Monitoring early and carry-over signals of insect outbreaks on an Alpine Scots Pine stand using a multi-proxy approach

Forest monitoring is essential to understand and forecast forest dynamics in a climate-change scenario. High altitude forest stands are affected by temperature and precipitation variability but also by the increase in frequency and severity of abiotic and biotic disturbances. Attack by phytophagous insects is one of the most widespread biotic disturbance agent affecting forests, causing profound effects on carbon balance, species composition and ecosystem functioning; however, the early-warning, immediate and carry-over effects on forest productivity and dynamics are still largely unknown. In this study, we aimed to assess trees susceptibility and quantify the impact of a pine bark beetle (Ips acuminatus Gyllenhal) outbreak on Scots pine (Pinus sylvestris L.) stands in the Alps. We combined dendrochronology and remote sensing analyses. Time series of ring width (RW) were obtained from 97 living trees and 58 dead standing trees of P. sylvestris collected at three sites in the outbreak area in the Boite Valley (Eastern Dolomites). In parallel, we detected anomalies in satellite-based Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) related to insect attacks and identifying past changes in photosynthetic activity across the valley. We identified outbreaks as distinctive years with reduced RW, without being directly related to climate but matching with years of parallel reduction in NDVI and NDWI. Dead trees showed a consistent growth declines prior to the outbreak occurrence and presented lower resistance, recovery, and resilience index compared to the surviving trees. All sites presented a significant increase in NDVI and NDWI after the outbreak period. Our results highlight the possibility of reconstructing the spatio-temporal dynamics of insect attacks and identify the parameters (e.g. growth resilience) that can be used to better forecast tree mortality and the recovery phase. This multi-proxy approach allows us to detect early warning signal of tree mortality, to assess the long-term effects of insect attacks on forest productivity and to define adaptive management strategies to cope with incoming climate change scenarios.