Climatic Drivers for the Variation of Gross Primary Productivity Across Terrestrial Ecosystems in the United States

Temperature and water stress are important factors limiting the gross primary productivity (GPP) in terrestrial ecosystems, yet the extent of their influence across ecosystems remains uncertain. This study examines how surface air temperature, soil water availability (SWA) and vapor pressure deficit (VPD) influence ecosystem light use efficiency (LUE), a critical metric for assessing GPP, across different ecosystems and climatic zones at 80 flux tower sites based on in situ measurements and data assimilation products. Results indicate that LUE increases with temperature in spring, with higher correlation coefficients in colder regions (0.79-0.82) than in warmer regions (0.68-0.78). LUE reaches a plateau earlier in the season in warmer regions. LUE variations in summer are mainly driven by SWA, exhibiting a positive correlation indicative of a water-limited regime. The relationship between the daily LUE and daytime temperature shows a clear seasonal hysteresis at many sites, with a higher LUE in spring than in fall under the same temperature, likely resulting from younger leaves being more efficient in photosynthesis. Drought stress influences LUE through SWA in all ranges of water availability; VPD variation under moderate conditions does not have a clear influence on LUE, but extremely high VPD (exceeding the threshold of 1.6 kPa, often observed during extreme drought-heat events) causes a dramatic reduction of LUE. Our findings provide insight into how ecosystem productivities respond to climate variability and how they may change under the influence of more frequent and severe heat and drought events projected for the future. The terrestrial ecosystem assimilates carbon through photosynthesis, and its ability to convert sunlight energy to primary production during photosynthesis is commonly measured by light use efficiency (LUE). To understand how temperature, water in the soil, and atmospheric aridity impact carbon assimilation through photosynthesis, we analyzed the drivers of LUE at 80 sites across different ecosystems and different climate regimes in the United States. We found that temperature is the main factor influencing LUE of grassland and forest in spring, and has a greater influence on LUE in colder regions than in warmer regions. Under the same temperature, LUE in the spring is higher than in the autumn season, likely because of new leaves. In summer, deep soil water availability is the primary driver of LUE variations. In addition, extreme aridity of the atmosphere contributes to a dramatic decrease in LUE during compound heat-drought events. Our findings contribute to advancing our understanding of how climatic factors influence LUE across different ecosystems in a changing climate. Light use efficiency (LUE) variation is generally driven by temperature in spring and by water stress in summer Soil water availability dominates the LUE response to moderate droughts, but vapor pressure deficit becomes important under extreme drought The primary drivers of LUE variation also depend on background climate and vegetation types