Impacts of Shrub enctoachment on soil nutrient properties, plant diversity and herbivory in Lochinvar National Park, Zambia

Abstract

This study aimed at assessing impacts of Dichrostachys cinerea and Mimosa pigra encroachment on soil nitrogen, phosphorous and carbon pools as well as plant species diversity, vegetation composition and food supply for herbivores in Lochinvar National park of Southern Zambia. The study hypothesized that shrub encroachment a) increases soil nitrogen and carbon pools but leads to a reduction in soil phosphorous pools, b)reduces plant species diversity and changes the composition of the understory vegetation and c) reduces biomass production of the understory vegetation, and that this would lead to a reduction in food supply for large herbivores, particularly the Kafue Lechwe. In order to assess the impacts, 20 and 19 field plots were selected along gradients of increasing cover of D. cinerea and M. Pigra shrubs respectively in Lochinvar National Park. In each of the plots, soil samples were collected and measured for soil pH, bulk density and soil nitrogen, phosphorous and carbon pools. Nitrogen mineralization rate as well as nitrogen and phosphorous availabilities were also investigated. Plant species composition and biomass production were measured for each of the plots in the D. cinerea and M. pigra plots. The results of this study showed that encroachment of D. cinerea shrubs not only increased the soil pools of nitrogen and carbon linearly, but also that of phosphorous, whereas no such associations were observed in the M. pigra gradient. Furthermore, the encroachment of D. cinerea and M. pigra had significant impact on plant species diversity and richness and altered the understory vegetation composition. Both these encroaching species largely reduced cover of grasses and grass biomass production. This suggests that shrub encroachment had reduced food supply for grass-eating herbivores, particularly the endemic Kafue Lechwe, in Lochinvar National Park. These results as well as an analysis of shrub encroachment based on satellite images show that shrub encroachment had likely not reached its end yet, and hence might even further reduce the food for these herbivores in the future.