Local site conditions drive climate-growth responses of Picea mariana and Picea glauca in interior Alaska

The growth response of boreal forest trees to projected changes in climate will have wide ranging and cascading impacts on disturbance regimes, climate feedbacks, carbon storage, and habitat ranges for flora and fauna. Recent findings in Alaska suggest the boreal biome is shifting in response to changes in climate. It is unlikely such a shift will occur in a uniform manner over a landscape variable in topography and site conditions, although there is little consensus of how local site characteristics will interact with climate to alter tree growth patterns. Few studies of tree growth responses to climate in the boreal forest have used randomized study designs encompassing the full range of site conditions, leaving open the question of whether observed patterns in tree growth at local scales are wholly representative of the boreal forest. We addressed these issues using a systematic sampling design to quantify the landscape-scale patterns in annual growth for Picea mariana and Picea glauca, the two most abundant tree species in interior Alaska. We used measurements of annual tree growth based on increment cores taken from trees distributed across a 1.28 million-ha study area in Denali National Park and Preserve. We found site-specific variables, namely near-surface permafrost, slope angle, and elevation, strongly modified the magnitude, shape, and, in some cases, the direction of growth response to climatic conditions for both species. With predicted warming, our results suggest both P. mariana and P. glauca will decrease growth in areas underlain by shallow permafrost. P. mariana growth may increase on flat terrain, but decrease on steep slopes, while P. glauca growth will show the greatest increases at high elevations during very warm summers. Our results suggest future shifts in these species distributions in response to a changing climate will be conditional on variation in site factors operating at local scales.