Climate change influences on vegetation photosynthesis in the Northern Hemisphere

Changes in ecosystem productivity affect terrestrial carbon sequestration. In previous research on the effects of climate change, it has been determined that prolonged growing season length (LOS) increases vegetation productivity in ecosystems. In addition to the duration of vegetation growth, the intensity of photosynthesis is another factor influencing the annual accumulated vegetation productivity. Nevertheless, the impact of climate change on productivity through photosynthetic intensity of vegetation remains uncertain. Here, we utilized the photosynthetic phenology extracted from solar-induced chlorophyll fluorescence (SIF) to investigate the influence of climate change on the annual peak value of vegetation photosynthesis (SIF_max), as well as the contribution of SIF_max to annual accumulated gross primary productivity (GPP_ann) in the Northern Hemisphere (>30° N). Furthermore, the influence of changes in LOS and SIF_max on GPP_ann were compared. The results showed that vegetation SIF_max increased in 73.0% of the areas, and that different climatic factors (radiation, precipitation and temperature), and the advanced start of the growing season (SOS) contributed to an increase in SIF_max. GPP_ann was more sensitive to the peak of photosynthesis than LOS, with SIF_max being the dominant factor affecting GPP_ann in 39.9% of the study area, compared to 13.7% of the area dominated by LOS. Our results demonstrated that climate change increases GPP_ann primarily by increasing SIF_max rather than by extending LOS. While temperature was the largest contributor to GPP_ann among all climate factors, precipitation and radiation can also have an obvious effect on GPP_ann through SIF_max. Our study highlights the important mediating role of peak photosynthesis in the influence of climatic factors on the annual accumulated productivity of vegetation. The results provide implications for understanding the characteristics of vegetation response to climate change, and for the development of ecosystem restoration and carbon management strategies.