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Plant Water Use Efficiency Response to the Atmospheric CO2 Concentration is Greater in High Altitude Environments


Intrinsic water-use efficiency of plants (A/g ratio, where A is CO2 assimilation rate and g is stomatal conductance of H2O) quantifies the amount of carbon assimilated per unit leaf area per unit time per unit cost of water. There has been a large body of work showing that intrinsic plant water use efficiency (WUE) increases with increasing atmospheric CO2 concentration. This conclusion has strong implications for quantifying the effects of terrestrial carbon sequestration and plant transpiration under the condition of continuously increasing anthropogenic CO2. Less attention has been given to assessing whether the plant response to the atmospheric CO2 increase differs as a function of environmental variables, such as temperature, precipitation and altitude. One would expect interactions between the CO2 concentration and other environmental variables, and the joint effects on the plant WUE might be different from the effect of CO2 concentration alone. However, these interactions can be quite complicated and difficult to predict; even the sign of response remains uncertain. For example, one would expect that plants growing under a dry climate may benefit from the CO2 increase more than those under wet climate, and thus A would increase more in a dry climate. Stomata density of leaves typically decreases with increasing CO2 concentration, causing g to decrease, and A/g to increase. However, it is not known if stomata density decreases more or less under dry or wet climate conditions. Similar uncertainties or lack of knowledge apply to temperature effects. In this work, we adopt an empirical approach using carbon isotopic ratios in tree rings. Over 50 tree-ring δ13C series are compiled from the literature. The response of δ13C to atmospheric conditions (CO2 concentration and δ13C) is obtained, and the rates of change of the WUE are obtained at several different times between AD 1800 and 2000. These rates are then compared statistically with location’s mean annual temperature, annual precipitation and altitude, in addition to the rate of change in the atmospheric CO2 concentration. The multiple linear regression results show that the majority of the WUE increase is explained by the change in CO2 concentration, and the magnitude of the CO2 effect is close to the theoretically predicted value. The rate of change of WUE has no significant response to either temperature or precipitation. However, there is a significant positive contribution of altitude to the rate of increase of WUE; i.e., the WUE increases faster in high altitude than in low altitude locations. This is probably caused by the fact that high altitude plants are CO2 starved due to low CO2 partial pressure. It is possible that an increase in CO2 concentration induces a greater boost in A in high altitude plants relative to low altitude ones.