside protected areas). It could be as simple as not clearing these species when they are naturally regenerating or actively planting them (Oliver et al., 2012). Managed forests provide a great opportunity for replanting after harvesting with other species or genotypes that are better adapted to new and changing climatic conditions (Gray et al., 2016; Schreiber et al., 2013; Torssonen et al., 2015).

Eventually, mixed forest stands are more resilient to climate change from a production perspective and they provide multiple ecosystem services because they promote higher overall biodiversity relative to single-species stands (Gao et al., 2014).

Alternative management regimes for warm-favored species

With a shifting climate and new dominant tree species, it is natural to consider alternative management regimes as a tool of the transformation strategy. Imagine that the previously dominant species has a regeneration niche adapted to large-scale disturbances, such as fire, but the new dominant species and its associated biodiversity are adapted to small-scale internal disturbances, such as gap dynamics. If so, a critical climate-adaptation tool would be to apply selective cutting rather than clearcutting.

Sometimes, climate change can even cause a shift from forest to shrubland or even grassland systems (Stenberg, 2001). In cases of such regime shifts, it might be necessary to totally rethink the conservation strategy and apply appropriate management for biodiversity in those systems (Henderson et al., 2016).

Connectivity

Functional connectivity across landscapes is important for species in a stable climate because many species benefit from genetically and demographically connected populations. Moreover, most species have patchy distributions and need to be able to recolonize after local extirpations to maintain stable metapopulations. Such recolonizations may need to be more frequent if local extirpations due to extreme events become more common. Thus, especially in the face of climate change, there is an urgent need to increase habitat connectivity in landscapes fragmented by human land use. Connecting patches of old and new habitat would also support advancing warm-favored species with high conservation value (Jennings et al., 2020). Here, the same microclimate heterogeneity that can create potential climate refugia at a species’ warm-range margin (e.g., relatively cold microclimates) can also create potential stepping-stone habitats at a species’ cold-range margin (e.g., relatively warm microclimates; Hannah et al., 2014; Saura et al., 2014; Lembrechts et al., 2018).

COMBINING RESISTANCE AND TRANSFORMATION STRATEGIES AT A LANDSCAPE SCALE

Adaptation of biodiversity conservation in a changing climate needs to take a landscape perspective. We suggest working with tools from both the resistance and transformation strategies to slow adverse effects on cold-favored species and support expanding warm-favored species (Figure 3). Suitability of the strategies differs among landscape types, but often strategies can be combined in the same landscape (Stralberg et al., 2020). Some tools can be implemented at small scales, such as buffer zones around climate refugia, whereas others are by nature landscape tools, such as increasing connectivity. The protection of small and large areas has been a major tool in biodiversity conservation and will also play a critical role in a changing climate. If the protected areas are large and topographically diverse, they might also in the future encompass suitable microclimatic conditions for at least some of the species (see “Identifying Current and Future Climate Refugia”) (Hanson et al., 2020; Stralberg et al., 2020). However, climate will continue to change, and changes in species composition must be accepted even in protected areas. Yet, protected areas often have quantities and qualities of substrates that are rare in managed stands, which will be equally important under new climate conditions, but perhaps for different species. For example, it is likely that certain species relying on dead wood might be locally extirpated, and other species will colonize the dead wood. Thus, the dead wood still acts as an important substrate supporting biodiversity. If the dead wood is long-lived, it could even be the same particular snag or log that continues to play this role but with a changed associated species composition. Some management actions might be appropriate to speed up the transformation of protected areas (see “Promoting Warm-Favored Tree Species and Genotypes”).

We suggest that the implementation of both the resistance and transformation strategies is a way to enhance the resilience of the forests (i.e., their capacity to remain functional ecosystems despite a changing climate). We suggest that adaptation should target the functions driven by common species, species richness, and rare and threatened components of warm- and cold-favored biodiversity. Finally, implementing many different adaptation tools can be an effective way of dealing with the uncertainties that come with environmental change (Clifford et al., 2020).

ACKNOWLEDGMENTS

This study was supported by funding from Formas (grants 2014–530 and 2018–2829 to K.H.) and the Bolin Centre for Climate research, Stockholm University (to K.H.).