How does Acer negundo invasion shape plant species diversity in the understory of early oak-hornbeam forests in proximity to a primeval ecosystem?

The impacts of numerous non-native trees on temperate forest understories are still poorly quantified, despite growing interest in invasion ecology. Acer negundo exemplifies this gap as one of the most widespread invasive trees in Europe, with little evidence on how it alters understory diversity and composition. We examined how increasing A. negundo cover affects taxonomic diversity, phylogenetic relatedness structure and community assembly processes in the understory of early-successional oak-hornbeam forests embedded within the Białowieża Primeval Forest. Ordination and regression analyses revealed that A. negundo suppressed both taxonomic and functional diversity, while enhancing phylogenetic diversity in the understory. A decline in species richness was accompanied by a shift from niche differentiation and habitat filtering towards stronger interspecific competition, reflected in the loss of light-demanding species and an increase in nutrient-demanding taxa. A higher presence of competitive, phylogenetically distinct tall herbs may explain the observed high phylogenetic diversity beneath dense A. negundo canopy. However, from a successional perspective, this may hinder the colonization of understory by typical forest specialists and impair the native trees self-regeneration abilities. Our findings highlight the need for early intervention to control the spread of A. negundo, with a particular focus on early-successional forests, which are especially prone to invasion. Furthermore, quantifying the per capita impacts of A. negundo along its invasion gradient may support the identification of ecological thresholds beyond which natural recovery becomes increasingly limited.