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In the present study, we attempt to determine whether forest water-use efficiency (Wei) increases with atmospheric carbon dioxide concentrations ([CO2]) in tropical and subtropical regions by using long-term eddy covariance observations in three old-growth forests in China.We find a significant decreasing trend of Wei in subtropical forests and an increasing, but not significant, trend in tropical forests.Thus, the hypothesis that forest Wei increases as [CO2] rises in subtropical and tropical forests cannot be confirmed.Nevertheless, the hypothesis remains of interest, because the confounding effects of other environmental or biotic factors could not be ruled out.More data are needed to fully test the proposed relationship.