Understanding Climate Change Impacts in Temperate Forests

Written by Ronald Mahoney

Forest ecosystems are complicated and ever changing. Forest landowners and managers must consider a vast array of information to meet either specific stand objectives and/or broader goals of landscape level management. In many situations, land management objectives integrate measurable products, such as timber and forage, and less tangible assets, often collectively described as aesthetics. On other lands, production of timber or other products may be primary, but a broad consideration of ecosystem functions and processes is still required for sustainable success.

To conceive how climate change can and is affecting temperate and boreal forests, it is necessary to first understand how different species in these ecosystems relate to each other (synecology) and how individual species relate to their environment (autecology). Many of the fundamental ecological principles were developed from research and experience in more tropical ecosystems, which have had little climate change or large-scale disturbances. As a result, tropical species have co-evolved to extreme specialization with highly developed adaptations to specific ecological niches and a finely tuned interdependence.

As you move north, more regular and dramatic disturbances occur. For example, the plants and animals of the Inland Northwest have been associated for less than 10,000 years, and in boreal and arctic regions for far less time. Consequently, the synecology of these plant and animal communities is much less developed. Most species are linked more by competition and adaptation to disturbance than by the refined interdependence we see in tropical ecosystems. Many of the pathogen/host interactions in this temperate region would seem to be a result of co-evolution, although many pathogens show the ability to infest diverse hosts. As examples, the white pine weevil (Pissodes strobi) infects mostly spruce (Picea spp.) and lodgepole pine (Pinus contorta), mountain pine beetles (Dendroctonus ponderosae) have success in several pine species, and the spruce budworm (Choristoneura spp.) can shift from grand fir (Abies grandis) to Douglas-fir (Pseudotsuga menziesii) to hemlock (Tsuga spp.) depending on availability and host condition. There may be more selection pressure for “generalist” pathogens and other opportunistic adaptations of many plants and animals because of more frequent and dramatic disturbances.

Moving from temperate to boreal to arctic forest ecosystems uncovers an increasing ability of organisms to adapt to change. These forests also experience more dramatic disturbances and their effects on species survival are often evident in epidemic pathogen outbreaks with some species being reduced or eliminated. Other species in these changing situations may greatly increase their range, vigor, and percent of the population. Rather than the current focus being on species decline as a result of human activity, perspective on the bigger picture is needed to understand and accommodate changes in species and environments. While human impacts on climate change should be modified, there are many interrelated, but inevitable, changes to understand and plan for to reduce the undesirable effects of climate change.

These generalizations about climate change effects on large-scale ecosystems are only part of a very complex and dynamic interaction of the physical and biological environments. However, they can guide decisions about how specific sites may be affected and how these changes may affect silvicultural objectives and prescriptions.

Species that require very specific ecological conditions to succeed and those that are adaptable to a wide range of conditions and are at the fringes of their tolerance will show the first and most dramatic climate change impacts.

Changes in temperature and water availability can create problems for many of these sensitive species. Periods of unseasonably warm temperatures followed by dramatic freezing periods can leave trees susceptible to frost damage and death (Saxe et al. 2001); two impacts that have already been seen in Alaskan yellow-cedar and in some Inland Northwest larch (Larix occidentalis). Many of these changes in climate are not directly manifest in warming, but in when and where precipitation occurs, particularly in having rain instead of snow during winter and in very early or late severe cold. Additionally, changes in water availability can create drought conditions, affecting tree growth during time periods when trees are most in need.

While forests represent a major opportunity for carbon storage to reduce greenhouse gases, their ability to do so is largely dependent on the overall system’s response to changes in environmental conditions. The intricate interplay between species range, disturbance regimes, and species’ life cycle will all play a role in how well temperate forest ecosystems respond to changing climatic conditions.

References Cited
Saxe H., M.G.R. Cannell, O Johnsen, M.G. Ryan, and G. Vourlitis. 2001. Tree and forest functioning in response to global warming. New Phytologist. 149: 369-400.

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