Risk Assessment of Alien Woody Plant Species Recommended for Restoration of Parks Parkide restaureerimiseks soovitatud võõrpuittaimeliikide riskihindamine

Abstract

Alien woody plants have become naturalized or invasive in many regions around the world, including Estonia. Nevertheless, there are no lists of alien woody plant species analyzed in terms of invasiveness in Estonia.

Therefore, the aim of the master thesis, which is the basis for this article was to analyze the relevance of the list of woody plant species recommended for restoration of parks (84 species) to explain the dangers of alien woody plants to natural communities. To achieve that aim, biological-ecological and invasive characteristics of alien woody plant species were explored, and answers were given to the questions that are the basis for the risk assessment. According to the risk assessment and general analysis, the relevance and possible disadvantages of the list in terms of alien species threatening natural communities were found. The adjusted list of species recommended for restoration of parks was formed without alien species which could potentially threaten natural communities.

The risk assessment of alien woody plants (53 species) was based on Weber & Gut methodology. The risk assessment scheme consists of 12 questions, each with a specific score. The final score of species allows to identify the risk class: “low risk”, “intermediate risk” or “high risk”.

According to the results of risk assessment, 5 species (9 %) are in risk class I (“low risk”), 30 species (57 %) are in risk class II (“intermediate risk”) and 18 species (34 %) are in risk class III (“high risk”).

Species in risk class I (“low risk”) are unlikely to pose a threat to natural communities, but they must be occasionally monitored. If necessary, the risk assessment can be repeated. Species in risk class II (“intermediate risk”) require further observation. They must be regularly monitored to detect the changes in distribution and to avoid the dispersal of species into the wild. The risk assessment must be repeated if the distribution of species changes. Species should be relocated to a more suitable risk class according to the results of the repeated risk assessment. Particular attention must be paid to species that spread successfully by seed and/or by vegetative reproduction (e.g. Syringa vulgaris).

Species in risk class III (“high risk“) are or will most likely become a threat to natural communities if naturalized. Some of them have already escaped from cultivation and are growing in natural communities (e.g. Amelanchier spicata, Caragana arborescens, Larix decidua, Sambucus racemosa). In the future, other species in risk class III will probably cause problems as well. Current reproduction and spread of these species is closely related to the original planting place (parks, gardens, cemeteries etc.). Therefore, the use of alien woody plant species in high risk class must be avoided.

The adjusted list of woody plant species recommended for restoration of parks (66 species) comprises native species and alien species with low and intermediate risk. To show the difference in potential threat to natural communities, there are statuses and inferences of the species in the adjusted list: native species are allowed without restrictions, alien species with low risk must be occasionally monitored, and alien species with intermediate risk must be regularly monitored to detect changes in distribution and to avoid the dispersal of species into the wild. Native species should be preferred wherever possible.