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Gross primary productivity (GPP) plays an important role in mediating the feedback between land carbon (C) cycle and climate warming. However, knowledge about the mechanisms regulating the response of GPP to warming is still limited. Based on a four-year field manipulative experiment, we examined the warming effects on GPP and the potential mechanisms in a typical alpine steppe on the Tibetan Plateau. Our results revealed that elevated temperature significantly increased GPP over the study period. The warming-induced increase in GPP was driven by the increased leaf area index, which was derived from the increase in aboveground biomass of forb. By contrast, warming had no significant effects on photosynthesis per leaf area, likely due to the offset of warming-induced increase in leaf nitrogen concentration and decrease in soil moisture. These results demonstrate that leaf area rather than leaf photosynthetic rate determines the response of ecosystem productivity to climate warming in this alpine steppe. More studies in other ecosystems are called for to test the observed mechanisms responsible for warming effects on GPP, so as to improve predictions of terrestrial C cycle under changing environment.