Ecological histories from Alaskan tree lines provide insight into future change

Ecosystem responses to past climate change can provide insight into plausible scenarios of response to future change and can elucidate factors that may influence the overall predictability of such responses. I explore the utility of paleoecological studies for addressing questions about the predictability of ecosystem responses to climate change using Alaskan tree line ecosystems as a case study. Published studies were used to develop a regional analysis of patterns of recent tree line advance, and to estimate lags between recruitment onset and forest development beyond tree line. Tree line advance is ubiquitous, but asynchronous in time, occurring significantly earlier in the White Mountains in interior Alaska than in western Alaska or the Alaska Range. The mean lag between initiation of recruitment and forest development was estimated at approximately 200 years, similar to what modeling studies have found. Although continued advance of white spruce forests is the most likely scenario of future change, variability in the rate of forest response to warming may be likely due to limitation of spruce establishment in highly permafrost-affected sites, changes in seed dispersal and early establishment, and recent changes in the growth responses of individual trees to temperature. All of these factors may cause spruce populations to exhibit nonlinear responses to future warming, and uncritical extrapolation from recent trends is thus unwarranted.