Aridity‐dependent regulation of water‐use efficiency by leaf vein traits across grassland communities

Abstract Water transport is a fundamental constraint on plant growth in arid regions. Leaf vein architecture regulates carbon assimilation and transpiration through leaf hydraulic conductance ( K leaf ), thereby influencing ecosystem water‐use efficiency (WUE) via community‐level hydraulic integration. However, most studies have focused on species‐level traits, and how community‐level leaf vein traits vary along aridity gradients and regulate ecosystem WUE remains poorly understood. We selected 20 grassland sites along aridity gradients across the Mongolian Plateau (MP) and Tibetan Plateau (TP), measured leaf vein traits of 185 species, and estimated K leaf using a modelling approach. Community‐weighted trait analyses were conducted to examine variation in leaf vein traits along aridity gradients and their relationships with ecosystem WUE. On the MP, community‐level leaf vein traits exhibited a nonlinear threshold response to aridity, with a breakpoint at ~0.8. Under less arid conditions, vein length per area (VLA), vein diameter (VD) and vein area index (VAI) declined with increasing aridity, whereas vein efficiency (VE) remained relatively stable, and K leaf explained most variation in WUE. Under more arid conditions, VLA and VE increased, VAI remained stable, and aridity became the dominant driver of WUE variation. This transition coincided with a shift in community composition from high vein‐investment, drought‐tolerant perennial species to low‐investment, high‐efficiency, drought‐escaping annual species. In contrast, on the TP, where low‐temperature constraints are stronger, all traits declined linearly with increasing aridity, with no evident threshold response. These findings indicate that the regulation of WUE by leaf vein traits and K leaf is aridity‐dependent. Our results provide community‐level evidence for grassland responses to climate change and offer guidance for improving arid‐region ecosystem models. Read the free Plain Language Summary for this article on the Journal blog.