Paul Anderson, Pacific Northwest Research Station; Brian Palik, Northern Research Station
An archived version of this topic paper is available.
There is a growing consensus that management decisions need to consider how actions either enhance or detract from a forest's potential to adapt to changing climate. Uncertainty regarding the specifics of future climate conditions increases this need.
Silvicultural planning needs to embrace managing forests for adaptation to new conditions by promoting the resistance of a forest to change, resilience of a forest in the face of change, and response options that facilitate the transition of forests to new conditions (1). This may involve actions that restore or sustain compositional, structural, and functional diversity in stands. This diversified investment portfolio concept applied to forests, provides more management flexibility and capacity for forests to adapt to changing environmental conditions and societal values. Managing for adaptability is applicable to all uncertainties associated with forests, not only climate change.
Silvicultural planning considers factors that influence a forest stand's potential response to manipulation, including the structure of the ecosystem (2), current and potential range of variation in stand composition, history of disturbance or disturbance suppression, and stand development dynamics over time. It considers habitat suitability for threatened or endangered species that need to be sustained and exotic invasive species that must be discouraged. The landscape context of a stand, how its current and desired composition and structure compare to other stands in the surrounding landscape, informs how to meet broader landscape management objectives. Climate change warrants additional silvicultural considerations such as future habitat suitability for tree species currently comprising the stand, or for those species desired for the future.
The silviculturist must also consider how threats may increase under a changing climate. Warmer, drier growing seasons may reduce fuel moisture levels and increase risk of catastrophic wildfire. Milder winter temperatures and longer growing seasons may increase the risk of attack from insect and disease pests. Sustained climatic stress can increase the threat to overcrowded, older-aged forests predisposed to insect epidemics(3), as may be evidenced by the recent devastating impact of mountain pine beetle in western North America.
Societal expectations for ecosystem goods and services from forests may change little in the face of climate change. Silviculturists may be challenged to develop prescriptions that enhance adaptability to climate change, while still providing desired or expected ecosystem goods and services, such as merchantable wood, game species, native plants and animals, and forest composition and structure.
Although climate is changing globally, changes in temperature, precipitation, and atmospheric composition will vary over time and among continents, among regions, and locally. Climate changes will modify the environment and cause disturbances affecting forest communities. If these modifications override the adaptive capacity of the forest ecosystem, these forests and the goods and services they provide are vulnerable.
Silvicultural approaches to climate adaptation will be effective when they focus on stress factors that pose the greatest risks to forests. An awareness of how environmental stresses result in altered tree vigor and stand dynamics is critical to understanding these risks, including:
- Which physiological and developmental processes are most sensitive to a particular stress or suite of stressors?
- How do changes in these sensitive processes affect the survival, growth and productivity of individual trees and stands?
- At what temporal and spatial scales do stressors act and forests respond?
- What are the consequences for various goods and services expected from forests?
With climate change, an objective for silviculture is to manage the composition and structure of stands and landscapes to alleviate climate-related stresses and to enhance forest capacity to resist, tolerate and adapt to a dynamic environment. When and where silvicultural adaptation strategies are employed will be influenced by overarching management objectives, perceived risks, and the confidence that intervention will be effective, given various ecological, economic or social criteria.
Density management based on characteristics explicitly related to site resource demand may be an effective means to mitigate climate-related stressors. Silviculturists have long recognized the value of thinning and other forms of vegetation manipulation to maintain a desired balance between site resource availability and utilization. Conventional thinning commonly targets stand productivity and focuses on allocating site resources from a large number of smaller, less desirable trees to a smaller number of larger, more desirable trees. Thinnings may be repeated over time to maintain stand densities at levels that sustain cumulative productivity and preclude periods of low stand vigor. Density management can be practiced to achieve not only a balance of site resource availability and demand, but also to modify species composition and other environmental and structural features that influence climate-related stresses. For example, thinning may target the release or recruitment of species that provide diversity of adaptation traits. Recent studies are beginning to demonstrate the usefulness of variable density thinning to achieve more varied plant communities that provide a broader array of habitats and potentially greater biodiversity (4).
Changes in stand structure also may alter local environmental conditions that influence biotic and abiotic disturbance agents. Maintaining lower tree density can increase wind speeds within a canopy, making controlled flight difficult for some bark beetles (5). Thinning may decrease relative humidity, creating conditions less favorable to some pathogenic fungi, but potentially promoting infection by others such as white pine blister rust (6). Removing shrubs and other ground vegetation may decrease site resource demand, as well as reduce the risk of severe wildfire by decreasing the abundance and depth of fuels. It may be important to couple density management with understory vegetation control, so gains in site moisture balance are not negated by understory growth (7). Density management also may substantially decrease risks to individual tree and stand vigor (5, 8).
Restoring component species
Species composition has been altered in many forested areas by management and change in disturbance regimes, so that some species well-adapted to the historic range of variation have been diminished, while other poorly-adapted species have been added. Climate change may take decades to generate discernible effects. For the near-term, those plant communities best adapted to transitional climatic extremes will be comprised of the species and populations that evolved on site. Restoring species that have been lost due to land-use, management practices, or exclusion by invasive species is a reasonable objective for silvicultural intervention. The time to restore community composition is before substantial changes in climate occur, while there is still a relatively strong match between current site conditions and the adaptive potential of the species being restored. Approaches to restore target species include retention during thinning or other vegetation removal operations, removing competing or inhibiting invasive and non-native species, or active regeneration by planting or seeding.
Favoring adaptable species and genotypes
Promoting resistant and resilient forest communities includes favoring those species and genotypes that are adaptable to projected environmental changes. Adaptive characteristics that vary along climatic gradients, and are therefore likely to be of importance, include traits that permit a plant to survive and function when subjected to water deficits, temperature extremes or uncharacteristic disturbance (9). Drought stress is an important contributor to seedling mortality in many ecosystems and can be a limiting factor to successful reforestation (10). If fires become more severe, those species that have thick, insulating bark or that regenerate by sprouting from below-ground root systems may be more resistant and resilient.
A major issue is the degree to which current seasonal patterns of growth and development will remain synchronized in the face of rapidly changing and increasingly variable weather and climate patterns. For example, if pollen dispersal occurs out of synchrony with flower receptivity, then decreased seed production may limit natural regeneration for seed-regenerating species.
The limitation of this strategy is that capacity for adaptation has evolved in response to historical pressures, and may not be sufficient for dealing with projected future conditions. There may not be enough time for new adaptations to evolve in place, given the rapid changes in climate we are experiencing (9).
Adding new species and genotypes
An active approach to facilitate adaptation may be intentionally moving species or genotypes to match known adaptive characteristics with locations where these traits may be beneficial in the future environment (11). This "assisted migration" can be practiced with varying intensity and risk. Initially, movements of species or genotypes can be limited to relatively short "ecological" distances along a climatic gradient and focused on the transition zones from one ecotype to another. Caution must be used, because in the near term, some transferred sources may not be as well adapted to the current environment as local sources.
A more subtle approach to building resilience may be planting or sowing a greater variety of species and genotypes when reforesting after a harvest or natural disturbance event. The premise is the same - expand the gene pool, and therefore the probability of having adapted individuals on a site. Regeneration harvests and stand-replacing disturbances may be opportunities to enhance the adaptive capacity of the regenerated forest.
Silviculture can also be used to decrease some threats to vulnerable forest stands and landscapes. Biotic threats include some insects and diseases, pathogen vectors, and invasive plants or animals. Physical threats include fire ignition sources such as lightning strikes, windstorms, flooding or landslides. Silvicultural approaches to biotic threats include treatment of incipient infestation centers through "sanitation" harvests, chipping or burning excessive down wood accumulations from harvest or disturbance events, and integrated pest management for invasive plants and animals.. Silvicultural regimes can minimize slope destabilization and moderate runoff to mitigate potential landslides and flooding.
Effective silvicultural regimes address site specific issues in the broader temporal and landscape contexts. The treatment of threats across a landscape can be influenced by the spatial and temporal application of stand-level treatments. For example, fuels management is integral to restoration of fire resilience in some western forests. For fuels reduction to be effective, at least 20-30% of a landscape needs treatment in designed spatial patterns, with retreatment occurring after 15-20 years; random spatial application requires approximately twice the area treated to get the same effect (12). These larger-scale contexts are useful to prioritizing site and stand level actions to reduce threats.
Silviculture for Climate Change: Generalities Common to all Regions
Vulnerabilities differ by forest type and stage of development.
Individual trees and species differ in their degree of adaptability to any given suite of stresses. Capacity to cope with a stress depends on the physiology of individuals and how individuals interact as a forest community. For the individual, changes in size and maturation can influence resource demand, acquisition and storage, and the ability to buffer environmental challenges. Stage of stand development will influence the degree of inter-tree competition and the susceptibility of forest stands to various disturbances.
Interactions between climate and other biological and physical stressors will be important in determining forest response to climate change.
Forest ecosystems are complex. Stresses imposed by climate that decrease tree and stand vigor will often result in increased damage by secondary stress agents. Climate may also directly influence the abundance and voracity of pests, as well as promote physical disturbances such as fire.
Opportunities exist for vegetation management to enhance balance between site occupancy and resource availability.
Silviculture aims to manipulate the composition and structure of forests to meet an array of management objectives. If sustaining vigorous forests is a primary challenge imposed by a changing climate, then silvicultural activities that maintain a balance between the supply and demand for site resources or that mitigate local environmental stresses will have an important adaptation role.
Restoring composition and structure now will enhance adaptation capabilities for the future.
Adaptation to a changing climate will be facilitated by starting with communities that are diverse and resistant and resilient to the range of environmental conditions historically encountered. Most managed ecosystems are simplified in composition relative to their un-managed, reference condition. A near-term strategy is to restore a portion of the landscape to native plant community composition and structure within the natural range of variation.
Silvicultural treatments at a stand scale are most effective when conceived and applied in a landscape context.
Climate-related stresses occur at stand and landscape scales. To have a major adaptation impact, silviculture must be practiced strategically to best target threats and responses that occur at multiple spatial and temporal scales. The opportunity to do everything needed, everywhere, all of the time, is rare. Silviculturists must understand how vulnerabilities and threats operate at multiple scales in order to be most effective in using limited adaptation resources.