Position in the climatic niche modulates post-fire performance of Pinus halepensis regeneration

Fire is a primary driver in shaping Mediterranean ecosystems, being post-fire regeneration a key process for vegetation dynamics and resilience. However, how climatic variability modulates regeneration success after fire is not fully known. We use Pinus halepensis as a species case study to investigate how local climatic conditions relative to the species climatic niche, together with microsite conditions, influence the performance of post-fire regeneration. We characterized the climatic niche of P. halepensis from its records in the EU Forest Inventory, crossing the geographic occurrences of the species with the respective yearly values of 12 bioclimatic variables for the period 1979-2018. First, we performed a Principal Component Analysis (PCA) with all the extracted bioclimatic values and, subsequently, we applied kernel density estimates to determine the climatic niche edge and optimum within the environmental space defined by the two first PCA axes. We sampled, measured and aged 385 newly established individuals in nineteen sites across Catalonia that burned between 2002 and 2013. Then, we modelled plant height, basal area, crown diameter and proportion of green canopy in relation to the distance to the niche optimum and the dry edge, the cover of neighbouring vegetation, and soil stoniness. As expected, height, crown diameter and basal area were significantly and positively related to recruits' age. Microsite characteristics were significantly related to recruits' performance. Specifically, surface stoniness was negatively and significantly related to tree height, whereas a higher cover of conspecific individuals was significantly related to taller plants with a narrower crown and a lower proportion of green canopy. As such, intra-specific competition appears as an important factor for the performance of P. halepensis post-fire regeneration. Plant height also exhibited a significant, positive relationship with the mean distance to the dry edge of the climatic niche, indicating that recruits performance is sensitive to drought-related conditions after fire. Overall, our study reveals that position within the climatic niche offers a suitable framework to better understand, together with microsite conditions, recruits' performance and thus post-fire regeneration patterns in a fire-resilient species such as P. halepensis.