Determinants of tree regeneration in degraded temperate floodplain forests under recent passive conservation

Temperate floodplain forests are biodiversity hotspots that offer significant ecosystem services, although being negatively affected by river regulation, land-use change, novel tree diseases and climate change. Decades of hydrological modification and forest management have resulted in ‘novel floodplain forests’ with so far unclear effects on tree regeneration. Along the River Isar (S Germany), north of Munich, a large forest reserve (2312 ha) was established in 2020. This area allows for observing natural regeneration under unmanaged yet highly altered conditions. Across 122 stratified plots, we assessed tree regeneration in relation to abiotic factors (light, soil moisture) and biotic interactions (mature trees, structural cover by understory vegetation and deadwood, browsing). The study aims (i) to establish a baseline of regeneration dynamics and (ii) to identify the species-specific drivers of regeneration success. The results revealed distinct regeneration patterns: Species such as Alnus, Tilia and Picea benefit from conspecific overstory composition, while Fraxinus prefers heterospecific stands, and regeneration of Acer and Ulmus is largely independent of existing stands. Light availability above 2 m height had limited influence, while regeneration declined with increasing structural cover, both with species-specific variation. Soil moisture, likely due to its low variability, showed no measurable effect. Relative abundance of Fraxinus declined with increasing height classes; it also experienced the highest browsing rates. In contrast, Acer increased with age, indicating a shift in species dominance during regeneration. These findings reveal how biotic and abiotic factors shape regeneration in degraded temperate floodplain forests, resulting in a shift from stands dominated by F. excelsior toward A. pseudoplatanus dominance. Thus, further interventions alongside passive conservation may be necessary in hydrologically modified floodplain forests to restore a resilient and diverse near-natural regeneration. • Ash regeneration prefers heterospecific neighbours, alder favours conspecifics. • Maple regeneration is evenly distributed across forest neighbourhood types. • Light availability shows little effect on floodplain forest regeneration. • Understory vegetation strongly shapes tree regeneration patterns. • Maple may be overtaking ash in dominance in modified floodplain forests.