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

Abstract

Asymmetric diurnal warming broadly affects terrestrial ecosystem carbon cycling in the Northern Hemisphere. Vegetation photosynthesis mostly occurs in the daytime and respiration occurs during the whole day. Then the asymmetrical diurnal warming will certainly affect the ratio between net ecosystem productivity (NPP) against gross ecosystem productivity (GPP), i.e. vegetation carbon use efficiency (CUE). However, this kind of study has rarely been reported on a global scale. Using long-term productivity datasets from MODIS and land surface models, we investigated spatiotemporal response patterns of CUE to daytime (Tmax) and nighttime (Tmin) temperature changes across the Northern Hemisphere (>30°N) spanning 2000-2019. Regions exhibiting a positive correlation between Tmax and CUE were extensive throughout the growing season in most northern ecosystems, with a greater magnitude in summer than in spring and autumn. Tmin tended to cause a positive impact on CUE in spring and autumn, while being correlated negatively with summer CUE, resulting in a weak positive relationship between CUE and Tmin. Divergences in the strength and direction of productivity and respiratory responses to diurnal warming lead to spatial and seasonal patterns between CUE and Tmax or Tmin, which is regulated by soil moisture. Our findings provide an in-depth understanding of how the interaction between asymmetric diurnal warming and water limitations regulates the spatiotemporal variations of vegetation product allocation, which is vital for predicting future carbon dynamics under climate warming.

Keywords: vegetation carbon use efficiency, asymmetrical warming, gross primary productivity, respiration, seasonal compensation, Northern Hemisphere