The effect of high-density, short-duration stocking on soils and vegetation of mesic grassland in South Africa.

Abstract

High-density, short-duration stocking (HDG) is currently gaining popularity amongst farmers in South African mesic grasslands but little is known about its potential impact on soil properties and plant species composition, particularly the forbs (herbaceous dicotyledonous and non-graminoid monocotyledon) which contribute more to plant diversity than grasses. Under HDG, animals are stocked at higher stocking rates and densities than conventional grazing systems and burning is discouraged. This study used a fence-line contrast approach to compare the long-term impact of “real world” HDG systems with rotational grazing systems at a lower stocking density (LDG) on soils and vegetation composition including forb growth habits at two study sites, Kokstad and Cedarville. An experimental trial was set up at Ukulinga Research Farm to determine the short-term effects of HDG compared with no grazing on plant species composition and demography of the selected perennial forbs. Another field experiment was used to determine the response of three mesic grassland perennial forb species (Afroaster hispida, Gerbera ambigua and Hypoxis hemerocallidea) to intense defoliation and interspecific competition with a grass species (Themeda triandra). Soils were more compacted under HDG but soil chemical properties did not differ between HDG and LDG at both Kokstad and Cedarville. There was a low percentage cover of desirable palatable grasses and high forb species turnover under HDG at Kokstad and low grass and forb species responses at Cedarville. There was high litter accumulation under HDG over the long- and short-term period. High litter accumulation reduces irradiance for plants, and may lead to lower basal cover. The intense grazing and trampling due to the higher stocking rate and stocking density under HDG resulted in less erect forb growth habits and more prostrate growth habits at Kokstad. A study of demography revealed that HDG threatened future populations of the grazing-sensitive species Afroaster hispida, Agathisanthemum chlorophyllum and Gerbera ambigua through increased mortality or reduction in the recruitment of large from small individuals. Intense defoliation altered the competitive response of A. hispida, it had a high competitive response when undefoliated but when defoliated its competitive response was reduced. Gerbera ambigua and H. hemerocallidea were not affected by the interaction between defoliation and competition. Findings from this study has shown that HDG potentially has a negative impact on soil health and vegetation composition of South African mesic grassland.