The massive volcanic eruption of
Mount St. Helens 31 years ago provided
the perfect backdrop for studying the
earliest stages of forest development.
Immediately after the eruption, some
areas of the blast area were devoid
of life. On other parts of the volcanic
landscape, many species survived,
although their numbers were greatly
reduced. Reassembly began at many
different starting points along the
spectrum of disturbance. Within the
national volcanic monument, natural
regeneration generally has been
allowed to proceed at its own pace.
Charlie Crisafulli and Fred Swanson,
scientists with the Pacific Northwest
Research Station, along with numerous
collaborators, have found that the sunlit
environment, dominated by shrubs,
herbs, and grasses that characterize
early-seral ecosystems, supports complex
food webs involving numerous
herbivores. These biologically rich
areas provide habitat for plant and
animal species that are either found
only in these early-seral ecosystems or
reach their highest densities there.
Although much of the focus of forest
ecosystem management over the past
20 years in the Pacific Northwest has
been on protecting old forests and
hastening development of conditions
associated with older forests, the
research on Mount St. Helens points
to the ecological value of allowing a
portion of a managed landscape to
develop characteristics of a complex
early-seral ecosystem
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Climate change is anticipated to alter plant species distributions. Regional context, notably the spatial complexity of climatic gradients, may influence species migration potential. While high-elevation species may benefit from steep cli- mate gradients in mountain regions, their persistence may be threatened by limited suitable habitat as land area decreases with elevation. To untangle these apparently contradictory predictions for mountainous regions, we evalu- ated the climatic suitability of four coniferous forest tree species of the western United States based on species distri- bution modeling (SDM) and examined changes in climatically suitable areas under predicted climate change. We used forest structural information relating to tree species dominance, productivity, and demography from an exten- sive forest inventory system to assess the strength of inferences made with a SDM approach. We found that tree spe- cies dominance, productivity, and recruitment were highest where climatic suitability (i.e., probability of species occurrence under certain climate conditions) was high, supporting the use of predicted climatic suitability in examin- ing species risk to climate change. By predicting changes in climatic suitability over the next century, we found that climatic suitability will likely decline, both in areas currently occupied by each tree species and in nearby unoccupied areas to which species might migrate in the future. These trends were most dramatic for high elevation species. Cli- matic changes predicted over the next century will dramatically reduce climatically suitable areas for high-elevation tree species while a lower elevation species, Pinus ponderosa, will be well positioned to shift upslope across the region. Reductions in suitable area for high-elevation species imply that even unlimited migration would be insufficient to offset predicted habitat loss, underscoring the vulnerability of these high-elevation species to climatic changes.
Keywords: climate change, demography, dominance, forest inventory and analysis, productivity, suitability, tree species, upslope migration
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