Linking water use efficiency with water use strategy from leaves to communities

Limitations and utility of three measures of water use characteristics were evaluated: water use efficiency (WUE), intrinsic WUE and marginal water cost of carbon gain ( $\partial E/\partial A$ ) estimated, respectively, as ratios of assimilation (A) to transpiration (E), of A to stomatal conductance (g_s ) and of sensitivities of E and A with variation in g_s . Only the measure $\partial E/\partial A$ estimates water use strategy in a way that integrates carbon gain relative to water use under varying environmental conditions across scales from leaves to communities. This insight provides updated and simplified ways of estimating $\partial E/\partial A$ and adds depth to understanding ways that plants balance water expenditure against carbon gain, uniquely providing a mechanistic means of predicting water use characteristics under changing environmental scenarios.