Using Tree Rings, CO2 Fluxes, and Long-Term Measurements to Understand Carbon Dynamics in an Alaskan Boreal Forest

Abstract:

Decadal and centennial processes are usually poorly constrained by data, but many opportunities exist to combine disparate data sources such as tree rings, greenhouse gas fluxes from the soil to atmosphere, and long-term tree inventories. At high northern latitudes, permafrost (and its current degradation across large scales) is presumed to exert a strong control on long-term ecosystem carbon uptake and storage. We integrate a variety of data from both Canada and Alaska, focusing on two years of observations across a permafrost gradient in a black spruce Alaskan watershed (the Caribou/Poker Creek Research Watershed ~50 km northeast of Fairbanks, AK, USA). Permafrost depth changes were strongly associated with changes in vegetation and leaf morphology, as well as soil greenhouse fluxes (0.1-2.0 µmol/m²/s, with strong spatial dependencies) and aboveground net primary production (60-550 gC/m²/yr). We use tree-ring data covering the last century to examine how tree response to climate variability changes with elevation and permafrost depth, both along small-scale transects and across the entire 104 km² watershed. A weakness is that these results are from a single site and point in successional time; we quantify potential variability in this area using 16 years of observations from a Canadian boreal chronosequence. We emphasize that both short and long term observations and experiments, using multiple approaches, are necessary to constrain ecosystem carbon uptake and storage.