Climatic resilience to fire of the woody understory in Mediterranean seederand resprouter-dominated forests

Abstract

Fires can reshape Mediterranean forests and landscapes, yet their role in modifying the climatic disequilibrium (CD) of post-fire understory communities, quantified as the difference between species’ climatic optimum and the observed climate, remains unclear. We assessed whether wildfires alter the CD of woody understory communities in forests dominated by two Mediterranean tree species with contrasting regeneration strategies: Pinus halepensis, a post-fire seeder, and Quercus ilex, a resprouter. We used burned plots from the Spanish National Forest Inventory sampled before and after fire, selecting forests clearly dominated by either species and burned once between the two most recent inventories. CD was quantified as the Euclidean distance between observed climate and community-inferred climate. We compared CD before and after fire, assessed changes in woody understory composition, and analyzed the influence on CD changes of fire severity, time since fire, CD peaks, regeneration traits, and forest structure. Q. ilex understories showed higher CD than P. halepensis understories, and wildfires did not significantly modify understory CD in either forest type, although the mechanisms underlying this stability differed. In P. halepensis forests, fire markedly altered understory composition, but post-fire regenerating species had similar climatic preferences. In Q. ilex forests, understory composition showed little post-fire variation, reflecting species persistence. Pre-fire climatic conditions were most relevant in explaining understory CD in P. halepensis forests, whereas forest structure was the most relevant factor in Q. ilex forests. Results suggested that Mediterranean understories are climatically resilient after fire, maintaining CD through compositional persistence or turnover among climatically similar species.