Species climatic niche explains post-fire regeneration of Aleppo pine (Pinus halepensis Mill.) under compounded effects of fire and drought in east Spain.

Fire and drought are two major agents that shape Mediterranean ecosystems, but their interacting effects on forest resilience have not been yet fully addressed. We used Pinus halepensis to investigate how compound fire-drought regimes determine the success of post-fire regeneration. We measured the density of P.halepensis newly established individuals following fire in forty-three sites along the Spanish east coast, the wetter region of the species distribution. The climatic niche of P.halepensis was characterized by considering their populations across its Spanish distribution range. We used yearly values (1979-2013 period) of accumulated precipitation, mean temperature and the warmest quarter values of these two variables to generate the climatic space or climatic niche occupied by the species. Kernel density estimates were then applied to determine the niche centroid, which would correspond to the species' climatic optimum within its Spanish distribution range. Then, we computed the pre- and post-fire climatic deviations of each sampling site as the difference between site-specific climate conditions respect to the species niche centroid, and assessed their relationship with the success of post-fire regeneration. We found highly variable patterns of post-fire regeneration density of P.halepensis over the studied sites, ranging from 7 to 42,822 tree pines ha-1. Generalized linear models indicated a positive relationship between fire severity and the density of P.halepensis regeneration. Positive temperature deviations - warm conditions - before fire were positively related to pine regeneration. This effect increases under higher fire severity. By contrast, warm temperatures after fire showed a negative effect on the density of pine trees. Positive precipitation deviations - wet conditions - after fire enhanced pine regeneration, while precipitation before fire did not had any significant effect. Though P.halepensis is considered a species adapted to fire and drought, the interaction between these two disturbances can alter the success of its post-fire recovery patterns limiting the species' resilience in the future.

Relationship between heatwave-induced forest die-off and climatic suitability in multiple tree species.

In recent decades, many forest die-off events have been reported in relation to climate-change-induced episodes, such as droughts and heat waves. To understand how these extreme climatic events induce forest die-off, it is important to find a tool to standardize the climatic conditions experienced by different populations during a specific climatic event, taking into account the historic climatic conditions of the site where these populations live (bioclimatic niche). In this study, we used estimates of climatic suitability calculated from species distribution models (SDMs) for such purpose. We studied forest die-off across France during the 2003 heatwave that affected Western Europe, using 2,943 forest inventory plots dominated by 14 single tree species. Die-off severity was estimated by Normalized Difference Vegetation Index (NDVI) loss using Moderate-resolution Imaging Spectroradiometer remote sensor imagery. Climatic suitability at the local level during the historical 1979-2002 period (HCS), the episode time (2003; ECS) and suitability deviance during the historical period (HCS-SD) were calculated for each species by means of boosted regression tree models using the CHELSA climate database and occurrences extracted from European forest inventories. Low HCS-SD and high mean annual temperature explained the overall regional pattern of vulnerability to die-off across different monospecific forests. The combination of high historical and low episode climatic suitability also contributed significantly to overall forest die-off. Furthermore, we observed different species-specific relationships between die-off vulnerability and climatic suitability: Sub-Mediterranean and Mediterranean species tended to be vulnerable in historically more suitable localities (high HCS), whereas Euro-Siberian species presented greater vulnerability when the hot drought episode was more intense. We demonstrated that at regional scale, past climatic legacy plays an important role in explaining NDVI loss during the episode. Moreover, we demonstrated that SDMs-derived indexes, such as HCS, ECS and HCS-SD, could constitute a tool for standardizing the ways that populations and species experience climatic variability across time and space.