The effects of soil conditions and grazing strategy on plant species composition in a semi-arid savanna

Abstract

The tree:grass ratio of savannas is important, especially in the semi-arid savannas of South Africa. An increase in tree cover leads to reduced productivity and profitability of rangelands. We investigated the effects of soil type and grazing strategy on the presence and distribution of plant species in the Kimberley Triangle through vegetation classification. Heavy livestock grazing is generally believed to lead to increases in the tree:grass ratio, although we found that grazing strategy is not as important as soil type in determining species presence and distribution. We did, however, find that of the grazing strategies studied, communal grazing has the greatest potential to cause general vegetation degradation. Soil texture is a crucial determinant of the tree:grass ratio due to its effects on plant growth, soil moisture and nutrient presence and availability. These, in turn, alter the competitive relationships between N-fixing woody species and non-N-fixing grasses. We suggest that habitats with sandy soils and clay-pan habitats are resistant to bush encroachment by N-fixing species due to moisture deficiencies, soil texture and/or heavy utilisation. Heavy utilisation by game in rocky areas and in loam-sand-clay combination soils may remove the suppressive competitive effect of grasses on the woody vegetation and lead to bush encroachment. Hence these areas should be managed conservatively. Because of the importance of soil moisture in limiting woody plant germination and initial survival, we suggest that heavy grazing in above-average rainfall years should be limited in all habitats.

Keywords: Braun-Blanquet vegetation classification, bush encroachment, canonical correspondence analysis, communal grazing, soil nutrients

African Journal of Range & Forage Science 2007, 24(2): 51–61