Climate Impacts on Regional Boreal Forest Productivity Inferred from Solar-Induced Chlorophyll Fluorescence

The uptake of carbon by the land via photosynthesis, also known as Gross Primary Productivity (GPP), may differ across Arctic continents. For example, previous studies have shown increased vegetation greening in Eurasia relative to North America, with an associated relative increase in Net Ecosystem Exchange (NEE) amplification. Amplification of NEE in boreal and arctic ecosystems, due to a changing seasonal cycle of atmospheric CO2 concentration, implies that the component fluxes of NEE - namely GPP and the release of carbon via respiration - are changing. We seek to determine whether variations in GPP are driven by top-down climate conditions across high latitude boreal forests in both Eurasia and North America. Understanding the sensitivity of GPP to climate and how these sensitivities may differ across regions of the globe will help us gain insight into what is affecting the uptake of carbon and the future of the net carbon sink in the Arctic Boreal Zone. To investigate these impacts, we use a global spatially contiguous solar-induced chlorophyll fluorescence (CSIF) data product from a multi-year, multi-platform satellite record, which has been shown to correlate strongly with tower-based GPP. We found that max air temperature is highly correlated with CSIF variations during the late spring and early summer, but explains less than 10% of the variance in CSIF for the growing season months of July and August in boreal ecosystems. Despite high variability of CSIF during late summer, soil moisture explains less than 20% of the variance in CSIF for July and August in the same boreal ecosystems. These results suggest that during late summer, ecosystem memory effects that depend on intrinsic ecosystem characteristics, rather than top-down climate drivers, likely control CSIF variance during late summer.