Changes in ecosystem energy and water limitations in the growing season on the Tibetan Plateau

Characterizing the partitioning of water fluxes in terrestrial ecosystems is crucial for understanding the mechanisms of the water cycle under climate change. Ecosystem function on the Tibetan Plateau (TP) depends on the availability of heat and water, yet how climate change will alter ecosystem energy and water limitations on the TP remains largely unclear. Thus, an ecosystem limitation index (ELI) derived from evapotranspiration (ET), surface soil moisture (SM), and temperature (TEMP) was applied to diagnose ecosystem water-energy limitation regimes using eddy covariance observations, remote sensing products, and land surface model simulations. Observations reveal strong spatial contrasts in ET controls across the TP. TEMP dominates ET variability in the humid and cold northeastern TP, whereas soil water availability plays a critical role in semi-arid areas. Even during SM flash droughts, ET in the northeastern sites remains primarily constrained by thermal conditions rather than water availability. Multi-dataset analyses further indicate a two-phase evolution of ecosystem limitation, characterized by increasing water limitation during 1998–2018, largely driven by declining SM. The critical SM threshold separating water- and energy-limited regimes ranges from 0.203 to 0.261 m3 m−3. Land surface model simulations further suggest that vegetation greening strengthens both ELI and the critical SM threshold through modifications of surface energy partitioning, albedo, and soil water availability.