Estimation of Soil Moisture Thresholds for Aggravation of Global Terrestrial Carbon Uptake Losses

Abstract

Most ecosystems have resistance to soil moisture (SM) deficit, which is termed drought resistance. Drought resistance can be invalid and global terrestrial carbon uptake losses can be aggravated when SM deficit exceeds a critical threshold. However, soil moisture thresholds (SMTs) that detrimentally impact global terrestrial carbon uptake are still unclear. We performed numerical simulations using the Community Earth System Model, and estimated the SMTs by the back propagation neural network method for the years 2004–2014. The SMTs represent the inflection point for vegetation changes from high to low drought resistance phase, and terrestrial carbon uptake losses from low to high rate. Soil moisture‐limited ecoregions have higher SMTs than energy‐limited ecoregions, indicating the increased vulnerability and sensitivity of SM‐limited ecoregions to SM deficit and more easily aggravated terrestrial carbon uptake losses during drought. SMTs varied in different vegetation types and broadleaf deciduous trees displayed the highest SMTs and C3 arctic grasses have lowest thresholds. Humid and high vegetation coverage rate regions have lower thresholds. The SMTs increase with the increase of clay content and the decrease of sand content. In addition, land‐atmosphere feedback caused by SM deficit has a large impact on terrestrial carbon uptake and may be one of the main reasons for the aggravation of vegetation carbon uptake losses. Our results provide a unique perspective for investigating the impact of drought on vegetation.