Water utilization patterns around isolated Acacia karroo trees in the False Thornveld of the eastern Cape.

Abstract

The effect of various vegetation treatments on two soil moisture regimes (i.e. the proportion of the experimental period where the soil had sufficient water for: (a) growth, and (b) to keep the plants turgid in the vicinity of experimentally isolated A. karroo trees) was monitored over a two-year period. Removal of all vegetation had the greatest effect on soil moisture, increasing regime by around 200%. Grass removal had the next most significant effect, increasing moisture regimes within 9 m of the tree by around 100%. Removal of the tree had the smallest significant effect, increasing the moisture regime by less than 20%. There was no significant difference in the moisture regime surrounding trees with heights ranging between 1, 4 m and 2, 5 m, or where various combinations of tree and/or grass defoliations were implemented. We conclude that water supply to the trees is enhanced when soil water extraction is reduced (e.g. during winter or when the sward is harmed) and argue that this may be a mechanism of accelerating bush encroachment in semi-arid savannas.

Keywords: acacia karroo; bush encroachment; competitive relationships; defoliation; eastern cape; false thornveld; grass defoliation; semi-arid savanna; shrub encroachment; soil moisture; soil water; south africa; trees; water supply; water utilization