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We compared early seral development between stands subject to single and repeated high-severity wildfire in low-elevation, mixed conifer--hardwood forests in the Klamath Mountains, California, USA. We used a before-after, control--impact (BACI) approach to assess changes in the density of conifer regeneration and the cover of multiple components of vegetation structure (conifers, hardwoods, shrubs, forbs, and graminoids) and compare pathways of seral development between plots that burned once and plots that burned twice. Fifty-three field plots were established 6 years following a high-severity fire in 2004. Nineteen of these plots experienced a second high-severity wildfire 11 years later (2015), and all plots were remeasured in 2016-2017. Conifer regeneration was abundant following the first fire but was greatly reduced by the second fire. Plots that did not reburn increased in conifer, hardwood, and shrub cover, whereas plots that reburned increased in forb cover and decreased in shrub, hardwood, and conifer cover. Despite conifer loss, we found little evidence of shifts to nonforested states following repeated fire due to resilience of resprouting hardwoods. Our results indicate that repeated high-severity fire has the potential to protract early seral development and catalyze transitions from mixed conifer--hardwood forest to hardwood-dominated early seral conditions. Key words: reburn, state change, Douglas-fir, early seral vegetation, hardwood resilience, before-after control-impact (BACI), compound disturbance. Nous avons compare le developpement de peuplements en debut de succession qui ont ete exposes a un ou plusieurs feux de foret de severite elevee dans les forets mixtes de feuillus et de coniferes situees a faible altitude dans les monts Klamath, en Californie, aux Etats-Unis. Nous avons utilise une approche de type avant-apres avec temoin (BACI) pour evaluer les changements dans la densite de la regeneration des coniferes et le couvert de plusieurs composantes de la structure de la vegetation (coniferes, feuillus, arbustes, plantes herbacees non graminoides et graminees) dans le but de comparer les parcours de l'evolution de la succession entre des parcelles qui ont brule une ou deux fois. Cinquante-trois parcelles experimentales ont ete etablies sur le terrain six ans apres un feu d'intensite elevee en 2004. Dix-neuf de ces parcelles ont subi un deuxieme feu d'intensite elevee 11 ans plus tard (2015) et toutes les parcelles ont ete remesurees en 2016-2017. La regeneration de coniferes etait abondante apres le premier feu mais a connu une forte diminution apres le second feu. Le couvert de coniferes, de feuillus et d'arbuste a augmente dans les parcelles qui n'avaient pas rebrule tandis que le couvert de plantes herbacees non graminoides a augmente et celui de la vegetation ligneuse a diminue dans les parcelles qui avaient rebrule. Malgre la perte de coniferes, nous avons trouve peu d'indices d'evolution vers une disparition de la foret a la suite de feux recurrents etant donne la resilience des repousses de feuillus. Nos resultats indiquent que la recurrence de feux d'intensite elevee a le potentiel de prolonger le developpement successionnel et de declencher la transition de la foret mixte de feuillus et de coniferes vers des conditions de debut de succession caracterisees par la dominance des feuillus. [Traduit par la Redaction] Mots-cles : rebrulage, changement d'etat, douglas de Menzies, vegetation de debut de succession, resilience des feuillus, etude experimentale avant-apres avec temoin (BACI), perturbation composee., Introduction Larger and more frequent wildfires are altering landscapes throughout forested regions of western North America (Westerling et al. 2006; Miller et al. 2012; Dennison et al. 2014). Increased stand [...]

Intraspecific variation can be an important driver of ecological interactions in species‐rich communities. Predicting the effects of intraspecific variation in different environments, however, remains a major challenge. This is because we often do not quantify both the effects of functional traits on associated communities and the extent to which trait variation is due to genetics (genotype effects) vs. plasticity (environment effects). As a consequence, the relative importance of trait plasticity vs. genetic variation in structuring associated communities remains unclear.We sought to fill this gap by conducting common garden experiments with the plant Salix hookeriana across biotic (ant–aphid interactions) and abiotic (wind exposure) environmental gradients in a coastal dune ecosystem. In each experiment, we simultaneously measured plant traits and species richness of associated above‐ and below‐ground communities. We then used statistical models to quantify the relative importance of trait plasticity vs. genetic variation in structuring communities.Our major finding was that trait plasticity was more important than genetic variation in determining the number of species in associated communities. This result was consistent across different environmental contexts (experimental manipulations of ant–aphid interactions and wind exposure), multiple years, and for above‐ground arthropods and root microbes. This occurred because the traits that had the largest effect on species richness were also the most plastic.Synthesis. These results indicate that trait plasticity can be a dominant driver of above‐ and below‐ground biodiversity. We conducted multiple field experiments to quantify the relative importance of trait plasticity vs. genetic variation in structuring plant‐associated communities. We found that plasticity in plant traits was more important than genetic variation, a result that was consistent across different environmental contexts, multiple years, and for above‐ground arthropods and root microbes. [ABSTRACT FROM AUTHOR]