Louis Iverson, Northern Research Station; Don McKenzie, Pacific Northwest Research Station
Why are species located where they are? This is a primary question ecologists have long been asking, and the classic answer is that vegetation depends on climate, parent material, organisms, disturbance, topography, and time (1). Prehistoric records show that the climate has changed through time, as have the roles of organisms (including humans) and disturbances such as fire. In present times, however, the climate is changing rapidly. Human-caused greenhouse gas emissions have raised peak levels of atmospheric carbon dioxide above 400 ppm for the first time in at least 3 million years. The changing climate is affecting species distributions via changes in growth, reproduction, and mortality, with increasing likelihood of more marked changes in the coming decades. Climate changes can act to directly influence species distributions (e.g., drought, floods, wind) as well as indirectly (e.g., temperature and weather related changes in patterns of wildfire, insects, and disease outbreaks).
Some species ranges have shifted in recent decades, very likely in response to climate changes. For example, a meta-analysis of 764 species (mostly arthropods) found an average rate of poleward migration of 16.9 km/decade (2). An earlier meta-analysis, using 99 species of birds, butterflies, and alpine herbs, reported an average poleward migration of 6.1 km/decade (3). For tree species, direct evidence of latitudinal shifts is more limited. Indirect evidence is apparent in some studies in the eastern US (4, 5, 6) and in the far north at the treeline ecotones of black spruce (7) and white spruce (8, 9) or Siberian pine (10). Even though suitable tree habitats appear to be changing, actual tree range shifts can take decades or more to detect. In the past however, we know that tree ranges shifted polewards in response to warming climate. The pollen-estimated rates for tree migration during the last late glacial period 10,000 -20,000 years ago show movement of about 1-10 km/decade (11, 12), rates much faster than experiments and mechanistic models have been able to account for (Reid’s paradox, 13). The explanations for rapid post-glacial colonization thus far include rare long-distance dispersal and recolonization from persistent isolated populations (14). Post-glacial migration occurred when species were not slowed by forest fragmentation, which can reduce expected migration rates by more than half (15, 16).
A major concern today is that tree migration may not be able to keep up with rates of climate change. The average velocity of changing global temperatures for 2050-2100, for an ensemble of middle range emissions (A1B) scenarios, is estimated to be 3.5 km/decade for the temperate broadleaf and mixed forests, 1.1 km/decade for temperate coniferous forests, and 4.3 km/decade for boreal forests and taiga (17). Terrain can have a significant effect on required migration rates. Forests in flat terrain must migrate ~145 km in latitude to reach similar zones with a 1oC temperature difference, whereas forests in mountainous terrain need only migrate ~167 m in altitude (18). These and many other indicators suggest that it is unlikely that trees and other organisms will be able to migrate fast enough to keep up with the rate of climate change, without human assistance.
Models provide useful methods to estimate potential changes in species distributions, as long as the caveats in interpreting the models are considered (“all models are wrong, some are useful”!). Predictive models of vegetation change are often divided into two categories: process-based models, which are usually simulations of vegetation dynamics at the taxonomic resolution of species or life forms, and empirical models (often called species distribution models, or SDMs) that establish statistical relationships between species or life forms and (often numerous) predictor variables. More and more there are hybrid models, extensions to SDMs that include elements of process models that provide additional scope and power to take advantage of the best of both worlds. However, there always will be trade-offs between using complex mechanistic models versus the simpler empirical models to associate changes in species habitats with forecasts of environmental change (19, 20). A further discussion on the merits of each category of model can be found in the next sections.
Management under climate change increases complexity, but the basic toolbox is the same as for current management. As discussed elsewhere, forest management under climate change can be categorized as mitigation, adaptation, or both. Some specific actions when considering how species may shift in response to climate include 1. Encourage increased connectivity for species modeled to increase with climate change; 2. Evaluate potential for assisted migration; 3. Encourage retention of refugia which may allow persistence of species modeled to decline under climate change; 4. Prepare for additional costs likely required to maintain forest health due to increased stress and disturbances (e.g., insect pests, diseases, fire, ice, drought); and 5. Identify species likely to be especially vulnerable.