Impact of alien tree species Quercus rubra L. on understorey habitat and flora: a study of the Silesian Upland (Southern Poland)

One hundred and one stands of non-native red oak Quercus rubra L. were examined across various forest types with the objective to relate the influence of environmental variables on the abundance of this species and to characterise its impact on the species richness and proportion of functional groups of native plant species. Amongst randomly selected stands more than 50% were self-regenerating ones, i.e. seedlings were present. The cover of Q. rubra seedlings was positively correlated with light intensity (rs = 0.38, P <0.05) and negatively with the content of nitrogen (rs = –0.20, P <0.05) and loss on ignition (rs = 0.20, P <0.05) in soil. The cover of mature trees was positively correlated with the value of pH (rs = 0.22, P <0.05). Separate correlations with DCA scores of phytosociological relevés and the cover of red oak in each layer showed that the species influenced the floristic composition of forest vegetation. The cover of mature trees of red oak negatively affected both the number of understorey species (rs = –0.39, P <0.01) and the cover of shrubs (rs = –0.21, P <0.05) and herbaceous species (rs = –0.22, P <0.05). Q. rubra in the form of a mature tree had a negative influence on the cover of barochores, endozoochores and species of Grime’s CSR strategy. The cover of red oak’s seedlings was negatively correlated with the cover of annual species (rs = –0.21, P <0.05). Saplings of Q. rubra had a negative influence on the cover of dyszoochores (rs = –0.21, P <0.05) and was positively correlated with competitors (rs = 0.31, P <0.01), megaphanerophytes (rs = 0.27, P <0.05) and therophytes (rs = 0.25, P <0.05). The study showed that Q. rubra is a competitive species both as a mature tree and in the form of seedlings or small saplings and thus it can contribute to the reduction of biodiversity in forest communities.