Using forest inventory to understand climate change impact on tree populations

Background/Question/Methods

Biogeographic responses of plant species to climate change are determined by the requirements of juveniles, which can limit spread to new environments. By contrast, most predictions of climate response are calibrated to adults, which may not reflect the climate that determined their establishment. Using USDA Forest Service’s Forest Inventory and Analysis (FIA) data, we analyzed the distribution and abundance of both juvenile and adult trees, so as to understand climate change impact on tree populations. First, we directly compared juvenile vs. adult distributions in geographic space based on the assumption that a migrating population is characterized by offspring extending beyond adults at leading edges and the opposite at trailing edges. The difference in juvenile and adult geographic ranges suggests whether or not species are shifting their distributions to track climate warming. Second, we jointly modeled juvenile and adult abundance distributions in climate space based on the hypothesis that warm and moist climates generally benefit tree species with high turnover rates. The relative abundance of juveniles and adults in climate space identifies the environments where species have high relative recruitment.

Results/Conclusions

The first geographic space analysis suggests that there is essentially no evidence that latitudinal migration has yet occurred for more than half of ~ 100 tree species in eastern US forests. Instead, results suggest that the majority of the tree species examined show the pattern expected for a population undergoing range contraction, rather than northward expansion. There appears to be a substantial migration lag behind climate change velocity. The second climate space analysis suggests that the majority of tree species are benefiting from warm and moist environments by having high turnover rates. Both relative recruitment and mortality are high in these environments. Moreover, it rejects the hypothesis of large-scale northward migration, which predicts species having high relative recruitment in cold climates. Taken these two analyses together, tree populations respond to climate change not by migrating at biogeographic scales, but by having faster turnover rates. By comparing juveniles and adults using forest inventory data, these conclusions should increase concerns for the risks posed by future climate change.