Spring Hydrology Determines Summer Net Carbon Uptake in Northern Ecosystems

Abstract:

Increased photosynthetic activity and enhanced seasonal CO₂ exchange of northern ecosystems have been observed from a variety of sources including satellite vegetation indices like NDVI and atmospheric CO₂ measurements. Most of these changes have been attributed to strong warming trends in the northern high latitudes (≥50°N). Here we analyze the interannual variation of summer net carbon uptake (NCU) derived from atmospheric CO₂ measurements and satellite NDVI in relation to surface meteorology from regional observational records. We find that increases in spring precipitation and snow pack promote summer NCU of northern ecosystems independent of air temperature effects. However, satellite NDVI measurements still show an overall benefit of summer photosynthetic activity from regional warming and limited impact of spring precipitation. We hypothesize this strong sensitivity of NCU to spring hydrology may be caused by a strong regulation of spring hydrology on soil respiration in relatively wet boreal and arctic ecosystems, and the reduced sensitivity of NCU to temperature is likely due to similar responses of photosynthesis and respiration to warming. We use a combination of in-situ tower eddy covariance CO₂ flux measurements, satellite soil moisture retrievals, and a process model to test this hypothesis. Our results document the important role of spring hydrology in regulating northern ecosystem carbon uptake, and indicate potentially stronger coupling of boreal/arctic water and carbon cycles with continued regional warming trends.