The effect of prescribed burning on the drought resilience of Pinus nigra ssp. salzmannii Dunal (Franco) and P. sylvestris L

Key message: Prescribed burning applied after a dry year increased (fall burns) or had no effect (spring burns) on pine resilience, measured as the capacity to reach pre-drought growth levels compared with unburned pines. In fall burns, there was a larger increase in resilience when the impact of drought and burning on pines was low and burning caused a significant release from tree competition. Context: Prescribed burning after a dry year can decrease a tree’s resistance (inverse of growth reduction during disturbance); however, burning can increase resource availability and consequently tree resilience (capacity to reach pre-disturbance growth levels). In addition, the burning season can affect the latewood to earlywood ratio (latewood:earlywood) and thus have an impact on tree-ring density. Aims: To study the effects of two consecutive disturbances (drought and burning) on Pinus nigra spp. salzmannii and Pinus sylvestris in terms of (i) total tree-ring resistance, (ii) total tree-ring, earlywood and latewood resilience, and (iii) post-stress latewood:earlywood. Methods: We selected drought-affected trees (control) and drought-and-burning-affected trees (burned) for tree-ring sampling. For each tree, we measured total tree-ring, earlywood, and latewood widths to determine resilience and resistance indices and calculated pre- and post-stress latewood:earlywood as indicators of tree-ring density. We used linear mixed-effects models to relate these response variables to the pine species, burning characteristics, and competition release. Results: Resistance was higher in control trees than burned trees. P. nigra showed lower resistance than P. sylvestris but higher resilience. Resistance positively influenced resilience. Specifically, as resistance increased, total tree-ring and latewood resilience increased significantly in pines burned during the fall compared with those burned in spring or control pines. In fall burns, the pines’ resilience increased, especially when pines were significantly released from tree competition. In P. nigra, post-stress latewood:earlywood increased after fall burns as pre-stress latewood:earlywood decreased. Conclusion: Prescribed burning can be a valuable management tool for overcoming the effects of a dry year on tree growth by increasing resilience, especially in P. nigra.