Survey of invasive alien species in the flora of Lozenska Mountain, Bulgaria

Abstract

This study explores the invasive alien flora of Lozenska Mountain, southwestern Bulgaria (maximum height 1190 m, area 80 km2). The aims were to (i) describe the taxonomic and ecological structure of the invasive alien flora in Lozenska Mountain, and (ii) provide data on the local distribution of the globally most widespread invasive species. Ten transects of different lengths (from 4 to 12 km) in different parts of the mountain covering the most characteristic habitats were surveyed between April 2017 and October 2018. Altogether, 27 invasive alien species (IAS) and 4 potentially invasive alien species (PIAS) belonging to 17 families of flowering plants were encountered. These represent 3.46% of the flora of Lozenska Mountain, and 45% of all the IAS in Bulgaria. Seven species (Acer negundo, Ailanthus altissima, Amorpha fruticosa, Bidens frondosus, Fallopia × bohe mica, Opuntia humifusa and Robinia pseudoacacia) are highly aggressive. In the biological spectrum of IAS, therophytes predominate (29%), followed by hemicryptophytes (19.4%). The majority of IAS originates from America (65.6%) and Asia (19.5%). 64.5% of the IAS were deliberately introduced. The established invasive species on the territory of the mountain are predominantly heliophytes (90.3%); microthermophytes (71.0%); high humidity species (38.7%); mesophytes (54.8%), mesooligotrophs (58.1%) and basophilic species (80.6%). The most widespread species are Robinia pseudoacacia, Erigeron canadensis and Xanthium italicum. Most of the IAS were found in one transect only. The highest percentage of species cover (over 80%) was reached by plants adapted to specific habitats: aquatic (Elodea canadensis), riparian (Impatiens glandulifera, Fallopia × bohemica) and anthropogenically heavily disturbed terrains (Sorghum halepense). There is a substantial threat of IAS irruption on the territory of Lozenska Mountain for the following habitat types (reference codes according to Annex I of the Directive 92/43/EEC): 3150 (Natural eutrophic lakes with Magnopotamion or Hydrocharition vegetation) and 3270 (Rivers with muddy banks with Chenopodion rubri p.p. and Bidention p.p. vegetation) included in Natura 2000. A map of IAS occurrences in the mountain was prepared and it was found that IAS predominantly appear in areas around rivers with permanent water regime such as Iskar, Rakita and Gabra, and near urbanized areas adjacent to settlements, along roads and in abandoned mines. The populations of the species Amaranthus hybridus, A. retroflexus, Elodea canadensis, Erigeron annuus, E. canadensis, Fallopia × bohemica, Impatiens glandulifera, Oenothera biennis, Opuntia humifusa, Robinia pseudoacacia, Solidago gigantea, Sorghum halepense and Xanthium italicum occur in certain parts of Lozenska Mountain. Other IAS have so far formed small populations only being accessorial elements in natural plant communities.