Non-native shrub overrides understory assembly rules through ecosystem engineering

Non-native plants are increasingly recognized as ecosystem engineers across a wide range of ecosystems. While their impacts on understory composition have been widely documented, consequences for community assembly remain largely unexplored. We investigated the engineering impacts of Sorbaria sorbifolia, a naturalized Rosaceae shrub, on abiotic conditions and understory community assembly in Central European forests. Across 60 plots spanning coniferous and deciduous stands, we quantified light availability, organic layer C/N ratio and thickness, and understory functional diversity along an invasion gradient. Using ordination and linear regression, we found that increasing S. sorbifolia cover reduced light availability, thickened the organic layer, and altered C/N ratio in contrasting, forest-type-specific ways. In coniferous stands, invasion increased C/N and strongly suppressed understory light, whereas in deciduous stands it reduced C/N with weaker shading effects. These changes translate into consistent declines in functional richness and dispersion, particularly in coniferous forests where invasion led to homogenization of the understory. Trait shifts indicated stronger habitat filtering under invasion, with declines in specific leaf area (SLA) and increases in leaf dry matter content and seed mass, reflecting exclusion of light-demanding species and persistence of stress-tolerant taxa and woody seedlings. Remarkably, once S. sorbifolia exceeded ~50% cover, positive relationships between habitat properties and understory functional diversity collapsed, replaced by uniform light limitation. This threshold effect highlights S. sorbifolia as an ecosystem engineer, capable of transforming forest structure and regeneration trajectories. Early detection and management are therefore crucial to prevent long-term homogenization and the formation of novel, invasion-driven forest states.