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

Advances in spring phenology (SOS) and increases in terrestrial gross primary productivity (GPP) are well-documented. However, the impact of an earlier SOS on the duration and rate of carbon uptake (GPPrate) during the early growing season (EGS) remains unclear, leaving the overall influence of these factors on EGS GPP uncertain. Using satellite and carbon flux data across the Northern Hemisphere, we found that an earlier SOS extended EGS, but reduced the mean daily GPPrate during this period, primarily due to lower temperatures and soil-water content. While the extended EGS increased total EGS GPP, the reduction in EGS GPPrate lowered these gains by 30% on average, with a more pronounced reduction in arid regions (~ 85%) compared to humid areas (~ 15%). The diminished contribution of an advanced SOS to early-season productivity suggests that the current increase in carbon uptake in northern ecosystems may be offset by future warming and drought conditions. An earlier spring phenology reduces daily carbon uptake rates during the early growing season, diminishing the potential gains in productivity from the extended duration during this period, based on an analysis integrating satellite and ground-based carbon flux data.