-1 Acacia karroo responses to early dormant season defoliation and debarking by goats in a semi-arid subtropical savanna Peter Scogings 1, * and Mfundo Macanda 2 1 Department of Agriculture, University of Zululand, Private Bag X1001, KwaDlangezwa 3886, South Africa; 2 Do ¨hne Agricultural Development Institute, Private Bag X15, Stutterheim 4930, South Africa; *Author for correspondence (e-mail address: pscoging@pan.uzulu.ac.za; phone: +27-35-902-6063; fax: +27-35-902- 6056) Received 12 July 2004; accepted in revised form 14 December 2004 Key words: Bark stripping, Browse, Coppice, Induced responses, Tannins Abstract Research was conducted to investigate responses of Acacia karroo to severe defoliation, with or without debarking, in a semi-arid, subtropical savanna. A herd of 250 goats reduced available foliage by 87% in a 10.5-ha area of Acacia woodland over 3 weeks during the early dormant season (May). The goats debarked 37% of the trees. On average, bark was stripped less than halfway round a stem and 11% of the bark area was affected. Trees 1.6–2.5 m tall and 11–30 cm in basal circumference were most susceptible to debarking. A year later, there were no detectable effects of defoliation on the number of browseable shoots, internode length, shoot length, spine length, number of seed pods, neutral detergent fibre (NDF) or nitrogen (N). Defoliation tended to increase condensed tannin concentration (CT), but the effect was not significant (p > 0.05). The absence of goats during the growth season following defoliation allowed the trees to recover. Debarking had greater impacts on the trees than defoliation had. Debarking reduced the number of browseable shoots, while ringing at the base reduced CT and increased N. Debarking tended to reduce shoot and spine length, and halve the number of pods, but the effects were not significant (p > 0.05). Responses of basally ringed trees indicated rapid vertical growth when the trees were highly vulnerable to browsers and fires, suggesting strong selection for herbivore/fire tolerance. Among all treatments, excluding basal ringing, there were significant variations in NDF and CT between years. There was a positive relationship between N and NDF, and a negative relationship between N and CT, in 1 year, but not in the other. Among basally ringed trees, CT was negatively related to N, but NDF was not. The strong trade-off between N and CT in basally ringed trees probably indicates that large changes in root:shoot ratio are required to effect definite changes in defence. Introduction Given predictions of increased woody plant pro- duction in southern Africa under climatic change (Bond and Midgley 2000), and the observed im- pacts of human activities on savannas (Scholes and Archer 1997), predictive understanding of woody plant ecology in subtropical regions is critical for the conservation and management of savannas. However, relative to the research effort given to browse–browser interactions in boreal and tem- perate regions, few studies have explored woody plant responses to browsing in savannas (Scogings 2003). Furthermore, the understanding of savanna browse–browser interactions is dominated by hypotheses developed in boreal and temperate Plant Ecology (2005) 179:193–206 Ó Springer 2005 DOI 10.1007/s11258-004-7809-1