Stronger influence of growth rate than severity of drought stress on mortality of large ponderosa pines during the 2012-2015 California drought event

Friday, December 13, 2019 - 13:40
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In recent decades, climate warming has had negative impacts on forest health in the western United States. Here we report on tree-ring growth and carbon isotope discrimination (∆13C) of ponderosa pines (Pinus ponderosa) in the southern Sierra Nevadas near the epicenter of the 2012-2015 California drought. During this event, severe drought stress was exacerbated by an epidemic-scale outbreak of western pine beetle (Dendroctonus brevicomis), resulting in widespread mortality of ponderosa pine. Mortality events of this scale have substantial and widespread effects on forest structure and can have disastrous consequences for local and regional economies for decades. A better understanding of how trees have responded to severe drought events may allow us to better mitigate future large-scale mortality events via forest management. Toward this end, we compared pairs of large surviving and beetle-killed ponderosa pines following the drought to evaluate physiological characteristics related to survival. Our ∆13C records showed that all sampled trees experienced severe drought stress during this event, and that sensitivity to drought-related climate variables has increased over the past century as the climate has warmed. However, there were no detectable differences in ∆13C or climate sensitivity between surviving and beetle-killed trees. Surviving trees had higher growth rates than beetle-killed trees and grew in plots with lower ponderosa pine basal area. These results indicate that variation in drought stress had little influence on local, inter-tree differences in growth rate and survival of large ponderosa pines during the 2012-2015 drought, likely due to the unprecedented severity of stress experienced by all trees. Although previous studies have shown that large trees are more likely to be targeted for bark beetle attacks than small trees, our data suggest that among large ponderosa pines, those within in the upper echelon of growth rates within a stand and across a landscape were targeted less and/or were more resistant to bark beetle attacks. Therefore, silvicultural strategies emphasizing lower tree densities and the protection of faster-growing trees could provide greater resistance and resilience to future drought events and bark beetle attacks.

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