The Flora of Urban River Wallscapes

Abstract

Urban river walls represent some of the most common habitat available to riparian species within heavily engineered urban river corridors, but research into the characteristics and diversity of river walls is very rare. In this study, the flora of urban river walls was surveyed at 16 sites (92 walls) along 32 km of the river Thames through central London to determine the characteristics and habitat origins of species found on these walls, whether these species indicated 'an urban cliff effect' occurring on wall habitats and whether plant species richness varied between different wall types. A total of 90 species were found on the walls, with the majority of species associated with disturbed areas and riparian habitats, supporting the idea of a 'mass effect' occurring, whereby the flora is maintained by propagule pressure from remnant or introduced habitats within urban or peri-urban areas, rather than walls selecting for cliff/rock species. There was an increased incidence of plants from cliff/rock habitats found in the flora however, suggesting that an urban cliff effect does still occur. Species were organized along the river walls based on riparian/terrestrial gradients and stress tolerance, probably reflecting a gradient of wall height above river flows. Plant species richness was significantly higher on brick walls than that on sheet piling, indicating key differences between materials probably based on surface integrity. Large areas of connected river walls ('wallscapes') through heavily engineered urban rivers represent potential sites for the enactment of reconciliation ecology via habitat improvement in urban ecosystems.

Vegetation of riprapped revetments along the freshwater tidal Hudson River, New York

“…The species composition of the community was correlated with revetment slope, roughness, and particle size, as well as site exposure. These results support previous findings that shore defenses with relatively small particle sizes are heavily vegetated by natural means (Bariteau et al 2013), whereas vertical walls with few crevices support little vegetation (Francis and Hoggart 2012).…”

“…The flora contains many non-native species (36 % of all species and 58 % of common species of known origins), and vines are a conspicuous part of the community. Many of the species are ruderals, and the flora in general is similar to that of disturbed mineral soils in the region, with a modest addition of wetland species, a result similar to that found for walls along the River Thames by Francis and Hoggart (2012). Despite the fact that our sites are adjacent to the water, their floras do not closely resemble those of local wetlands (e.g., Phragmites and wetland ferns such as Onoclea sensibilis were absent, and Carex was rare), presumably because of the much higher physical energy inputs and more severe disturbance regimes on the revetments than at typical wetland sites.…”

Urban rivers: novel ecosystems, new challenges

“…Likewise, the novel habitats found in urban rivers may support atypical species (native and nonnative), such as the wide range of terrestrial species found on river walls. 16 It is important not to rely on changes in water quality and the presence/absence of particular species to qualify intervention success, but rather to improve resilience, particularly given the uncertainties of ongoing environmental changes, which may be exacerbated in urban regions (such as climate change). More resilient systems that are self-sustaining, maintaining assemblages that have colonized available habitat naturally, are most likely to persist in a desirable state.…”

An overview of non‐native species invasions in urban river corridors

“…Interestingly, this perception is most often challenged in urban ecosystems, because of the recognition that non‐natives will continue to be part of urban ecological communities, and because urban communities do not hold to traditional community typologies (Rotherham, ). Indeed, study of cosmopolitan, recombinant communities—those formed of species that originate from a range of habitats, and which often include non‐native species—has often taken place in urban ecosystems, including urban rivers (Francis & Hoggart, ; Rotherham, ). Urban river conditions can create locally unique communities, especially around novel habitats, as observed by Nelson () for macroinvertebrates and McLellan, Huse, Mueller‐Spitz, Andreishcheva, and Sogin () for microbes in sewerage systems.…”