Diminished contribution of spring phenology to early-season carbon uptake in a changing climate

Abstract

Climatic warming advances the start of the growing season (SOS) and enhances terrestrial gross primary productivity (GPP). The consequences of this earlier SOS on the daily rate of GPP (GPPrate) and total GPP (GPPtotal) during the early growing season (EGS), however, are poorly understood.We used data for solar-induced chlorophyll fluorescence and carbon flux across the Northern Hemisphere to determine that an earlier SOS extended EGS, i.e. the period from SOS to the peak of the growing season, which increased GPPtotal during this extended period. The earlier SOS, however, decreased GPPrate during EGS due to reduced mean temperature and soil-water content. The extended EGS increased GPPtotal, but the decrease in GPPrate restricted these increases by ~25% and was more pronounced in arid (~40% reduction) than humid (~10%) areas. The diminished contribution of an advanced SOS to early-season vegetation carbon uptake suggests that the current increase in carbon uptake in northern ecosystems may be offset by future warming and drought conditions.