Evidence for range contraction of eastern North American trees

Background/Question/Methods

Tree species are expected to track warming climate by shifting their ranges to higher latitudes. However, current evidence for range shifts of multiple species comes either from output of climate envelope models, or from highly indirect observations. Large-scale forest inventory data provides an opportunity to compare the present latitudes of seedlings and adult trees at their range limits, implying the future latitudes of tree establishment. We quantified the number of species having seedling ranges beyond the current ranges of adult trees, as would be predicted for populations migrating in response to climate change, and compared seedling vs. tree patterns with dispersal properties of seed. Using USDA Forest Service’s Forest Inventory and Analysis (FIA) data, we directly compared 92 species seedling and tree extreme and 90% latitudes in 30 longitudinal bands for 43,000 plots across eastern North America. We further compared this latitudinal difference with 20th century temperature change and species functional traits of seed size and seed spread rate. Climate-change induced migration is not the only factor that could influence latitudinal limits of seedlings and adult trees, so we considered our results in the context of effects that could arise from source-sink populations, successional trends, and sampling designs.

Results/Conclusions

More than half of the tree species examined showed the pattern expected for a population undergoing range contraction, rather than expansion, at both northern and southern boundaries. Slightly more species showed tendency of northward shift than southward shift, and only few species showed expansion at both range limits. Artifacts due to sampling designs were minimized by the result that both extreme and 90% latitudes showed the same patterns, and by the fact that they were replicated across 30 longitudinal bands. An observation that seedling extreme latitudes exceed those for trees could indicate sink populations beyond the range where populations are self-replacing. The fact that we found the majority of extreme seedlings to be less extreme than adult trees only emphasizes the lack of evidence for migration that might keep pace with climate change. When compared with 20th century temperature changes that have occurred at the range boundaries themselves, we found no consistent evidence that population spread was greatest in areas where climate had changed most. Nor were patterns related to seed size or dispersal characteristics. Lack of evidence for climate-mediated migration should increase concerns for the risks posed by climate change.