Short-term responses of herbaceous vegetation to bush clearing in semi-arid rangelands of South Africa

Abstract

Bush thickening reduces herbaceous species diversity and forage production, leading to a decline in grazing capacity. Hence, altering the micro-climate and woody-grass competition in bush-encroached rangelands using bush clearing is instrumental for the restoration of herbaceous vegetation. The short-term recovery potential of herbaceous species composition, basal cover and biomass production was assessed following bush clearing at Makapaanstad, North West province, South Africa. The mean density was 6908 woody plants ha−1, with Vachellia tenuispina encroaching at 6301 plants ha−1. Paired observations were conducted in bush-cleared sites (where woody plants were cut using saws, with stumps treated with picloram at 1% concentration) and uncleared sites (where the woody cover was left intact). Fortynine different herbaceous species were recorded, with grasses being more abundant in cleared (62%) than in uncleared sites (50%). Herbaceous species showed interspecific responses, with Brachiaria eruciformis and Panicum coloratum colonising in cleared sites, whereas the frequency of other species remained comparable between cleared and uncleared sites. Graminoids, largely bunch grasses, had a significantly greater basal cover in cleared (496 cm2 m−2) compared to uncleared (301 cm2 m−2) sites. Likewise, total biomass production was significantly (p = 0.05) higher in cleared (760 kg DM ha−1) than in uncleared (636 kg DM ha−1) sites. The principal component analysis (PCA) showed that most herbaceous species were associated with soils having high total nitrogen and nitrite. Seedlings of forbs (r2 = 0.81), grasses (r2 = 0.97) and succulents (r2 = 0.98) were highly associated with nitrate-rich soils. Our results revealed that even at a short term, bush clearing is important for the restoration of herbaceous vegetation.