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 degrees 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. Plain Language Summary Spring phenology shifts not only change the length of growing season, but modify local and regional climate that in turn affects the magnitude of ecosystem carbon uptake, which is not well understood, particularly in the high-latitude ecosystems. Here, we investigated the effects of a shifting spring phenology on subsequent seasonal carbon uptake and explored the underlying mechanisms in the northern high latitudes (>50 degrees N) using satellite data and climate data sets. We found that warming-induced earlier spring resulted in an enhanced carbon uptake in the early season (April-June), but slightly weakened carbon uptake in the late season (July-September). Early season warming caused advanced spring greenup dates and drier conditions with improved vegetation water use efficiency (WUE). The warm and dry conditions improved vegetation WUE because of the low water requirement, thus enhancing vegetation productivity. The dry conditions extended to the late season due to less snow. With the higher temperature in the late season, water stress reduced the WUE of vegetation and therefore resulted in a slight decrease in vegetation productivity.

Automated summary

This study investigates the effects of shifts in the start of the growing season on carbon uptake in high-latitude ecosystems. It finds that an earlier start of the growing season leads to increased carbon uptake in the early season, but slightly decreased carbon uptake in the late season. This is due to the warmer and drier conditions in the early season, which improve vegetation water use efficiency and enhance photosynthesis. However, the dry conditions persist in the late season and reduce vegetation water use efficiency, resulting in a slight decrease in photosynthesis.