Long-term monitoring of sap flow and dendro-anatomy indicate increase mortality risk in fire-damaged Pinus pinaster Aiton forests of southern Italy

Forest fires are becoming more frequent and intense due to climate change, leading to serious impacts on forest ecosystems. Beyond the immediate damage, they can trigger complex and long-lasting effects on forest health, leading to episodes of tree mortality that can occur even years after the fire event. Understanding these long-term processes and identifying early warning signs of post-fire mortality is a major challenge for forest management in the context of global warming.  Multidisciplinary approaches, such as dendro-anatomy and high-resolution sap flow monitoring, have emerged as key tools for studying these processes. Dendro-anatomy provides data on growth and structural changes in xylem, while sap flow monitoring offers insights into water and carbon use dynamics in trees.  In our study, we focused on a Pinus piaster Aiton forest in the Vesuvio National Park, southern Italy, which was impacted by a fire in 2017. Since 2021, we have been monitoring dominant trees in both a burned area and an adjacent control site using Tree Talker devices that measure daily sap flow and microenvironmental variables. Our goal is to follow the post-fire dynamics of tree sap flow and integrate these data with dendro-anatomical analyses of wood cores to assess the effects of fire on tree growth and xylem function in the years. This approach aims to allow the identification of potential signals of decline that could precede tree mortality. Preliminary results of sap flow monitoring showed different eco-physiological responses between burned and control trees. In the years immediately following the fire, burned trees exhibited increased sap flow, suggesting a strategy of increased stomatal opening to counteract carbon starvation caused by severe defoliation. However, in the last years, sap flow has decreased, falling below control site levels, indicating a progressive physiological decline, that could suggest these trees have entered a critical phase, approaching a tipping point. Although dendro-anatomy analyses are ongoing, we expect to observe reduced growth and alterations in xylem functionality in burned trees that could corroborate the observed eco-physiological trends and provide further insights into carbon reserve depletion and mortality thresholds. This case study aims to provide an integrated view of the long-term eco-physiological processes in tree species hit by fire, offering valuable tools for adaptive forest management in the face of climate change.