Strong Inhibiting Effect of Daytime Warming But Weak Promoting Effect of Nighttime Warming on Carbon Use Efficiency in Northern Hemisphere

Asymmetric daytime and nighttime warming broadly affects terrestrial ecosystem carbon cycling in the Northern Hemisphere. Photosynthesis mostly occurs in the daytime and respiration occurs during the whole day. Then AWD-N will certainly affect the ratio between net primary productivity (NPP) against gross primary productivity (GPP), i.e. vegetation carbon use efficiency (CUE). However, how night and daytime warming differentially affects CUE remains unclear on a global scale. Using long-term productivity datasets from MODIS and terrestrial biosphere models, we investigated spatiotemporal patterns of CUE response to daytime (Tmax) and nighttime (Tmin) temperature changes across the Northern Hemisphere (>30 degrees N) spanning 2000-2019. Regions exhibiting a negative correlation between Tmax and CUE were extensive throughout the growing season in most northern ecosystems, with a stronger correlation magnitude in summer than in spring and autumn. Tmin warming tended to cause a positive impact on CUE in spring and autumn, while being correlated negatively with summer CUE, resulting in an overall weak positive relationship between CUE and Tmin. Divergences in the strength and direction of productivity and respiratory responses to AWD-N lead to spatial and seasonal patterns between CUE and Tmax or Tmin, and this pattern is regulated by soil moisture. Our findings provide an in-depth understanding on how the interactions between AWD-N and water limitations regulate the spatiotemporal variations of vegetation gross primary product allocation, which is vital for improving model performance.