1. Forests play a strong role in the global carbon cycle by absorbing atmospheric carbon dioxide through increasing forest biomass. Understanding temporal trends of forest net aboveground biomass change (ΔAGB) can help infer how forest carbon sequestration responds to on-going climate changes. Despite wide spatial variation in the long-term average of climate moisture availability (CMIaverage) across forest ecosystems, temporal trends of ΔAGB associated with CMIaverage remains unclear. 2. We tested the hypothesis that the extent of negative impacts of climate change on ΔAGB would decrease with CMIaverage using the data from permanent sample plots of varying ages from 17 to 210 years, monitored from 1958 to 2011 in western boreal forests of Canada. 3. We found that ΔAGB on average increased with CMIaverage. Temporally, ΔAGB declined sharply between 1958 and 2011 in plots with low CMIaverage owing to increased biomass loss from mortality accompanied by little growth gain, whereas ΔAGB changed little in plots with high CMIaverage. The temporal decrease of ΔAGB in drier areas was attributable to its negative responses to warming-induced temporal decreases in climate moisture availability. 4. Synthesis. Our results indicate that large-scale changes in forest carbon functioning associated with climate change depend on the long-term average of climate moisture availability. Our finding suggests a possible retreat of boreal biome at the drier distribution limits with predicted declines in water availability in the 21st century.