Tree growth and mortality responses to climate and disturbance across a broad forest gradient in southwest Alaska

Background/Question/Methods

Rapid warming and drought in the last few decades have been identified as contributing factors to changes in tree growth, widespread tree mortality, and an increasing incidence of disturbances (i.e., wildfires and insect outbreaks) across mid- to high latitude forests in the Northern Hemisphere.  However, forest response to warming in Alaska has been complex, varying with respect to the biophysical and geographic setting.  For example, recent studies of Alaskan forests show that rapid warming is causing some trees to increase in radial growth, while others are in decline due to moisture stress, loss of snowpack due to earlier spring melt and other stressors (e.g., disturbance), resulting in forest mortality. In this study, we use tree-ring records to characterize the growth response and mortality of trees to disturbance and climate variability across a range of forest and woodland sites (n = 30) on the Alaska Peninsula. We specifically evaluate on the relationships and interactions among tree growth rates, mortality, climate variables (temperature, precipitation), and disturbance history (i.e., spruce beetle outbreaks).  

Results/Conclusions

Across the Alaska Peninsula, tree growth appears to show relatively consistent positive growth associated with winter-spring temperatures, with the strongest relationships in open woodland sites. Many sites also show negative growth associated with spring precipitation (i.e., extended periods of snow pack), dependent on forest structure, disturbance history and biophysical setting. These preliminary results suggest that, in general, tree growth is responding favorably to recent warming across both open and closed forest ecosystems in southwest Alaska. However in many low elevation-closed canopy stands, recent warming-related spruce beetle outbreaks have significantly affected tree growth (i.e., stressed trees) and mortality levels throughout the Alaska Peninsula.  Results of the study provide information on the extent to which recent warming has influenced tree growth and mortality across a range of representative forested sites in southwest Alaska.  This study, and comparative studies on stand dynamics and regeneration, provides a basis for assessing the current condition of, and potential changes to, representative spruce ecosystems in Alaska.