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Abstract Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought‐related mortality derived from measurements of tree‐ring growth (ring width index; RWI) and carbon isotope discrimination (∆13C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012–2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆13C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆13C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine‐dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management—particularly in drier regions—may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.
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This content will become publicly available on October 24, 2026
Biogeography modulates insect defoliation effects on climate-growth relationships in pines
Forest ecosystems are increasingly threatened by interacting biotic and abiotic stressors, yet how insect defoliation interacts with climate to influence tree growth remains poorly understood. In this study, we hypothesized that biogeography modulates the influence of defoliation on tree growth responses to climate, with two possible outcomes: defoliation may amplify climate sensitivity and increase vulnerability to stress, or alternatively, it may neutralize or even reverse typical climate–growth relationships. We compared pine forests in four biogeographic regions: two water-limited Mediterranean lowland regions in Portugal, and two temperature-limited regions, one in a Mediterranean mountain in Portugal, and the other in a humid subtropical forest in New Jersey (USA). Using dendroecological techniques, we reconstructed defoliation events, marking the first such reconstruction for Portuguese pine forests, and assessed growth responses to seasonal climate variables. In the absence of defoliation, tree growth aligned with the primary climatic limitation at each site, whether temperature or water availability. However, during defoliation events, these typical climate-growth relationships were disrupted. In temperature-limited regions, defoliation reversed the positive effects of warming and increased vulnerability to moisture stress. Conversely, in water-limited regions, defoliation reduced growth sensitivity to warming and did not increase drought vulnerability, suggesting a buffering effect. These results demonstrate that defoliation can decouple tree growth from climatic drivers, with biogeographically distinct outcomes shaped by underlying environmental constraints. Our findings emphasize the need to integrate biotic disturbances into models of forest climate sensitivity to improve predictions of forest resilience under global change.
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- Award ID(s):
- 2224545
- PAR ID:
- 10659479
- Publisher / Repository:
- Elsevier B.V.
- Date Published:
- Journal Name:
- Forest Ecology and Management
- Volume:
- 599
- Issue:
- C
- ISSN:
- 0378-1127
- Page Range / eLocation ID:
- 123267
- Subject(s) / Keyword(s):
- Dendroecology Outbreaks reconstruction Forest disturbances Mediterranean Humid subtropical Global challenges
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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