Rapid chemical responses of Acacia karroo to early dormant season defoliation in a semi-arid subtropical savanna

Research was conducted to investigate rapid chemical responses of Acacia karroo to severe defoliation in a semi-arid, subtropical savanna. A flock of 250 goats reduced available foliage by 87% in a 10.5-ha area of Acacia woodland over 3 weeks during May (early dormant season). During the first 10 days, concentrations of nitrogen (N), neutral detergent fibre (NDF) and condensed tannin (CT) did not change. The concentration of potentially hepatotoxic alkaloids (PHAs) decreased. There were no immediately detectable effects of defoliation intensity or recovery time on any of the measured chemical variables. There were no relationships between N and NDF, CT or PHA. Given that the treatments were applied at the start of the dormant season, the metabolism of mature or senescing leaves may have precluded rapid increases in defence. Because a previous study under similar browsing conditions showed increased defence in A. karroo at the same time of year, it was concluded that intra-annual variations in rainfall, and the resulting variations in plant growth activity, lead to intra-annual variations in plant responses to defoliation in semi-arid subtropical savannas. Such variations need to be accounted for in future experiments aimed at investigating induced responses to browsing in savannas.