The boreal forest biome is experiencing dramatic changes in climate with important implications for ecosystem biogeochemical cycles and global feedbacks. With a vast amount of belowground carbon (C) that may be particularly vulnerable to climate change, understanding the distribution of carbon among heterogeneous forest types and ecological gradients, including the influence of fire, may improve predictions of future C pools. To capture the breadth of soil C stocks in Alaska’s boreal forest we examined soil C data collected from throughout the Tanana River Basin – an area of over 13 million hectares – that was inventoried between 2014 and 2018 by the USDA Forest Service, Forest Inventory and Analysis (FIA) program. We analyzed C data from the spatially balanced and randomly selected plot network to explore the spatial patterns among and within forest types, and the relative influence of topographic, ecological and climatic factors on belowground C pools in Interior Alaska.
Results/Conclusions
Of the 690 inventoried plots, complete soil cores were obtained from 606 forested plots among aspen (Populus tremuloides; 59 plots), Alaska paper birch (Betula neoalaskana; 123 plots), black spruce (Picea mariana; 307 plots) and white spruce (Picea glauca; 117) forest types. Forest type accounted for approximately 29% of the variance in soil C, reflecting differences in soil physical properties such as bulk density and organic layer thickness among forest types. Overall, live tree basal area, slope and elevation were negatively associated with soil C, whereas stand age and summer precipitation positively influenced soil C. Random forest analysis revealed the relative importance of aspect and slope among plots that had burned, although slope and live tree basal area were the two most important variables among plots without a history of fire. Basal area had opposing effects on soil C between burned and unburned plots. Soil C increased with high basal area on burned plots, however unburned areas with low basal area were associated with high soil C, likely reflecting poor site conditions for tree growth but favorable settings for soil organic matter accumulation (cool, saturated soils). Our study leverages the power of the FIA soils dataset by randomly capturing the broad range of conditions in Alaska’s boreal forest and reveals the importance of landscape position and dynamic ecological variables contributing to regional soil C storage.