Seeing the forest through the trees: how site conditions mediate white and black spruce responses to climate in Interior Alaska

Abstract

The boreal forest provides essential ecosystem services and helps regulate global climate. With climate change occurring at a faster rate at high latitudes, including in the boreal forest biome, it is critical to understand how boreal forests are responding to these unprecedented changes. Despite much effort, uncertainty remains as to how boreal forest productivity has and will change with ongoing climate changes. Some of the uncertainty reflects the complex mosaic of regional climatic patterns, direct and indirect species-specific responses to regional climate, and heterogenous local site conditions that affect boreal forest productivity. I focused on the latter uncertainty: the potential role of topographic, edaphic, and biotic conditions in mediating the climate-growth responses of boreal tree species. My overarching goal was to quantify the radial growth response of black spruce (Picea mariana) and white spruce (Picea glauca), the two most common tree species in interior Alaska, to climate variability across a suite of site conditions to better understand the observed and predicted variation in climate driven productivity across a variable landscape. I employed a systematic sampling design to quantify the landscape-scale patterns in both environmental conditions and incremental annual growth of trees distributed across a 1.28 million-ha study area in Denali National Park and Preserve (and beyond in Chapter 4). I also used targeted sampling of carbon isotopes in tree rings to investigate potential drought stress. I found that near-surface permafrost, slope angle, and elevation strongly modified the magnitude, shape, and, in some cases, the direction of radial growth response of both species. For white spruce, the negative growth response to warm and dry summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. During years with high cone and seed production, white spruce shifted its current year's carbon resources from radial growth to reproduction and showed signs of drought stress. I also observed differences between black and white spruce climate-growth responses, with near-surface permafrost driving their contrasting responses to June-July temperatures and with black spruce growth showing an overall more positive response to summer precipitation. These results demonstrate that local site and stand variables can force contrasting growth responses to similar climate conditions and help predict how future black and white spruce growth may play out with climate changes across a heterogeneous landscape. My results underscore the pivotal role of near surface permafrost in both the climate-growth responses and competitive dynamics of black and white spruce. Consequently, my results emphasize the importance of ongoing and predicted changes in the distribution and prevalence of permafrost for the future of the boreal forest.