Assessing the influence of changing vegetation distributions on increasing seasonal CO2 exchange in high latitudes

Vegetation change, including changes in vegetation density, functional types, and distributions, has been inferred as an important factor in explaining the increasing amplitude of seasonal CO2 cycling in high latitudes. However, the extent to which vegetation change has driven changes in CO2 amplitudes has not been explored with comprehensive, long-term, observed vegetation data. This study seeks to determine the impact of vegetation changes on seasonal CO2 fluxes using remote sensing-based vegetation products and an improved version of the Community Land Model (CLM) 5. We used long-term vegetation cover data derived from AVHRR, including fractional tree, short-stature vegetation, and bare ground cover, as well as time series of deciduous vs. evergreen fractions derived from Landsat as input land surface data for CLM during the past 30-40 years. Here we describe the impact of these observed vegetation changes on gross and net fluxes of CO2, with the latter being compared to surface atmospheric CO2 measurements using a pulse response transport operator based on atmospheric dynamics from the Community Earth System Model. We then compare vegetation influences to those from changing climate, CO2 fertilization, nitrogen deposition, and land use.