Drivers of spatial variability in post-fire forest regeneration

This study examines spatial distribution of post-fire forest regeneration in a Mediterranean case study. Wildland fires have a profound and long-lasting effect on forest ecosystems, but Mediterranean forests show adaptations to this natural disturbance, and forest regeneration, i.e. juvenile trees, may onset early after fire. Naturally occurring forest regeneration rarely distributes evenly in burnt areas, leveraging the best micro-sites to germinate and survive; in addition, post-fire biological legacies, especially remnant mature trees, have a pivotal role in determining seed dispersal, and consequently regeneration distribution. In this study we recorded the geographical coordinates with centimetrical precision of all regeneration of Pinus halepensis Mill. (Aleppo pine) in an area burnt first in 2006, and successively in 2015, near Spotorno (Savona, Italy). This pine species is adapted to severe fires thanks to its serotinous cones, specific fruits that preserve seeds from fire heat and release them after fire extinction: this strategy guarantees an early colonisation of burnt areas, although post-fire distribution of regeneration was heterogeneous also for this species. To identify the areas with the highest suitability for post-fire pine regeneration we set up a spatial modelling framework combining environmental predictors and regeneration presence data to feed a Random Forest model calibrated with ecologically weighted pseudo-absences. We observed a direct relationship between sapling occurrence and favorable micro-site conditions (Fig. 1), such as proximity to mature remnant trees, moderate wetness values quantified through the Topographic Wetness Index (Beven & Kirkby, 1979), and high potential solar radiation estimated through the Heat Load Index (McCune & Keon, 2002). The findings add to the literature on post–fire forest regeneration dynamics and shed light on the ecological response of a fire-adapted species to successive burns, showing an early response after the event and unexpected trends which will require special attention.