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Experimental warming in situ suggests that warming could lead to a loss of biodiversity. However, species that remain in situ and experience climate change will interact with species tracking climate change, which could also affect patterns of biodiversity. The relative contribution of species gains and losses to net changes in species richness is still unclear. We use transplanted plant communities to test the hypothesis that both the change in climate and ecological communities tracking climate change will influence how species richness responds to climate change. Three intact alpine plant communities were reciprocally transplanted to create scenarios in which species experienced warmer and wetter conditions (transferred to lower elevations) and cooler and drier conditions (transferred to higher elevations) over 10 years on the Tibetan Plateau. Communities transplanted into the same elevation as controls represent species tracking climate change. Transferring to lower elevations generally caused a net increase in richness and a higher rate of gains relative to the control plots; the magnitude of this effect depended on the specific elevation. Transferring to higher elevations lead to either net increases or decreases in richness and gains, depending on elevation. Species gains predicted much more variation in changes in species richness (50%) than did species loss (9%). Species richness at the receptor site and the donor site were both important predictors of variation in species richness, and the abiotic environment did not explain additional variation. Changes in cover of dominant plant species in response to transfers did not predict changes in species richness, species gain or species loss. Our results suggest that species gains from species tracking climate change at the receptor sites, rather than species loss from the donor sites, predicted changes in species richness. Synthesis. Warming experiments with physical barriers to dispersal may overestimate the negative effect of warming on plant diversity by not accounting for species gains. Our study highlights the importance of biotic factors in addition to the abiotic environment, when considering how climate change will affect plant diversity.