Urban hedges facilitate spontaneous woody plants

Most trees and other woody plants growing within urban areas are deliberately cultivated native or non-native plants. Opportunities for spontaneous establishment and spread of woody species are scarce in an urban environment, especially if the establishment begins from seeds. We hypothesized that urban hedges might offer safe sites for the spontaneous establishment of woody plants, which reach the hedges through propagule dispersal. We recorded the species composition of urban hedges in Maribor (NE Slovenia) in a gradient from the city centre outwards and evaluated the species' invasive potential and ecological requirements, along with hedge characteristics. Results show that hedges facilitated the survival and growth of 69 woody plants from 28 families. Among these, 62% were non-native and 10% were locally invasive. Most of recorded non-native woody plants were ornamentals. Moreover, for native species, we show that the stressful, warmer urban environment mostly facilitates the establishment of species in the genera Hedera and Acer. Comparison of non-native and native plants showed the former are more drought and heat tolerant than the latter. We further show that with distance from the city centre, the number of bird-dispersed species increases, in contrast to the wind-dispersed species. Additionally, our results suggest evergreen hedges may be less permeable to wind-dispersed propagules than deciduous hedges. Next, we stress the importance of paved surfaces and urbanization, selecting for the thermophile, drought tolerant, wind-dispersed woody plants that find refuge in urban hedges. Our results suggest urban hedges can facilitate the establishment of a wide range of woody plant species.