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The Effect of Severe Drought on Tree Mortality in Old-Growth Mixed Conifer Forests of the Sierra Nevada California

Released: 2016
Das, A.J. and N.L. Stephenson. 2016. The Effect of Severe Drought on Tree Mortality in Old-Growth Mixed Conifer Forests of the Sierra Nevada California. 2016 Mountain Climate Conference,  Leavenworth, WA. 17-21 October 2016.

    California is experiencing a drought of a severity unprecedented in the instrumental record. Not surprisingly, the drought has had substantial effects on California forests, including large increases in tree mortality, particularly at lower elevations. Some evidence suggests that such droughts may become more common in a changing climate. In that light, the consequences of the current drought may be highly relevant to our understanding the future of forests in the Sierra Nevada.

    We took advantage of long-term forest demography plots, some of which are experiencing highly elevated drought-related mortality, to study the similarities and differences between typical background mortality and mortality during a severe drought. We found that that the size structure of mortality was not substantially different than that of background mortality but that there were marked differences in the species composition of that mortality. Increases in overall mortality were associated with strong increases in biotic attack, and there was evidence of substantial increases in the activity of insects normally considered minor contributors to tree mortality.

    These results suggest that drought mortality likely differs in some respects from those of other disturbances, such as fire, and that previously unexamined biotic factors made lead to unexpected outcomes. In addition, our results may have implications for understanding drought vulnerability, particularly with regard to tree size. We argue that understanding biotic interactions will be critical for improving our ability to forecast future mortality.

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