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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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The hazard components of representative key risks. The physical climate perspective
The framework of Representative Key Risks (RKRs) has been adopted by the Intergovernmental Panel on Climate Change Working Group II (WGII) to categorize, assess and communicate a wide range of regional and sectoral key risks from climate change. These are risks expected to become severe due to the potentially detrimental convergence of changing climate conditions with the exposure and vulnerability of human and natural systems. Other papers in this special issue treat each of eight RKRs holistically by assessing their current status and future evolution as a result of this convergence. However, in these papers, such assessment cannot always be organized according to a systematic gradation of climatic changes. Often the big-picture evolution of risk has to be extrapolated from either qualitative effects of “low”, “medium” and “high” warming, or limited/focused analysis of the consequences of particular mitigation choices (e.g., benefits of limiting warming to 1.5 or 2C), together with consideration of the socio-economic context and possible adaptation choices. In this study we offer a representation – as systematic as possible given current literature and assessments – of the future evolution of the hazard components of RKRs. We identify the relevant hazards for each RKR, based upon the WGII authors’ assessment, and we report on their current state and expected future changes in magnitude, intensity and/or frequency, linking these changes to Global Warming Levels (GWLs) to the extent possible. We draw on the assessment of changes in climatic impact-drivers relevant to RKRs described in the 6th Assessment Report by Working Group I supplemented when needed by more recent literature. For some of these quantities - like regional trends in oceanic and atmospheric temperature and precipitation, some heat and precipitation extremes, permafrost thaw and Northern Hemisphere snow cover - a strong and quantitative relationship with increasing GWLs has been identified. For others - like frequency and intensity of tropical cyclones and extra-tropical storms, and fire weather - that link can only be described qualitatively. For some processes - like the behavior of ice sheets, or changes in circulation dynamics - large uncertainties about the effects of different GWLs remain, and for a few others - like ocean pH and air pollution - the composition of the scenario of anthropogenic emissions is most relevant, rather than the warming reached. In almost all cases, however, the basic message remains that every small increment in CO2 concentration in the atmosphere and associated warming will bring changes in climate phenomena that will contribute to increasing risk of impacts on human and natural systems, in the absence of compensating changes in these systems’ exposure and vulnerability, and in the absence of effective adaptation. Our picture of the evolution of RKR-relevant climatic impact-drivers complements and enriches the treatment of RKRs in the other papers in at least two ways: by filling in their often only cursory or limited representation of the physical climate aspects driving impacts, and by providing a fuller representation of their future potential evolution, an important component – if never the only one – of the future evolution of risk severity.
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- Award ID(s):
- 2103754
- PAR ID:
- 10499595
- Publisher / Repository:
- Elsevier B.V.
- Date Published:
- Journal Name:
- Climate Risk Management
- Volume:
- 40
- Issue:
- C
- ISSN:
- 2212-0963
- Page Range / eLocation ID:
- 100516
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.crm.2023.100516
