Estimates of White Spruce density at two elevations from 1600-present

Paleoecological investigations of 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. There is particular interest in the rate at which subarctic Picea populations respond to climate change, as the rate of conversion from tundra to forest may affect the rate of future climate warming. We investigated recent changes in the structure and distribution of treeline white spruce forests in the White Mountains and the Alaska Range. Treeline advance was ubiquitous, but asynchronous in time, occurring significantly earlier in the White Mountains in interior Alaska than in 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 Picea is the most likely scenario of future change, the recent history of treeline advance suggests that nonlinear responses to warming may be likely due to nonlinear growth responses of individual trees to temperature, limitation of Picea establishment in highly permafrost-affected sites, and decreasing seed availability as the advancing front moves farther from dense seed sources. All of these factors may cause Picea populations to respond nonlinearly to future warming, and these potential nonlinearities caution against uncritical extrapolation from recent trends.