Isotopic and anatomical signals for interpreting fire-related responses in Pinus halepensis

Our study aims to define isotopic and anatomical responses to fires of P. halepensis . Main results: decrease in tree growth and relative conductivity and increase in water use efficiency. We investigated the ecophysiological responses of a Pinus halepensis Mill. stand surviving two wildfires in southern France. Basal area, isotope composition and anatomical traits were analysed before and after fires, using tree rings to assess the ecological responses of trees to heat-related damage. The years were determined based on the presence of fire scars. Stable isotopes (delta C-13 and delta O-18) were measured in tree rings before and after the "fire years". Anatomical observations allowed qualitative analysis of the scar region and quantification of tracheid size in tree rings before and after the fire years. Relative and percentage conductivity of earlywood and latewood far from the woundwood were estimated. Results showed a decrease in tree growth after the fire events accompanied by an increase in C-13-derived water use efficiency (WUEi) and a decrease in relative conductivity. The positive relationship between delta C-13 and delta O-18 suggested that both isotopic variations are mostly driven by changes in stomatal conductance following fire events. P. halepensis proved to be a strong isohydric species, able to survive frequent fires with temporary ecophysiological modifications and anatomical adaptations. Our findings afford new insights into post-fire survival strategies of this species in an environment where fires are predicted to increase in frequency during the twenty-first century.