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Key messageThis work studied the response of tree growth to the increased drought and carbon dioxide in semi-arid areas of northwestern China.AbstractForests ecosystems play a key role in the global carbon cycle. However, effects of increased drought and rising atmospheric carbon dioxide (C-a) on it are complex and there are many uncertainties. In this study, we evaluated this issue through dendroecology in semi-arid regions of northwest China. Results found the basal area increment of trees decreased by 28% (from 7.59 to 5.43cm(2)year(-1)) as the intrinsic water use efficiency increased 32% (from 98.61 to 130.28 mu molmol(-1)). Despite both C-a and drought positively affected intrinsic water use efficiency, they did not promote tree growth. Meanwhile, values of intercellular [CO2] increased 14% (from 158.83 to 180.89ppm) while the ratio of intercellular [CO2] to C-a decreased 7% (from 0.50 to 0.46). Our study advances understanding of the variability of the global carbon cycle under climate change.