Effect of long-term exclusion of fire and herbivory on the soils and vegetation of sour grassland

Abstract

The effect of long-term exclusion offire and herbivory, as mediated by aspect, on soils and the structure, composition, and diversity of vegetation was investigated in DOhne Sourveld (34 years). Piet-Retief Sourveld (45 years) and Southern Tall Grassveld (49 years). Exclusion had no effect on the soil chemistry of the surface 15cm of soil in Dohne and Piet-Retief Sourveld. Exclusion promoted the ingress of woody elements, including heath-like species, but the extent of ingress was related to water availability (rainfall, aspect), proximity of propagules, duration of protection, and accidental fires. Alien invasive woody species, in particular Acacia mearnsii established at all sites. Woody species were mostly bird- or wind dispersed, some had no known nearby population, although some Acacia species were herbivore-dispersed. The wettest grassland, Piet Retief Sourveld on a southern aspect, was transformed to a shrubland matrix (mainly Helichrysum splendidum) with tree clumps of forest precursor species (Rhus dentata, Halleria lucida, Myrsine africana, Buddleja salviifolia, Leucosidea sericea) and A. mearnsii. A species-rich grassland on the northern aspect had become dominated by tall Hyparrhenia rufa. with scattered shrubs of L. sericea and Helichrysum rogulosum. The southern aspect exclosure in Dohne Sourveld had developed an A. mearnsii - B. salviifolia savanna. with Cymbopogon excavatus, Themeda triandra and Tristachya leucothrix as common grasses. The northern aspect exc/osure on Dohne Sourveld maintained a grassland character, although heath-like elements (Stoebe vulgaris, Erica aspalanthifolia) established. Protection of the Southern Tall Grassveld promoted an open savanna of exotic and indigenous Acacia and broadleaf species, an increase in life forms including the succulent Aloe maculata, and the transformation of a T. triandra - T. leucothrix sward to an Aristida junciformis sward. Ferns benefitted from exclusion of fire and herbivory. Fire and herbivory rather than soil properties would seem the primary determinants of the structure of sour grasslands.

African Journal of Range & Forage Science 17(1, 2&3): 70-80