Data on habitat properties, species and functional composition of understory assemblages depending on two non-native Prunus ssp. invasion

Non-native trees contribute to biotic novelty in forest ecosystems, but their combined impacts on understory assembly remain insufficiently understood. We examined how two congeneric invaders, Prunus serotina and Prunus virginiana, influence habitat properties and understory community structure when occurring separately and together. We established 60 vegetation plots across four canopy variants including stands dominated by each species, mixed stands, and native reference forests. We quantified canopy openness, litter and understory biomass, soil properties, and assessed taxonomic, functional, and phylogenetic diversity of the understory. Prunus virginiana imposed stronger effects on habitat properties than P. serotina. Its dense canopy reduced light availability and was associated with higher litter accumulation and thicker organic layers. Under its dominance, understory composition became more functionally similar and phylogenetically clustered. In contrast, P. serotina supported more diverse assemblages with higher functional richness and dispersion. When both species co-occurred, understory structure closely resembled that observed under P. serotina. Species composition, functional diversity, and phylogenetic structure showed no further reduction relative to this variant, indicating a weakening of the stronger effects of P. virginiana in mixed stands. This pattern contrasts with the invasional meltdown hypothesis, predicting that interactions among non-native species should amplify their impacts on ecosystem functioning. Across all invaded stands, shifts in functional traits indicated consistent filtering of plant strategies, with both species shaping understory communities in a similar way regardless of species identity. At the same time, differences among canopy variants show that the magnitude of these effects depends on how each species engineers habitat properties and alters understory functional composition. Thus, co-occurrence of congeneric invaders may not necessarily intensify understory reorganization. Instead, it may produce outcomes reflecting weaker engineering influence, leading to non-additive and less predictable effects on forest ecosystem functioning.