Abstract
The critical soil moisture threshold (SMth) of plant water stress distinguishes whether vegetation is in an energy-limited or water-limited regime. Accurate quantification of SMth for grasslands holds significant implications for drought monitoring and efficient grassland management. However, our understanding of SMth within global grasslands remains limited. In this study, we investigated the relationship between SMth and environmental factors and mapped the global SMth for grasslands based on the estimated SMth across FLUXNET sites. Firstly, SMth was identified and estimated for 18 distinct grassland sites. Then, the factors significantly affecting SMth were investigated and the global SMth for grasslands was predicted using an empirical linear model. Our results showed that SMth was significantly positively correlated with mean annual soil moisture (SM), mean annual precipitation (PRE), and maximum annual LAI (LAImax), while negatively correlated with vapor pressure deficit (VPD) and shortwave radiation (RAD). These factors collectively constrain SMth well, with the coefficient of determination of 0.79. The estimated mean SMth for global grasslands from 2000 to 2021 is 27.8%, with a range from 1.3% to 55.2%. The SMth values are lower in the northern hemisphere than that in the southern hemisphere. The highest values of SMth, exceeding 40.0%, are mainly distributed in Brazil, Peru, Southern Europe, New Zealand and the Tibetan Plateau. In contrast, the lowest values, failing below 20.0%, are mainly distributed in Northern Canada, Greenland, Central African Savannah and Kazakhstan. The SMth is markedly lower in the arid region than that in the tropical and temperate regions. The quantified SMth in this study could provide a direct way to determine whether and to what extent grasslands are subjected to water stress, which would greatly facilitate the study of drought or water stress effects on vegetation growth for grassland ecosystems.
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