The threat of alien invasive grasses to lowland Cape floral diversity : an empirical appraisal of the effectiveness of practical control strategies : research in action

Abstract

European annual grasses in highly fragmented natural ecosystems along the South African west coast are displacing wildflowers, which form the basis of a growing lucrative, nature-based tourist industry. We examined the cost-effectiveness of different labour-intensive strategies linked to a national poverty relief programme for controlling invasive annual grasses in renosterveld. The treatments tested involved combinations of grass mowing, hand-clearing, light and intense burning and pre-emergent herbicide application randomized over forty-eight 100-mlsupg2l/supg plots in the Tienie Versfeld Wildflower Reserve. Springtime vegetation responses were monitored over two successive years; labour, capital equipment and consumable costs were audited. Total clearing costs associated with intense burning of uncut grass (R415/ha), grass mowing (R924/ha) and light intensity burning of mowed grass (R1338/ha) were all less than those (up to R1 927/ha) reported for clearing dense stands of woody aliens. However, costs of hand-clearing of grass (R6 743/ha) and pre-emergent herbicide application (R13 380/ha) were much greater. Intense burning, the cheapest strategy overall, was ineffective as this promoted recruitment of both alien invasive annual and perennial grasses and inhibited recruitment of native geophytes. We conclude that mowing of grass-infested renosterveld prior to grass seed maturation, and the removal of the cut grass biomass for use as fodder in restricted feed lots to offset clearing costs, provides the most credible strategy for controlling the annual grass populations to conserve native floral diversity over the short term.