Effects of Shifting Spring Phenology on Growing Season Carbon Uptake in High Latitudes

Phenological shifts not only alter the growing season length, but also modify land-atmosphere exchanges of water and energy, which in turn affects ecosystem carbon uptake. However, how changes in the start of the growing season (SOS) affect carbon uptake throughout growing season has not been fully explored for high-latitude ecosystems. Here, we investigated the impacts of SOS shifts on carbon uptake and the mechanisms over 2000–2020 in the northern high latitudes (>50°N) using multiple satellite and climate data sets. We found a contrasting response of gross primary productivity (GPP) in the early (April–June) and late (July–September) growing season to SOS shifts. Advanced SOS resulted in increased early-season GPP, whereas slightly decreased late-season GPP. The earlier SOS resulted from a warmer early season, which induced a decline of snow to precipitation ratio, therefore drier soil and atmosphere. In the early season, when water requirement of vegetation was low, the warm and dry conditions ensured favorable solar radiation and improved vegetation water use efficiency (WUE), thus enhancing photosynthesis. The dry soil and atmosphere conditions extended to the late season due to less snow and excessive water utilization by vegetation in the early season, which therefore slightly suppressed photosynthesis by weakening WUE. Conversely, the delayed SOS resulted in reduced early-season GPP and slightly enhanced late-season GPP due to the opposite thermal and moisture conditions to that of advanced SOS. The SOS is likely to be further advanced as warming continues in the high latitudes, intensifying seasonal contrasts in vegetation photosynthesis capacity.