Temporal trends in the invasions of Austrian woodlands by alien trees.

Invasion of different habitats differs greatly with that of temperate woodlands being characterized by relatively low levels of invasion. However, evidence is accumulating that alien species of trees are increasingly colonizing woodlands, in particular floodplain woodlands. Here, we used 346 stratified relevés (154 plots in floodplain and 192 in non-floodplain woodlands) sampled between 1950–2014, combined with a control dataset of 369 uninvaded plots (97 plots in floodplain and 272 in non-floodplain woodlands), to analyse the invasion over time of Austrian woodlands by alien species of trees. The most frequent alien species of trees were Robinia pseudoacacia, Acer negundo and Ailanthus altissima. In addition, eight species of alien trees were recorded infrequently at the end of the study period. The average cover of alien trees has steadily increased since 1950.While the proportions of alien trees in floodplain relevés were significantly higher in 1950 than in non-floodplain relevés, the spread of alien trees was more pronounced in the latter. Average cover of native trees in the tree layer decreased over time in non-floodplain relevés, while for floodplain woodlands there was no temporal trend. Since 1950, indicators of human impact (mean levels of hemeroby and urbanophily) increased in both habitats, particularly in non-floodplain woodland, but remained stable in the control dataset. The frequency of nitrophilous and drought-tolerating species increased in non-floodplain and floodplain woodland, respectively, but both trends were also recorded in the control dataset. Further, changes in altitude and proportion of thermophilous species over time could indicate an effect of global warming. Our results point to anthropogenic habitat destruction, climate change, propagule pressure and deliberate planting of alien trees as the main drivers of alien tree invasions in the region studied.