The response of ecosystem marginal water use efficiency to soil drying

Marginal water use efficiency (λ) describes the increase in carbon assimilation per unit of water loss. The stomatal optimization theory posits that λ remains stable over short timescales, yet to what extent λ varies across space and time, and how it relates to ecosystem-level characteristics remains unclear. Here, we estimated daily λ using ecosystem-scale underlying water use efficiency (uWUE) derived from eddy covariance (EC) measurements and analyzed its trend along declining soil moisture. We found that λ generally increased with declining soil moisture, and its sensitivity to soil water deficit (λ gradients) also intensified under drier conditions. This response was more pronounced in forest and woody ecosystems compared to non-forest systems. The weakest λ gradients are observed in regions with intermediate aridity, moderate soil texture, lower tree cover, and shallower rooting depth. These different patterns of λ gradients along soil drying can inform the ecosystem-level plant water use strategies, with stronger gradients often corresponding to ecosystems dominated by more isohydric species. Our study highlights the necessity to consider spatiotemporal variations of λ to predict stomatal behaviors in response to drought.