Temporal declines in tree longevity associated with faster lifetime growth rates in boreal forests

Abstract

Global change has been linked to significant increases in tree mortality in the world's forests. Reduced tree longevity through increased growth rates has been suggested as one of the mechanisms responsible for the temporal increases in tree mortality, but this idea has not been directly tested. Here we explicitly defined two testable hypotheses: (i) the probability of ageing driven tree mortality increases with global change and (ii) the mortality probability associated with global change is higher for faster growing trees. To test these hypotheses, we examined the temporal changes of tree mortality probability in 539 permanent sample plots monitored from 1960-2009, with ages greater than 100 years at initial censuses, across the boreal region of Alberta, Canada. As expected, we found an overall temporal increase in tree mortality probability, indicating a loss in tree longevity with global change. We also found that trees with faster lifetime growth rates experienced higher temporal increases in mortality probability compared to slower growing trees. An analysis of the responses of tree mortality probability to increasing atmospheric carbon dioxide and temperature and decreases in water availability indicated that increasing atmospheric carbon dioxide and decreasing water availability were the major drivers of declining longevity. Our results suggest that tree longevity may further decline with the expected increase of atmospheric carbon dioxide and decreasing water availability in the region.