Colonization by Robinia pseudoacacia of various soil and habitat types outside woodlands in a traditional Central-European agricultural landscape

The invasiveness of Robinia pseudoacacia, one of the most invasive alien tree species in the agricultural landscapes of Central Europe, was studied in relation to its colonization of various soil and habitat types. The study area was a traditional agricultural landscape in Goricko Landscape Park (Slovenia), a hilly region, where half the territory is covered by forests and where R. pseudoacacia is also common. Habitat mapping in the field with a resolution of two meters was applied and further elaborated in GIS. It revealed 1307 patches colonized by R. pseudoacacia and belonging to 11 habitat types, covering 0.6% of the total study area. The most widespread were pure R. pseudoacacia stands, representing 69% of the total patch area. R. pseudoacacia is also abundantly present in small woodlots (19%) and in lowland and collinar riverine willow scrub (6%). The patches vary little in fractal dimension; the elongation index is highest in lowland and collinar riverine willow scrub, stream ash-alder woods and mixed stands. The most frequently invaded soil is pseudogley, with more than 38% of the total surface, followed by 21% for fluvisol and 17% for eutric cambisol. We confirmed that distance from nearest woodland is important, since almost 32% of the invaded patches are found at a distance of 1-100 meters from closed woodland.