Varying Northern Forest Response to Arctic Environmental Change

The sensitivity of northern forests to Arctic warming and related seasonality effects is likely to be highly complex, involving shifts in the timing and dynamics of multiple factors that can significantly impact tree growth. We investigate the response of northern boreal forests to changes in Arctic climate and seasonality, focusing herein on tree-ring data from sites across Alaska, one of the regions of greatest warming on the globe today. These data are being used to identify the nature of forest response to climatic and environmental changes for white spruce (Picea glauca), a dominant Arctic treeline species. A white spruce ring width record for the Firth River area of northern Alaska shows evidence for inferred low-frequency temperature variability for the past millennium that is consistent with large-scale temperature reconstructions for the northern hemisphere extra-tropics. There is also evidence for a weakening in the expected positive response to temperature in the latter part of the 20th century, termed the 'divergence' effect, which has been tentatively attributed to warming-induced drought stress. High-resolution satellite-derived vegetation observations indicate growing season 'browning' in interior Alaska, transitioning to 'greening' in the Alaskan tundra over recent decades, with an intervening region of relatively little trend in photosynthetic activity. Continued research will help generate additional tree-ring ring width and density records along a north-south gradient of the northeastern Alaskan forests for comparison with the satellite vegetation indices. The ultimate goal of this research will be a detailed, integrative assessment of how boreal forests are responding to recent warming-induced seasonality changes, and how these changes may feed back and influence the Arctic and global environmental systems in the future.