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 Sorbaria sorbifolia, 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 S. sorbifolia 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 S. sorbifolia 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 S. sorbifolia cover exceeded ~50%, positive relationships between habitat properties and functional diversity collapsed, giving way to uniform light limitation. This finding underscores S. sorbifolia’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.