Plant invasions in temperate forests: Resistance or ephemeral phenomenon?

Invasion patterns in Europe are uneven across habitats. In particular, temperate zonal forests are relatively little affected by invasions. However, long generation periods of forest species and the rarity of disturbance events may lead to long time lags, and so, invasion resistance of temperate forests may have been overestimated. Based on the inventory of the alien vascular flora of 15 study sites in East Austrian lowland forest patches adjacent to villages, we analysed diversity and temporal trends of their alien flora and the influence of within-habitat diversity, occurrence of forest areas dominated by alien tree species and adjacent land use diversity on the level of invasion. In total, we recorded 119 alien (neophyte) species. Species introduced by horticulture are of overriding importance (86% of all aliens). Using the year of first record in Austria as a proxy for local residence time, numbers of aliens have increased linearly by five species per decade since 1800. A GLM including the study site size and ratio of forest areas dominated by alien vs. native tree species explains 64% of the variance of the number of alien species. Size was more important, but the ratio of forest areas dominated by alien trees (Ailanthus altissima, Robinia pseudacacia) significantly increased alien species number. Our results show, that invasion of alien trees with strong impacts on ecosystem properties facilitates invasion in Central European forests, leading to invasional meltdown. Further, high levels of propagule pressure, created by adjacent settlements and gardens, may foster invasions. Currently observed low levels of invasion in Central European forests situated in greater distance to settlements may turn out to be an ephemeral phenomenon. Spread of alien forest plants on the landscape level may ultimately, although possibly only over long time periods, lead to increased levels of invasion.