Effects of Sicklebush (Dichrostachys cinerea (l.) wight and arn.shrub) Encroachment on Floristic and Vegetation Structure in Semi-arid Savannah of Southern Ethiopia

摘要

Anthropogenic drivers in Protected Areas particularly overgrazing in National Parks may enable the spread of native plant species into non-encroached areas, a phenomenon witnessed with sicklebush ( Dichrostachys cinerea (L.) Weight and Arn. Shrub) in Semi-arid Savannah Plains of Nech Sar National Park (NSNP). This study was conducted in the Savannah Plains of NSNP, to investigate the effects of sicklebush encroachment on native floristic and vegetation structures. Two sicklebush encroached patches, each greater than one hectare and two adjacent non-encroached sites of the same size were selected randomly to establish sampling units. A total of 32 (10m by 10m) plots were established being eight from each of the four encroached and non-encroached sites. Five 1m by 1m quadrats were laid in each plot to collect vegetation data except for aboveground biomass. Aboveground biomass was clipped from 0.25m 2 sub-quadrats of each quadrat. Species composition, richness, diversity, vegetation live cover, vegetation dead cover, bare ground cover and vegetation height and aboveground biomass were compared between encroached and non-encroached sites. A total of 46 plant species were identified in the study area, of which 27 were found only in non-encroached site and 19 were common for both sites, implying that 58.7% of the species were absent in sicklebush encroached sites. Non-encroached sites harboured a total of 18 families and 43 genera, whereas encroached sites harboured only 7 families and 18 genera. There were significant differences ( p 0.05) in species richness, species diversity, species composition, vegetation live and dead cover, vegetation height between encroached and non-encroached areas, with non-encroached areas recording higher values than encroached areas. Vegetation height did not show significant difference between the compared sites for the plant growth forms but it did show for few of dominant species. On the other hand, bare ground cover record was high in encroached areas compared to non-encroached areas. This implies that the native plant species of the savannah plains of the study area were under pressure due to sicklebush encroachment unless treated properly in short term.