Trends of phanerophyte encroacher species along an aridity gradient on Kalahari sands, central Namibia

摘要

Poor rangeland management, especially overstocking and under-burning coupled with climate change on southern African savannas, have brought about a serious ecological problem of bush encroachment. Bush encroachment leads to many ecological implications such as extirpation or extinction of plant species and a colonisation by opportunistic species leading to unwanted changes in plant species composition, structure and loss of species diversity. Furthermore, bush encroachment has a negative impact on the country’s progress in terms of conservation efforts, economic stability and livelihood. Namibian livestock ranchers forego an estimated N$ 700 million loss yearly linked to bush encroachment. Studies focusing on particular bush encroacher species enable the devise of ecologically sound management strategies by land manager, farmers and scientists for the prevention and control of bush encroachment. Therefore, this study was undertaken to determine the main encroacher species and their relationship to the environmental factors along an aridity gradient on Kalahari sands in central Namibia. Results disclosed that Acacia erioloba E.Mey., Acacia mellifera (Vahl) Benth. ssp. dentines (Burch.) Brenan, Combretum collinum Fresen., Terminalia sericea Burch. Ex DC., Grewia spp., Bauhinia petersiana Bolle ssp. macrantha (Oliv.) Brummitt & J.H. Ross were the main encroacher species, and mean annual rainfall was the main environmental factor influencing their distribution. Nanophanerophyte from different encroacher species were recorded mainly from 400 mm to 500 mm mean annual rainfall, mesophanerophyte recorded from 280 mm to 450 mm, while microphanerophyte were widely distributed over the rainfall gradient. Bush encroachment was recorded at 440 mm mainly due to the poor rangeland management. Information from this study should be used as a baseline for conservation and restoration attempts towards savanna rangelands.