The effects of stand characteristics on the understory vegetation in Quercus petraea and Q. cerris dominated forests

Different types of forest use significantly changed the structure and species composition of European temperate forests. Herbaceous species and seedlings are important parts of the forest ecosystem, thus it is necessary to understand the effects of stand characteristics on the species composition of the understory. In our study we assessed the main factors that affect the species composition of herb and tree seedling assemblages in Quercus petraea and Q. cerris dominated stands (age 50-150 years) in the Bukk Mountains, Hungary. The relationship between the studied assemblages and explanatory variables (tree species composition, stand structure, canopy closure and topography) were explored by Redundancy Analysis (RDA). The occurrence of herbaceous species was affected by canopy closure, stand structure (mean DBH and DBHcv of trees), topography and the density and diversity of shrub layers. Oak forest species were associated with more open stands with sparsely distributed large trees, while mesic forest species were positively associated with heterogeneous stand structure, low shrub density, and western exposure. Seedlings of trees and shrubs showed a dispersal limited phenomenon. The composition of seedlings was significantly influenced by the mean DBH of trees, the structural heterogeneity of the overstory, the tree species diversity and the density of shrub layers. However the seedlings of both dominant oak species required the same stand structure, sessile oak was able to regenerate almost exclusively in those stands where it was dominant in the overstory, which is significant for the management of the species. Generally, forest management affects species composition and structure of the overstory, accordingly it had direct and indirect effects on the understory community as well.