Weakening amplification of grassland greening to transpiration fraction of evapotranspiration over the Tibetan Plateau during 2001-2020

Evapotranspiration plays a key role in regulating the water cycle in terrestrial ecosystems. Transpiration fraction to evapotranspiration (TF) characterizes the role of vegetation function in evapotranspiration and is a pivot feature for the interactions between soil, ecosystem, and the atmosphere. Generally, the amount of vegetation has the most direct impact to TF, but the sensitivity of TF to vegetation change is not known, which could be a metric to evaluate the role of greening in land-atmosphere water transportation. To address this key knowledge gap, we used remote sensing observations of leaf area index (LAI) as a proxy for vegetation amount and derived the sensitivity of TF to LAI (θ = ∂TF / ∂LAI ) for the grasslands over the Tibetan Plateau (TP). Results showed there is a substantial decline of θ over the TP during 2001–2020, decreasing from over 4% m⁻² m² to below 1.5% m⁻² m². To analyze the drivers of θ, we introduced a diagnosis framework for soil-atmosphere hydraulic stress shifts based on trends in soil moisture and atmospheric vapor pressure deficit. Results implicated the θ decline over the TP is likely not induced by the dry-wet balance shifts by soil and atmosphere, nor by the vegetation's stomal conductance. Instead, we speculate that the θ decline is induced by a marginal damping effect in case the TF rise is approaching to grassland's limit over the TP. Our finding appeals future studies on terrestrial evapotranspiration to consider the role of vegetation in controlling transpiration and emphasize that greening may not lead to higher TF.