Invasive alien plant species, fragmentation and scale effects on urban forest community composition in Durban, South Africa

Abstract

Background: Urban forests are under increased pressure from invasion by exotic (alien) species. The vegetation present in the matrix of urban sites is a rich source of alien invasive propagules, which increases the risk of alien invasion in forests within an urban space, leading to a decline in indigenous species. Therefore, determining the distribution patterns of native and exotic species as influenced by environmental factors can assist in quantifying the impact of exotic species at broad scales based on responses on a finer scale. Quantifying the effects of multiple environmental factors on the distribution patterns of both indigenous and alien species in the ecosystem may help in prescribing suitable management efforts. Methods: Fifteen forest patches were sampled in the eThekwini (Durban) Municipality and data collected from 74 100-m 2 plots with different degrees of invasion. Indigenous and alien species of trees, shrubs and climbers occurring in ten and more plots were considered for analysis and the CANOCO 5.1 package was used to run various constrained ordination analyses. Variation partitioning analysis was used to assess the impact of environmental variables at different spatial scales, namely the plot and patch scales. Results: Canopy gaps are the major controlling factor for invasive alien plants (IAPs) occurrence at fine scale. At patch level, residential and industry areas outside the boundaries (buffer area) of forest patches have a high influence on the distribution of IAPs. Communities dominated by the invasive Chromolaena odorata (L.) R.M. King are most common on the lowland coastal forests while communities dominated by either Litsea glutinosa (Lour.) C.B.Rob. or Cardiospermum grandiflorum Sw. can prevail in both lowland coastal and scarp forests. Conclusions: Canopy gaps in lowland forests can facilitate the transition of native forests to novel communities containing a variety of alien plant species. Communities of shrub, climber and tree IAPs occur in lowland coastal forests while climber and tree IAPs dominate the high-elevation scarp forests. The resilience shown by some native species to the tree, shrub and climber IAPs by remaining when IAPs establish makes these species very suitable for restoration projects. Forest patches surrounded by a high incidence of residential and industrial areas in the buffer matrix are likely to have a high diversity of IAPs. Larger patch size and high connectivity to nearby native forests are key in reducing invasion by IAPs.

Forest regrowth reduces richness and abundance of invasive alien plant species in community managed Shorea robusta forests of central Nepal

“…Edge and gap formation generates considerable changes in the distribution and availability of environmental resources (Lockwood et al 2005). Although, gap dynamics in natural forests have been studied, applications of gap dynamics to forestry practice in relation to IAPS management are limited (Mavimbela et al 2018). Increase in canopy and closure of canopy gaps in the study forests after community management might have prevented establishment of new IAPS and suppressed the growth of previously established IAPS, particularly Chromolaena odorata as reported by local people.…”

“…Similarly, forest edges trap airborne propagules of IAPS and facilitate invasion (Joshi et al 2015). Thus, areas with small patches or more edge density are more vulnerable to invasion by IAPS (Mavimbela et al 2018).…”

Forest canopy resists plant invasions: a case study ofChromolaena odoratain sub-tropical Sal (Shorea robusta) forests of Nepal

“…Invasive Alien Plant species (IAPS) prefer to grow in open areas in forests and forest ecotones (Mavimbela et al 2018). Open areas in forests provide sites for regeneration and growth of IAPS and have higher proportion of IAPS density and coverage compared to closed-canopy areas (Charbonneau andFahrig 2004, Driscoll et al 2016).…”

Plant species diversity in urban areas of Northwest Siberia

“…Urbanization is a global process which is expanding from year to year, so urban ecology is becoming a fastdeveloping field of scientific study. Generalized and detailed information on urban biodiversity and ecological processes is mostly connected with European cities and towns (Niemelä et al, 2011); thereby any data on urban ecology of Siberian localities situated in northern areas are valuable. Our results have shown some specific traits of floristic diversity in all study towns and settlements of the northern taiga and forest-tundra.…”

“…Urban habitats, especially roads, often become corridors for alien and invasive species (Hao et al, 2013;Uchida et al, 2014). Control measures for invasion processes can be developed only on the basis of a comprehensive assessment of the impact of numerous environmental factors on the distribution of indigenous and alien plant species in urban ecosystems (Threlfall et al, 2016;Mavimbela, Sieben and Procheş, 2018).…”