Raw data on habitat properties and understory species composition of Central European Scots pine and oak-hornbeam forests, used in the publication: „Non-native shrub overrides understory assembly rules through ecosystem engineering"

This study investigates the ecosystem engineering effects of <i>Sorbaria sorbifolia</i>, a naturalised shrub from the Rosaceae family, on abiotic conditions and understory community assembly in Central European forests. While non-native plants are increasingly recognised for their transformative roles in ecosystems, their influence on community-level processes remains underexplored.We conducted fieldwork across 60 plots located in both coniferous and deciduous forest stands, capturing a gradient of <i>S. sorbifolia</i> invasion intensity. The environmental variables measured included:Light availability (above vs. below canopy),Organic layer thickness,Carbon-to-nitrogen (C/N) ratio in the organic layer,Understory plant composition and functional traits.Using ordination and linear regression models, the study revealed that increasing cover of <i>S. sorbifolia</i> consistently reduced light availability, increased organic layer thickness, and altered C/N ratios in forest-type-specific ways. In coniferous forests, invasion led to higher C/N ratios and stronger shading, while in deciduous forests, it reduced C/N with milder light suppression. These abiotic shifts translated into declines in functional richness and dispersion of understory vegetation, especially in coniferous stands where homogenization was most pronounced. Trait-based analyses indicated intensified habitat filtering under invasion, with reductions in specific leaf area and increases in leaf dry matter content and seed mass—suggesting exclusion of light-demanding species and persistence of stress-tolerant taxa and woody seedlings. A notable threshold effect was observed: once <i>S. sorbifolia</i> cover exceeded ~50%, positive relationships between habitat properties and functional diversity collapsed, giving way to uniform light limitation. This finding underscores <i>S. sorbifolia</i>’s role as an ecosystem engineer capable of reshaping forest structure and regeneration pathways. The study highlights the urgency of early detection and management to prevent long-term ecological homogenization and the emergence of novel, invasion-driven forest states.