Raw data on understory species composition of early-successional oak-hornbeam forests differing in invasive Acer negundo in canopy

Despite growing interest in the impacts of invasive trees, their effects on understory composition remain unevenly explored across taxa and regions. The consequences of invasions by less-studied tree species in temperate forests are still poorly understood. One such species is <i>Acer negundo</i>, a highly recognizable and widespread non-native tree in temperate Europe, but its effects on diversity of forest understories remain unexplored. We examined how increasing <i>A. negundo</i> 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. Using ordination and regression methods, we found that <i>A. negundo</i> negatively affected taxonomic and functional diversity, but a positive impact on phylogenetic diversity of 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. The increased presence of competitive, phylogenetically distinct tall herbs may explain the observed high phylogenetic diversity under dense <i>A. negundo</i> 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 <i>A. negundo</i>, with a particular focus on early-successional forests, which are especially prone to invasion. Furthermore, quantifying the per capita impacts of <i>A. negundo</i> along its invasion gradient may support the identification of ecological thresholds beyond which natural recovery becomes increasingly limited.