Climate Change and...
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Silviculture and Climate Change
Preparer: Paul Anderson, Resource Management and Productivity Program, PNW Research Station.
Newer (2011) version of this paper is available here.
Under some predictive scenarios, changes in climate may occur that will exceed the capacity of existing forest tree populations to adjust physiologically and developmentally. Furthermore, climate changes may occur at rates that will exceed the capacities of forest species to evolve in place to adapt to new conditions or to migrate to more favorable, future environments (Aitken et al. 2007). Being relatively long-lived, the forest trees living today will probably compose much of the forests of the next century. Long-term adaptation to climate changes will require healthy and productive forests in the short term.
Hundreds of thousands of acres in the Pacific Northwest are dense, overstocked forests with low structural and compositional diversity (Tappeiner et al. 2002). Such dense forests are prone to declines in tree vigor when the abundance of vegetation exceeds the capacity of the site to provide essential environmental resources such as water, nutrients, or light. Dense, uniform forest canopies are typically associated with lesser amounts of understory vegetation of relatively few species and may provide quality habitat for fewer organisms than more structurally and compositionally diverse forests (Wilson and Puettmann 2007). The susceptibility and resilience of these forests to fire or pest disturbances, as well as their ability to adapt to meet future climate challenges may be compromised by a lack of vigor or diversity.
It is important to determine what management actions can be undertaken to enhance forest ecosystem health and productivity in a changing climate.
Under many climate change scenarios, increased temperature and increased frequency of summer drought may result in more stressful forest environments owing to, among other reasons, an increased evaporative demand combined with limited soil moisture (Spittlehouse 2003). Likely affects of increased moisture stress include reduced growth and productivity and decreased vigor of forest stands. Declines in vigor may make forest more susceptible to large-scale pest attacks and more frequent or severe fires. Furthermore, existing plant species or genotypes may be poorly adapted to future climate conditions during all or various parts of their life cycles, resulting in increased risk of regeneration failures and altered trajectories of forest growth, development, and productivity. Projections for increased ambient air temperatures as well as increased frequency of seasonal drought may have substantial consequences for instream and riparian microclimates and habitats and the unique ecosystem functions provided by riparian systems (Anderson et al. 2007, Olson et al. 2007).
Options for Management
Active management of forest vegetation may increase forest resistance and resilience to climate changes and mitigate some ecosystem responses to climate changes. Silvicultural practices can be applied and modified adaptively as climate and disturbance regimes change over time. Several long-term, large-scale silviculture experiments are underway that are providing a strong base of scientific knowledge and tools needed by managers to address climate change issues (Oliver 2000, Ritchie 2005, Poage and Anderson 2007). Based on existing and developing knowledge, the following management options can be recommended:
- Thin to avoid overstocked stands susceptible to increased mortality from drought, insects, disease and wildfire. Maintain lower densities of canopy foliage as site resources, particularly soil moisture, become more limiting
- Underplant thinned stands with adapted species or genotypes when advanced regeneration is unacceptable for future conditions. Although understory regeneration is often limited in dense second-growth stands, overstory density management may provide opportunities to introduce regeneration better suited to a future climate through underplanting.
- Provide structural features at stand and landscape scales to meet the varying habitat requirements of plants and animals. Provide diversity of stand structures favoring a greater diversity of species and genotypes across the landscape.
- Use vegetative buffers to mitigate effects of harvest on stream and riparian microclimates and habitat. Protect unique instream and riparian habitats by maintaining riparian functions of energy and nutrient exchange, filtration, bank stabilization, and wood production. manage for prompt revegetation with adapted plant communities. Attain site occupancy with desired species or genotypes before weedy or invasive species establish. Explore options for establishing species or genotypes better adapted to future environments and disturbance regimes.
- Promote development of mixed-species or multiprovenance forests. Decrease risks associated with major pest outbreaks and promote greater genetic diversity and more broadly adapted populations. Consider deploying a mixture of seedlings including some provenances adapted to more stressful environments.
- Where practiced, use intensively managed plantations as an opportunity to facilitate the migration of adapted genotypes or species. Regeneration cycles characteristic of commercial timber production may provide an opportunity to relatively rapidly distribute commercial species and genotypes better adapted to future environments.
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USFS Climate Change Atlas
Western Forests Climate Change Taskforce
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Anderson, Paul. 2008. Silviculture and Climate Change. (May 20, 2008). U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. http://www.fs.fed.us/ccrc/topics/silviculture.shtml