Effects of wildfire on growth, transpiration and hydraulic properties of Pinus pinaster Aiton forest

In the last decades, forests have been seriously threatened by the effects of climate change. In Mediterranean regions, increasing fire occurrence and drought have a substantial impact on forests, possibly pushing them towards tipping points. Indeed, although trees could survive to fire events of medium severity in the short term, severe defoliation and environmental stressors could affect the capacity to transport water and to fix and use carbon, amplifying the possibility of a delayed post fire mortality. Therefore, it is urgent to better understand trees eco-physiological processes in response to wildfire in order to plan suitable forest management strategies. In this research an innovative continuous monitoring system of sap flow and micro-stationary climatic conditions of trees was combined with dendro-anatomical analyses and xylem cavitation vulnerability studies to assess fire impact on a Pinus pinaster forest affected by a severe fire and compared to an unburned control site. Xylem analyses showed that the forest-fire did not influence the xylem hydraulic parameters and did not increased vulnerability to embolism. However, burned trees with high defoliation rate, presented reduced assimilation capacity and carbon availability for growth, as well as a limited development of thicker xylem conduit walls compared to unburned trees. Furthermore, continuous monitoring highlighted an increase in sap flow of defoliated trees compared to the control ones during the hottest and driest months of the year indicating a higher stomatal conductance. Overall, our results suggested that although fire did not affect the xylem hydraulics properties of the vascular system, the severe crown damage caused growth reduction and transpiration dysfunction in the monitored period. Those effect could negatively affect the survival of burned trees, especially in areas subjected to hot and dry periods.