Drought-induced die-off triggers species composition shift at warmer and drier edges of climate gradient in Pinus sylvestris forests of Catalonia (NE Spain)

Abstract

Climate change is increasing the frequency and intensity of drought episodes, with raising forest die-off events globally. Impacts of drought on tree growth and mortality have been extensively investigated, but studies on regeneration after these episodes and their consequences for tree diversity and potential plant composition shifts are still scarce, especially analysing climate gradients. We analyse the impact of drought-induced die-offs on woody plant diversity in Pinus sylvestris forests in Catalonia (NE Spain), considering local temperature and precipitation effects. We compared plots affected by die-off to non-affected ones, surveying tree layers in 2013, 2017, and 2022, and tree recruits in 2022. Die-off plots showed greater richness and diversity in tree layer after 5 and 10 years of die-off, particularly in drier sites, while higher evenness was observed in warmer sites. Such pattern indicated a vegetation shift towards a Mediterranean-type forest, with increasing ingrowths of Quercus , Acer , Prunus , and other broadleaf trees which incorporated to tree layer, potentially replacing dead P. sylvestris . In die-off plots, species richness and evenness of larger recruits were 36% and 42% higher, respectively, than in non-affected ones. Species in tree layer and recruiting communities showed a 29% higher similarity in die-off plots than in non-affected ones, supporting that die-off provide better opportunities for recruitment by incorporation into the tree canopy and successful establishment of new recruits. These results indicate that drought-induced die-off is triggering forests transition towards a more adapted plant composition to climate change concurrent with ongoing local climate conditions.

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