Responses to sheep browsing at different stocking rates: Water relations, photosynthesis and carbon allocation in two semi-arid shrubs

The ecophysiological responses of two karoo shrubs to sheep browsing at different stocking rates were studied. The responses of a relatively unpalatable shrub (Pteronia tricephala), and of a shrub of greater palatability (Eriocephalus ericoides) were examined at high, intermediate and at low stocking rates. These responses were compared with the ecophysiological attributes in the absence of browsing (control). Despite the two species being of different palatabilities, the estimates of the extent of plant utilization confirmed that both species were browsed at all stocking rates. No significant changes in ecophysiological processes were apparent in E. ericoides at high stocking rates. However, non-structural carbohydrate status decreased at the highest stocking rates, which may signify a loss in plant vigour. In contrast, P. tricephala responded to high stocking rates with an improvement in plant water relations, and an elevation in net photosynthetic rates. The carbohydrate status of heavily browsed P. tricephala plants remained unchanged, and leaf polyphenol concentrations decreased. These findings suggest that relatively more palatable shrubs such as E. ericoides may not be able to survive continuous heavy browsing, and that less palatable shrubs, such as P. tricephala, have compensatory mechanisms that operate only at high stocking rates. These compensatory responses may increase the acceptability (low polyphenol levels) of relatively unpalatable shrubs, and may confer a competitive advantage (high carbon reserves and elevated photosynthesis) on these shrubs when subjected to browsing at high stocking rates.