Impacts of exotic invasive plants on the composition and structure of riparian woody vegetation in the lower Orange River- Tsau/Khaeb (Sperrgebiet) National Park, Namibia

Abstract

The adverse impacts of exotic invasive plants in protected areas, agricultural areas and riparian zones are a global concern. With particular focus on riparian zones, the invasive alien plant species displaces indigenous riverine vegetation, alters species composition and plant community structure. In totality, alien invasive species affects ecological functioning of natural systems as well as disturbs the ecosystem and habitat integrity. This study therefore investigates the impact of exotic invading plant species on the structure and composition of the resident woody vegetation communities within the riparian zones of the lower Orange River part of Tsau//Khaeb (Sperrgebiet) National Park, Namibia. A comparative methodological study approach was adopted and a 20 m x 50 sampling plots were used for intensity density and biomass assessment between the invaded and uninvaded sites. Differences in canopy cover stem, width and vegetation height between the invaded and un-invaded sites were determined by using a t-test for the equality of means which was performed at the 5% significance level. The results proved that there was a significant difference in the mean height, mean canopy cover and mean stem width of the vegetation (p = 0.00). The most common exotic invader recorded are Datura inoxia, Eucalyptus camaldulensis, Nicotiana glauca, Prosopis spp and Ricinus communis. It is concluded that invasive alien plants exerts adverse impacts on the characteristics of riparian vegetation communities of the Lower Orange River in the Tsau//Khaeb (Sperrgebiet) National Park. Thus, there is a need for an active management and control interventions of alien invasive plant species within the riparian zones of the Orange River in the Tsau//Khaeb (Sperrgebiet) National Park.