Mirjana Šipek and Nina Sajna
University of Maribor, Koroska c. 160, Maribor, Slovenia
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Introduction of alien species is considered as one of the major threats to native biodiversity after human influenced land use changes that cause fragmentation and habitat destruction. Diversity and composition of alien plant species (APS) differ according to specific land uses. APS are the most abundant in urban areas such as human settlements, industrial areas and in vicinity of transport infrastructures because roads are potential corridors dispersing APS propagules. Residential gardens are an important source of APS where numerous ornamental and edible plants are abundant and some of them could escape from cultivation, become naturalized and even invasive. The spread of APS into adjacent natural and semi natural habitats is influenced by settlement size and the distance to potentially invaded habitats. Especially small urban and periurban forest islands are under a high risk of invasion because of numerous house gardens and frequent road networks in the vicinity. Moreover, such forests are also under high disturbance pressure, which further increases the probability of invasions. Local inhabitants use forest fragments for forestry, recreational activities, cutting and harvesting of non-timber forest products, and finally, yet importantly, dumping garden waste in forest edges.
To analyse how adjacent land use influence diversity and composition of APS, we surveyed alien flora in 16 forest islands trapped in urban or rural matrix in NE Slovenia. We calculated the proportion of each land use adjacent to the forest island using Google Earth Pro. For forest islands in a rural matrix, we calculated the distance to the adjacent settlement to find a correlation with APS richness and proximity to the propagule source.
We found a positive correlation between APS richness and proportion of the residential area, while the trend was negative when the proportion of adjacent arable land was increasing. Further, a negative correlation was found between the proximity to settlements and the APS. The most common herbaceous aliens were Phytolaca americana L., Impatiens parviflora DC., Erigeron annuus (L.) Pers. and Duchesnea indica (Andrews) Th. Wolf, while the most common woody alien was Robinia pseudoacacia L. occurring in approximately 90% of forest islands regardless of adjacent land use type and other forest fragment features. In the understorey of plots near settlements, we found alien shrub species common for gardens such as Prunus laurocerasus L. and Symphoricarpos albus (L.) S.F.Blake. Our results show that urban and periurban forest fragments are under high risk of invasions and should be monitored in the future to prevent new establishments. Further, local inhabitants should be informed about activities that promote new invasions to minimize them, for example, depositing garden waste in nearby forests.
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