Predicting the dynamics of US forest distributions through data-constrained, individual-based demographic modelling

Background/Question/Methods

The geographical distributions of many species and assemblages are controlled by climate, but to date ecologists have been unable to predict how such distributions emerge from environmental and competition‑related variation in underlying demographic processes. Forest distributions are controlled by both the physical environment and competition for limiting resources, particularly light. These two sets of factors act upon tree vital rates to determine population dynamics in space and time, which in turn give rise to fundamental and realized geographic niches. We used repeat measurements from forest inventory plots across the eastern US to parameterize models for tree‑level growth, mortality, and recruitment for seven plant functional types (PFTs) across this region. We then implemented the resulting demographic models in simulations of joint stand-level population dynamics across the region. By controlling for climate-dependencies in growth, mortality, recruitment, and competitor performance in turn, we sought to identify the demographic processes that acted to control the limits to the distributions of individual PFTs.

Results/Conclusions

Based purely on data for low‑level demographic processes, the emergent, simulated, geographic distribution of each PFT approximated its current distribution and reproduced well‑known successional patterns. Simulation experiments showed that temperature‑related increases in mortality determined the southern boundaries of three out of four boreal and northern temperate PFTs, whereas temperature‑related decreases in recruitment controlled the northern limit of all three southern temperate PFTs. Changes in growth rates and competitor performance had only minor effects on distribution limits. This work highlights the potential for combining models and data on demographic processes to understand and predict the dynamics of forest distributions.