The response of marginal water use efficiency to drought stress

Yuanzhizi Deng
College of Urban and Environmental Sciences, Peking University, 100 Zhongguancun North Street, Beijing, China

Marginal water-use-efficiency (λ) describes the increase in carbon assimilation per unit increase in water loss, and is a key metric for modeling stomatal behavior since its introduction by Cowan and Farquhar in late 1970s. The stomatal optimization theory posits λ as constant on short timescales, yet it may vary on longer scales due to soil drying or ecosystem structural changes. The extent to which λ changes across space and time and its link to other plant hydraulic traits remains unclear. Here we estimated daily-scale λ based on the ecosystem-scale underlying WUE using eddy covariance data, and characterize its trend with decreasing soil water. We find that λ's sensitivity to drought increases as soil moisture declines for most ecosystems. Forest ecosystems exhibit a stronger increase in λ compared to non-forest ecosystems under water-limited conditions. Along the climatic gradients, the increase in λ is more pronounced in drier and warmer regions for non-forest ecosystems. Humid areas and regions with higher tree coverage tend to have higher λ values. However, the degree of this increase and its progression during periods of drought vary among ecosystems, potentially linked to their distinct water use strategies. We discussed the connection between the drought response patterns of λ in various ecosystem types and the water use strategies of plants, as well as the potential for associating λ with plant hydraulic traits to predict isohydricity. Our study highlights the necessity to take temporal variations of λ into account to predict stomatal behaviors in response to drought.