According to currently available data from the Slovenian Forest Service, the growing stock of non-native tree species is 0.99% of the total growing stock of forests in Slovenia, and black locust prevails among them with 0.60% of the total growing stock. Other more common non-native tree species are eastern white pine (Pinus strobus) (0.18%), Douglas fir (Pseudotsuga menziesii) (0.05%) and northern red oak (Quercus rubra) (0.03%) (Table 1). It is estimated that approximately 25 different non-native tree species in total are currently present in Slovenian forests.
One of the possible consequences of non-native species introduction is their invasiveness. In Slovenia the two most important invasive trees are black locust (Robinia pseudacacia) and tree of heaven (Ailanthus altissima). Black locust was often used in the afforestation of degraded karst soils as early as in 1810. In 1858 it was still recommended for its multiple uses but for the first time its high persistence in the natural environment was recognized as a potential problem. Its present status is controversial, however. Due to its multiple uses, such as durable wood and rich honey production, it is clearly a desired species among forest owners. In addition, its drought resistance might be important in the altered climatic conditions in the future. Similarly, tree of heaven (Ailanthus altissima) was also used for the afforestation of degraded lands such as Kras and was still highly promoted in the mid-20th century. However, the species has limited use and is considered undesirable by forest owners; moreover, after successfully establishing itself in harsh environments, it is recognized as one of the most dangerous invasive species and is thought to have negative effect on the biodiversity of local ecosystems. Both species clearly illustrate how human interference in natural ecosystems can generate long-term processes that normally cannot be stopped after a sudden change in social perspectives and values.
Most invasive tree species are also present in the urban forests of Ljubljana and are being studied within the APPLAUSE project led by City of Ljubljana. Department of Forestry of the Biotechnical Faculty, University of Ljubljana, as one of the project partners is providing knowledge and workshops of wide-spread identification of invasive alien tree species by citizens and public awareness raising as well as experiences and research of management of invasive tree species in the forests.
Table 1: Presence of the most important non-native tree species in Slovenian forests. * - numerical data acquired from the forest information system of the Slovenia Forest Service (ZGS 2012), - no data available, PF – planted in forest, PN – planted in non-forest plantations, AF – used for afforestation, IN – invasive spread reported.
Author: Prof. Robert Brus, PhD, Department of Forestry and Renewable Forest Resources, University of Ljubljana, Biotechnical Faculty