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Climate and other global environmental changes are major threats to ecosystem functioning and biodiversity. However, the importance of plant diversity in mitigating the responses of functioning of natural ecosystems to long-term environmental change remains unclear. Using inventory data of boreal forests of western Canada from 1958 to 2011, we found that aboveground biomass growth increased over time in species-rich forests but decreased in species-poor forests, and importantly, aboveground biomass loss from tree mortality was smaller in species-rich than species-poor forests. A further analysis indicated that growth of species-rich (but not species-poor) forests was statistically positively associated with rising CO2, and that mortality in species-poor forests increased more as climate moisture availability decreased than it did in species-rich forests. In contrast, growth decreased and mortality increased as the climate warmed regardless of species diversity. Our results suggest that promoting high tree diversity may help reduce the climate and environmental change vulnerability of boreal forests.