Shrub invasion drives herbaceous vegetation degradation in semi-arid rangelands of the Eastern Cape Province of South Africa

Abstract

Shrub invasion is a global concern for biodiversity conservation, and rangeland productivity. Since early 1900s, a dwarf shrub from the Asteraceae family, Pteronia incana has invaded vast areas of the Grassland and Thicket biomes of South Africa. However, it is unclear how this shrub affects herbaceous vegetation and whether its effects are modulated by topography in the Eastern Cape Province, South Africa. Three 1-hectare blocks were delineated in the mountain top, middle and bottom slopes in Lower Mgwalana and Qeto communal rangelands from which three 10 × 10 m plots were assigned to characterize three replicates of open, moderate and high invasion levels of <10 %, >20–40 % and >40 % shrub cover, respectively. We determined shrub cover and density in each plot and assessed herbaceous vegetation in three 1-m² quadrats per plot. In total, 22 grass species were recorded, with their total abundance declining with shrub invasion while the opposite was true for forbs. However, two palatable grass species (Digitaria eriantha and Eustachys paspaloides) remained dominant in invaded sites. Graminoid cover varied significantly with shrub invasion irrespective of slope position. Highly invaded sites (HIS) generally exhibited lower graminoid cover relative to moderately invaded (MIS) and open areas (OAs). Conversely, cryptogam and litter cover were higher in HIS relative to OAs. Plant density varied by invasion levels only in the bottom slope, whereas species diversity showed differences in the middle and top slopes, with both vegetation attributes being highest in OAs compared to MIS and HIS. Overall, shrub invasion facilitated herbaceous vegetation degradation, calling for active restoration interventions including strategic removal of shrubs in strips for herbaceous recovery and leaving some shrubs as contour buffer strips to minimize soil erosion. Brush-packing can be incorporated into shrub-cleared strips to improve infiltration and minimize impact of direct rain splash.