worldwide, forest ecosystem functioning and plant diversity have been altered by global environmental change. understanding the relationship between biodiversity and ecosystem functioning with long-term environmental change is important because maintaining diversity can mitigate the impacts of environmental change on ecosystem functioning. here, i i) developed a concept that can elucidate how enhancing plant diversity may help mitigate global environmental change impacts on ecosystem functioning; ii) empirically tested this concept in natural forest systems by examining whether higher tree diversity enhances and reduces positive and negative impacts of long-term environmental change on forest biomass dynamics (biomass growth, loss, and net biomass change); iii) explored effects of spatial variations in climate on the relationship between tree functional diversity and forest biomass dynamics; and iv) investigated how spatial variations in climate mediate the impacts of long-term environmental change on tree functional composition. in order to establish the concept of how tree diversity can mitigate the impacts of global environmental change on forest ecosystem functioning, i reviewed the field of climate change effects on biodiversity-ecosystem functioning. i found mixed evidence for positive diversity effects on ecosystem functioning persistent before and after experiencing changes in climates within grassland communities, but strong support in the few studies conducted in forest ecosystems which are more stable and resilient at higher levels of diversity. i identify the importance of future research combining investigations into climate change impacts on ecosystem functionality with the b-ef. i concluded that biodiversity can hold certain potential to be a solution to mitigate environmental change impacts. using inventory data of boreal forests of western canada from 1958-2011, i revealed that aboveground biomass growth of species-rich forests increased with the calendar year but that of species-poor forests decreased. moreover, species-rich forests experienced less aboveground biomass loss from tree mortality than species-poor forests. i found that the growth of species-rich forests, but not species-poor forests, was positively associated with elevated co2. mortality in species-poor forests increased more with decreasing water availability than species-rich forests. in contrast, growth decreased, and mortality increased as the climate warmed regardless of species diversity. the results of this study suggest that promoting high tree diversity may help reduce the climate and environmental change vulnerability of boreal forests.