The effects of Acacia mearnsii (black wattle) on soil chemistry and grass biomass production in a South African semi-arid rangeland: implications for rangeland rehabilitation

Globally, grasslands are under threat from woody encroachment by invasive alien plants and this undermines grass production. The study sought to determine the effects of Acacia mearnsii clearing on soil physico-chemical properties and grass production. Soil samples were collected from three A. mearnsii invasion statuses and analysed for representative physico-chemical variables. Using line transects and a disc pasture meter (DPM) approaches, equations were developed to predict annual grass dry matter production in areas cleared of A. mearnsii. A. mearnsii significantly (p < 0.05) altered most of the physico-chemical soil characteristics that affected grass production. Predicted grass dry matter production from sites cleared of A. mearnsii ranged from 284 to 362 g m(-2) y(-1). These production rates were similar to those from uninvaded landscapes, suggesting that background grass production can still be attained even if there is no active soil management. Autogenic recovery thresholds have not been surpassed despite a long history of landscape invasion in the area. Management of invaded grasslands should be informed by an appreciation of local soil background characteristics. The equations developed for grass dry matter prediction may enable farmers to non-destructively estimate forage availability and fuel loads.