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. • Defoliation reversed or neutralized typical climate-growth relationships. • Biogeography shaped how defoliation changed typical climate-growth relationships. • In water-limited regions, defoliation reduced tree growth sensitivity to warming. • In temperature-limited regions, defoliation increased tree vulnerability to drought.