Silviculture

Preparers

Paul Anderson, Pacific Northwest Research Station; Brian Palik, Northern Research Station

An archived version of this topic paper is available.

Issues

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.

Likely Changes

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.

Management Options

Density management

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).

Managing composition

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.

Reducing threats

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.

Regional examples of silvicultural adaptation strategies: Western Great Lakes Mixed-Pine Ecosystem

Preparers

Paul Anderson, Pacific Northwest Research Station; Brian Palik, Northern Research Station

Potential Vulnerabilities

The Great Lakes mixed pine ecosystem occurs on sand to loamy sand soils derived from glacial outwash and ice-contact topography (1). The soil moisture regime is dry-mesic to xeric and as such, a decrease in growing season soil moisture under a warmer, drier climate could negatively impact the system in several ways. Higher growing season temperatures, combined with reduced summer precipitation, will result in greater plant water deficits, if evapotransporation demand exceeds moisture availability more frequently and for longer duration. Reduced photosynthetic capacity, growth, and tree vigor are likely consequences. Increasing atmospheric CO₂ concentration may increase growth in the short term, but the long-term response is not clear at present. For example, increasing ground-level ozone concentrations, which can damage forest trees, as well as soil moisture limitations, may offset the positive effect of CO₂ fertilization on growth.

Odocoileus virginianus) is a significant problem in many parts of the region (2). In Minnesota, for example, it is virtually impossible to regenerate eastern white pine, and often jack and red pines, without browse protection of some type (3). It is difficult to predict how white tail deer will respond to a climate that is expected to be warmer, with potentially deeper snowpack in winter.

Dryer conditions during the growing season may result in reduced moisture of woody fuels more frequently in space and time, with a concurrent increased risk of wildfire. While the Great Lakes mixed-pine ecosystem is fire dependent, fuels build up due to long-term fire suppression poses a significant risk of catastrophic wildfire in many locations. This may be particularly true if fuels are drier. Soil moisture deficits during the growing season will also exacerbate the potential for wildfire. More frequent heavy rainstorms accompanied by high winds may result in increased blowdown, particularly of older pines, as both red and eastern white pine can reach super-dominant crown positions, with high wind exposure.

Warmer, drier growing season conditions, along with milder winter temperatures, may shift competitive advantage towards more generalist species, and those currently at the northern edge of their distribution in the region. For instance, habitat suitability for several currently minor oak (Quercus) species is projected in increase (4), as is that for species that currently do not occur in the region, such as eastern red cedar (Juniperus virginiana). In contrast, predictions suggests a moderate reduction in habitat suitability for eastern white pine and substantial reductions for red pine throughout much of the region, with both low and high emission scenarios (4). Other species that occur in the mixed-pine ecosystem, including trembling aspen, paper birch and balsam fir, are projected to have large declines in habitat suitability as well.

Potential compositional changes may lead to long-term effects on fuels and fire risk that are complex and unpredictable. If CO₂ fertilization and milder temperatures increase productivity, fuel loadings may increase and potentially fire risk along with this. Alternatively, drier growing season conditions could reduce productivity and thus the standing crop of fuels. Additionally, shifts in composition to species producing less flammable fine fuels, such as maples, could lead to decreased fire risk.

Insect and disease pests of red and eastern white pine may respond to climate change in ways that increase tree mortality. Changes in pest populations could result directly from changes in climatic factors. For example more frequent drought could increase bark beetle outbreaks, a potentially serious threat to eastern white and red pines. Climate change may result in more complex interaction with other environmental changes. For example, populations of Ips bark beetles are greater on red pine with bole scorch from surface fire, resulting in increased mortality (5). If fire is more frequent with climate change, bark beetle-induced loss of vigor and mortality may increase as well.

Management Options

Several of the general strategies outlined in ‘Silviculture for climate change’ may have relevance for managing Great Lakes mixed pine ecosystems in the face of climate change. These strategies can be broadly segregated into resistance, resilience, and response or facilitation strategies (6), although there is some ambiguity in assigning specific approaches to a broad strategy.

Resistance

Resistance strategies may include activities that, at least in the short-term, greatly increase regeneration of species that are expected to have reduced habitat suitability in the future, such as red pine and eastern white pine. Increased use of prescribed surface fire to create appropriate seedbeds and reduce competing understory vegetation may be one such strategy.

Corylus Americana, Corylus cornuta). This response may negate potential enhancement of soil moisture availability to overstory trees. As such understory control may need to be more widely practiced than it currently is. Greater use of growing season prescribed fire would help reduce hazel populations (7). As with other forest types, thinning may also help to reduce the potential outbreaks of density dependent pest species, such as bark beetles.

Resilience

A primary resilience strategy involves restoring and sustaining the native species and structural conditions currently associated with reference conditions for the Great Lakes mixed-pine ecosystem. The specific magnitudes of climate changes, especially for precipitation, are still uncertain and the exact response of various tree species is not well understood. As such, hedging one’s bet and increasing options for the future by maintaining the full suite of native species in the ecosystem is appropriate. There is tremendous opportunity to restore native species composition in the Great Lakes mixed pine ecosystem (8), as much of the current resource occurs in near mono-specific stands (often plantations) of either red pine or eastern white pine. Generalist species, such as red maple (Acer rubrum), and those with a more southerly distribution such as some oaks (Quercus spp.), that currently are a minor components of a stand may be favored by a resilience strategy, under the premise that conditions for their survival, growth, and fecundity may be enhanced in the future. Density management may be a tool to promote resilence of these species if they already occur in a stand. Specifically, thinning during the stem exclusion stage of stand development may prevent their competitive exclusion and allow them to recruit to the overstory.

The structural condition of many pine stands in the region is greatly simplified, relative to reference conditions. The later is inclusive of multi-cohort stands (8), as well as "even-aged" stands composed of one dominant cohort, along with a smaller population of older individuals. Concurrent with these age structures is greater complexity and heterogeneity in vertical and horizontal canopy distribution, greater amounts of large dead wood, and greater ranges of tree sizes, including very large individuals. The greater diversity of tree age and sizes may allow more flexibility of a species to persist in the face of climate change, if certain age or size classes are less susceptible to drought or other negative influences. As such, a resilience strategy may involve restoration of more complex age and size structures in forest of the region.

Response

Managing native species composition, as a strategy to adapt the system to climate change may meet with limited success in the long-term. Red pine, along with most of the associated "boreal" species, are predicted to face habitat conditions that are not suitable for long-term sustainability of populations. Thus, as a response strategy to facilitate the transition of forests to a new state, the silviculturist may also begin considering compositional manipulation that is currently outside the norm of practice. Initially, this may include the introduction of more southerly genotypes of contemporarily occurring species. This should be fairly straightforward when the species under consideration is widely regenerated artificially, as are both eastern white and red pines and some oaks. However, this approach may require relaxing seed source regulations, so as to allow material from outside of currently acceptable ranges.

A more controversial approach, but one that is gaining attention, is assisted migration of species not currently part of the contemporary ecosystem. In the Great Lakes mixed pine ecosystem, this might include, for example, eastern red cedar and short leaf pine. Assisted migration of species not currently part of the ecosystem should only be "experimented" with on a limited basis in the short-term. The problem with habitat suitability projections for these species is that the occurrence and magnitude of extreme climatic events, particularly extreme low temperatures, may not be well predicted in models. While average temperature or precipitation may begin to coincide with a species range requirements, the occurrence of even one deep freeze could decimate populations of these "formerly" southern species.