Plant diversity decrease and directional species turnover induced by shifting overstory dominance in the oak-hornbeam forest reserve over 50 years

• Long-term vegetation changes in the Krakovo forest reserve (Slovenia). • Quercus robur strongly declined in the upper tree layer. • Shade-casting tree Carpinus betulus significantly expanded in the lower tree layer. • Herb-layer decrease in species richness, accompanied by taxonomic homogenization. • A directional shift towards shaded and cooler conditions in the understory. Due to a long history of various anthropogenic pressures, lowland forest reserves are scarce and lack historical datasets suitable for analysing long-term vegetation trends. This study investigated changes in plant diversity and species composition in the long-untouched Krakovo forest reserve, the only preserved remnant of lowland oak-hornbeam forests with primary status in Slovenia. The original vegetation sampling was conducted in the 1970s. In 2024, we resurveyed 30 semi-permanent plots capturing the natural variation in soil moisture gradient. We found the decline of Quercus robur in the upper tree layer and the expansion of shade-casting tree Carpinus betulus in the lower tree layer, a pattern mainly attributed to the lowering of the groundwater table. This shift in overstory dominance manifested in a directional change of herb-layer composition associated with high species turnover (61 %). The deterioration of light conditions at the forest floor resulted in significant decrease of species richness, accompanied by the process of taxonomic homogenization. The analysis of Ellenberg indicator values revealed a reorganization of the plant communities in response to the increasingly shaded and cooler understory environment, benefitting a limited number of perennial herbs preferring closed canopies. The change towards nutrient-demanding species was probably caused by improved litter quality, nitrogen input from agricultural areas and nutrient release from accumulated deadwood. In terms of soil moisture, the reserve still exhibits a mosaic of interchanging distribution between wetter and more mesic habitats. However, given that oak natural regeneration is very poor, we anticipate even more drastic vegetation changes in the future.