No abstract

Public access to high quality green environments has become a key issue for city managers and a matter of environmental justice. Urban pressures on ecosystem remnants may act to favor the expansion of some invasive species in cities. Whilst the negative impacts of invasive species on ecosystem function is well documented, little is known about how invasive species influence the use of green space by people. Here, we examined one of the few remnants of urban riparian forests in Europe, the Vistula river valley in Warsaw, which has recently become an attractive recreation site. Despite their high ecological value, the poplar and willow forests have been increasingly taken over by the invasive tree species Acer negundo. We examined the status of the invasion process and the relationship between recreational ecosystem services and the characteristics of the tree stands—tree species, tree density, and age and NDVI values. We found the willow forest to be more susceptible to invasion by A. negundo than the poplar forest, which was revealed in significantly higher share of the maple individuals and their greater volume per unit area. Ash-leaved maples also prevailed in numbers in younger stands (<10 years) than in older ones. The presence of A. negundo affected biodiversity, resulting in decreased undergrowth density and biodiversity expressed in number of species. The use intensity by the public, assessed on the basis of soil compaction of existing informal tracks, as revealed by GLM analysis, was related to volume of invasive maple and distance from the main track. This study highlights the need to integrate invasive species management into green infrastructure planning and management.

Invasive species significantly impact ecosystems, which is fostered by global warming. Their removal generates high costs to the greenery managers; therefore, quick and accurate identification methods can allow action to be taken with minimal impact on ecosystems. Remote sensing techniques such as Airborne Laser Scanning (ALS) have been widely applied for this purpose. However, many species of invasive plants, such as Acer negundo L., penetrate the forests under the leaves and thus make recognition difficult. The strongly contaminated riverside forests in the Vistula valley were examined in the gradient of the center of Warsaw and beyond its limits within a Natura 2000 priority habitat (91E0), namely, alluvial and willow forests and poplars. This work aimed to assess the potentiality of a dual-wavelength ALS in identifying the stage of the A. negundo invasion. The research was carried out using over 500 test areas of 4 m diameter within the riparian forests, where the habitats did not show any significant traces of transformation. LiDAR bi-spectral data with a density of 6 points/m2 in both channels were acquired with a Riegl VQ-1560i-DW scanner. The implemented approach is based on crown parameters obtained from point cloud segmentation. The Adaptive Mean Shift 3D algorithm was used to separate individual crowns. This method allows for the delineation of individual dominant trees both in the canopy (horizontal segmentation) and undergrowth (vertical segmentation), taking into account the diversified structure of tree stands. The geometrical features and distribution characteristics of the GNDVI (Green Normalized Vegetation Index) were calculated for all crown segments. These features were found to be essential to distinguish A. negundo from other tree species. The classification was based on the sequential additive modeling algorithm using a multi-class loss function. Results with a high accuracy, exceeding 80%, allowed for identifying and localizing tree crowns belonging to the invasive species. With the presented method, we could determine dendrometric traits such as the age of the tree, its height, and the height of the covering leaves of the trees.

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.