Linking intra-annual density fluctuations to early-warning indicators of drought-driven tree mortality in the NE Iberian Peninsula

In recent decades, drought-driven tree mortality has become widespread, particularly in Mediterranean ecosystems, where prolonged drought accelerates forest-to-shrubland transitions. Detecting early-warning signals of tree decline is therefore essential for anticipating ecosystem changes and guiding conservation. We investigated massive 2023 mortality event in an Aleppo pine population ( Pinus halepensis ) at the Garraf Massif (NE Iberian Peninsula). We compared a set of previously proposed growth-related parameters (long-term growth trends, growth series autocorrelation, growth variability and climatic growth sensitivity) as potential early-warning signals of tree die-off between living and dead trees. We also introduce new parameters: frequency of intra-annual density fluctuations (IADFs), and IADF climatic sensitivity. Our results indicate that early-warning signals based on long-term growth trends, autocorrelation, growth variability, and climatic sensitivity performed poorly, with no significant differences between dead and living trees. Precipitation was the main climatic driver for trees of both statuses. In contrast, IADFs emerged as effective early-warning indicators: living trees exhibited significantly higher IADF frequency and greater climatic sensitivity. This suggests that living individuals had a higher capacity to plastically adjust to environmental conditions, reflected in a bimodal growth pattern. Overall, our findings highlight IADFs as growth-based early-warning signals of tree mortality, mechanistically linked to the individual ability to withstand unfavorable climatic conditions. These results provide new insights into the rapid shifts occurring in the Mediterranean forest–shrubland ecotone and offer a valuable tool for anticipating drought-driven tree decline.