Responses of Marginal and Intrinsic Water-Use Efficiency to Changing Aridity Using FLUXNET Observations

According to classic stomatal optimization theory, plant stomata are regulated to maximize carbon assimilation for a given water loss. A key component of stomatal optimization models is marginal water-use efficiency (mWUE), the ratio of the change of transpiration to the change in carbon assimilation. Although the mWUE is often assumed to be constant, variability of mWUE under changing hydrologic conditions has been reported. However, there has yet to be a consensus on the patterns of mWUE variabilities and their relations with atmospheric aridity. We investigate the dynamics of mWUE in response to vapor pressure deficit (VPD) and aridity index using carbon and water fluxes from 115 eddy covariance towers available from the global database FLUXNET. We demonstrate a non-linear mWUE-VPD relationship at a sub-daily scale in general; mWUE varies substantially at both low and high VPD levels. However, mWUE remains relatively constant within the mid-range of VPD. Despite the highly non-linear relationship between mWUE and VPD, the relationship can be informed by the strong linear relationship between ecosystem-level inherent water-use efficiency (IWUE) and mWUE using the slope, m*. We further identify site-specific m* and its variability with changing site-level aridity across six vegetation types. We suggest accurately representing the relationship between IWUE and VPD using Michaelis-Menten or quadratic functions to ensure precise estimation of mWUE variability for individual sites. Plants use diverse strategies for water utilization during growth. Marginal water-use efficiency (mWUE) quantifies how effectively plants gain carbon relative to the water they lose through their leaves. A scientific debate exists regarding how mWUE responds to dry conditions. To investigate this, we analyze data from various vegetation types worldwide, observing changes in mWUE under dry conditions. Contrary to common assumptions, mWUE is not a constant; it varies substantially based on moisture levels. Additionally, we show that a simpler measure called inherent water-use efficiency (IWUE) can help explain this complicated relationship, which is useful for predicting plant growth under different moisture conditions. The relationship between marginal water-use efficiency (mWUE) and vapor pressure deficit (VPD) is highly non-linear at a sub-daily scale in general Despite the highly non-linear relationship between mWUE and VPD, the relationship can be informed by simpler inherent water-use efficiency (IWUE) We identify the site-specific relationship between mWUE and IWUE and its variability with changing aridity across six vegetation types