Response of plants' water use efficiency to increasing atmospheric CO2 concentration.

This study assesses plants' adaptation to the elevated atmospheric CO(2) concentrations (c(a)) using 83 tree-ring δ(13)C series from the mid- to high-latitudes of the northern hemisphere. We found that the variation of Δ with the atmospheric CO(2) concentration is nonlinear and that the range of Δ change is relatively small. After 1950, the mean increase in Δ is 0.43‰, corresponding to the average coefficient of Δ-c(a) relationship to be about 0.006‰/ ppmv CO(2). In contrast to the changes in Δ, intercellular CO(2) concentration (c(i)) and intrinsic water-use efficiency (W(i)) both increase linearly with c(a). For the past two and a half centuries, changes in the intercellular CO(2) concentration (c(i)) and intrinsic water-use efficiency (W(i)) are, on average, both about 30%, while the mean change of the c(i)/c(a) ratio is 3%. Most changes have occurred after 1950. W(i) responds to c(a) linearly with sensitivities ranging from 0.06 to 0.6 μmol CO(2)/mmol H(2)O ppmv(-1), and an average 0.33 μmol CO(2)/mmol H(2)O ppmv(-1) during the past 50 years. Statistical analysis shows that the increase in c(a) accounts for 98% of the W(i) variation. The remaining small variance is explained by altitude and temperature. Trees at higher elevations show slightly higher increase in W(i), and they are also more sensitive to the CO(2) increase than trees at lower altitudes. Trees growing at low temperature environments are slightly more sensitive to CO(2) increase than those at higher temperature sites. No significant relationship between precipitation and plants' W(i) response to the atmospheric CO(2) increase is found with these data. Although the temperature and altitude both impact the W(i) response to elevated CO(2), the size of the impact is physically small and can be omitted from ecological models.