Climate Offsets CO ₂ Increase as the Main Driver of Tree Growth in Arid and Semi‐Arid Northern China

Abstract The combined contribution of CO 2 fertilization and climate variability to arid and semi‐arid forest growth remains unclear. To disentangle these multiple influences, we used a preexisting process‐based ecophysiological model (MAIDENiso) to simulate tree growth changes during 1956–2010 in arid and semi‐arid regions of China. Results revealed that simulated tree growth was more dependent on climate trends than on atmospheric CO 2 concentration. Mechanistic analysis showed that the regulation of stomatal conductance under water stress positively affected tree growth in the arid region, but had an opposite pattern in the semi‐arid region. Intrinsic water‐use efficiency (iWUE, measured from tree‐ring δ 13 C) has increased by 29% and 44% since 1900 CE in the arid and semi‐arid regions, respectively, but did not stimulate radial tree growth. This suggests that there will possibly be a continued increase (decrease) of radial forest growth in arid (semi‐arid) areas of northern China if the current climate trends remain in the next decades.