Drivers of Understory Plant Diversity in Thermophilous Oak Forests Dominated by Quercus frainetto and Quercus cerris: Topographic, Edaphic, and Stand Structure Factors

Oak forests are important components of temperate forest ecosystems, yet the factors shaping understory plant diversity in Southeast European thermophilous oak forests remain insufficiently understood. This study investigates understory plant diversity in forests dominated by Quercus frainetto and Quercus cerris in the protected area of Kosmaj (Serbia) and examines its relationships with topographic, edaphic, and stand structure factors. Species richness, Shannon diversity, and Pielou’s evenness were used to characterize diversity patterns. Understory diversity was shaped by multiple environmental factors, with stand structure variables showing the highest independent contribution to species richness and Shannon diversity, whereas topographic variables were more closely associated with Pielou’s evenness. Humus content showed positive associations with Shannon diversity and Pielou’s evenness, whereas higher phosphorus and potassium availability showed negative associations with diversity indices, particularly Shannon diversity. Younger stands showed a tendency toward higher species richness, while Shannon diversity showed a similar but weaker pattern, although differences among stand age classes were not significant. Geological substrate was associated with variation in diversity patterns, with limestone showing greater species richness, whereas silicate substrates showed slightly higher Shannon diversity and evenness. These findings highlight that variation in understory plant diversity in thermophilous oak forests reflects the combined influence of stand characteristics, soil conditions, and topographic gradients, providing insights for biodiversity conservation and adaptive forest management through the maintenance of stand structural complexity, soil quality, and environmental heterogeneity.