There is wide recognition that urban green space provides city residents with considerable benefits, inter alia of an environmental, economic and health-related profile. However, the different types of urban greenery include a rather large proportion taking the form of vegetation on abandoned sites that remain uncultivated, to the extent that a plant cover develops without much active human involvement. Almost by definition, there is only a poor level of recognition of the ecological potential such sites (here referred to as “informal green spaces” – IGS) enjoy, or of their capacity to render a variety of different ecosystem services. Against that background, the work presented here entailed in-depth study of the flora and plant communities present in Warsaw’s informal green spaces, the aim being to better ascertain their role in preserving biodiversity and delivering ecosystem services. Specifically, we identified the plant species composition present at 75 different locations within the urban green space of Poland’s capital city. To qualify for consideration, these sites had to be identified as entirely bereft of vegetation maintenance, or else only minimally subject to it, to the extent that it is largely processes of natural succession that are ongoing. The sites in fact range from wastelands with stabilised vegetation, via urban scrub and forest, through to non-forest habitats, sporadically cultivated and established over 20 years. We determined the density of vegetation present, and examined its structure in relation to various known classes of green space. We further determined the role of various vegetation types in rendering ecosystem services, be that surface cooling, substrate moisture maintenance or a capacity to remove particulate matter from the air. The informal green spaces we investigated are in fact found to comprise mainly-stable forest communities dominated by invasive species of tree (phytosociological Classes Robinietea and Salicetea purpureae), as well as non-forest communities (of Classes Molinio-Arrhenatheretea, Epilobietea and Artemisietea) again largely dominated by invasive plant species. The level of biodiversity here is average, it mostly being common forest and non-forest species that are preserved. However, in exceptional cases, the habitats constituted here do support species rare on a regional scale. It emerges that the forest vegetation is of weakly-diversified structure, as a reflection of the specific strategy invasive species pursue as they form monospecific communities. As noted already, the vegetation of the informal green spaces is seen to be largely dominated by such invasive species. Nevertheless, despite their evidently limited role in preserving biodiversity, these sites represent such a high density and volume of vegetation that their provisioning of ecosystem services is on a high level, especially where forest plant communities are involved. Sites that have come to be dominated by invasive plant species are shown to render ecosystem services comparable with (or sometimes even surpassing) those provided by native species, and this is especially the case when it comes to the removal of particulate matter from air and the exerting of a cooling effect.
Faculty Opinions recommendation of Invasive species triggers a massive loss of ecosystem services through a trophic cascade.