Divergent Responses of Canadian Boreal Forests to Recent Climate Warming

Abstract:

Climate warming at high latitudes is changing the productivity and function of Canadian boreal forests. Several recent studies have implicated drought stress as the major cause of declining photosynthetic rates and increasing tree mortality in these regions. To investigate the effects of a changing climate on Canadian boreal forests, we analyzed Landsat Thematic Mapper time series (1984-2011) at 46 sites. The study sites ranged in size from 3,500 to 15,000 square kilometers and spanned the entire range of ecoregions and disturbance regimes in the Canadian boreal forest east of the Rocky Mountains. Our results suggest a divergent response of these forests to recent climate warming, with declines in peak-summer vegetation indices (VIs) observed in the dry regions of western and central Canada and increasing VI trends observed in more humid eastern regions. Further, we show a strong relationship between the timing of disturbance and the magnitude and direction of VI trends in disturbed forests. Positive trends are associated with disturbances that happened at the beginning of (or prior to) the Landsat time series and negative trends are associated with disturbances that occurred in the latter part of the time series. Taken together, these two effects explain the major modes of variability in VI trends in Canadian boreal forests. Future decreases in springtime water availability related to climate change and associated changes to fire regimes have potentially large implications for carbon and water budgets in this geographically extensive and important biome.