Journal of Tropical Ecology (2012) 28:161–169. © Cambridge University Press 2012 doi:10.1017/S0266467412000028 Coping with herbivory at the juvenile stage: responses to defoliation and stem browsing in the African savanna tree Colophospermum mopane David C. Hartnett ∗,1 , Jacqueline P. Ott ∗ , Kathryn Sebes ∗ and Marks K. Ditlhogo† ∗ Division of Biology, Kansas State University, 104 Ackert Hall, Manhattan, KS 66506 USA † Department of Biological Sciences, University of Botswana, Private Bag UB0022, Gaborone, Botswana (Accepted 4 January 2012) Abstract: Responses of plants to herbivory are dependent on the type of damage and the ontogenetic stage of the plant. We compared the effects of stem pruning and defoliation on seedlings of Colophospermum mopane, an ecologically important tree species widely distributed in southern Africa. The growth of 160 greenhouse-grown juveniles were measured for 6-mo after germination and then 6-mo after treatments including 50% defoliation, 100% defoliation, 50% stem pruning and controls. Pruning resulted in 30% reductions in total leaf area, height and biomass. Partial defoliation resulted in 30% reductions in total leaf area and plant biomass. However, complete defoliation resulted in a 30% increase in biomass production, a doubling in leaf and lateral branch number, a 45% reduction in leaf size, and no change in total leaf area. Thus, completely defoliated seedlings showed greater performance than those that were only partially defoliated, indicating that C. mopane has become adapted to the chronic and severe defoliation inflicted by Imbrasia belina caterpillars. Comparison of our results with other studies indicates that C. mopane seedlings are less herbivory-tolerant than adults and that pruning has more negative effects than defoliation. Thus, seedling browsers may constrain recruitment in C. mopane, influencing its population dynamics and abundance. Key Words: Botswana, browsing, defoliation, herbivory, plant–herbivore interactions, savanna, seedlings, tree recruitment INTRODUCTION In addition to factors such as the timing, frequency and intensity of herbivory, the responses of plants to herbivory are influenced by both the type of damage and the ontogenetic stage of the plant. For example, consumption of woody stems and branches by mammalian herbivores (hereafter referred to as pruning) and leaf tissue consumption by insects (hereafter referred to as defoliation) typically result in very different responses in re-growth rates, shoot and leaf size and foliar chemical composition (Bergstrom et al. 2000, Bryant et al. 1991, Hrabar et al. 2009, Lehtila et al. 2000, Messina et al. 2002, Rooke & Bergstrom 2007). Contrasting effects of pruning versus defoliation on plant architecture are expected as only the former typically involves developmental changes in response to removal of the apical meristem and shifts in the root/shoot ratio. Woody plants show responses following pruning including increased branch length and 1 Corresponding author. Email: dchart@ksu.edu often larger leaf size (Bergstrom et al. 2000, Hrabar et al. 2009, Lehtila et al. 2000, Messina et al. 2002, Rooke et al. 2004), whereas defoliation often results in smaller branch and leaf size (Piene et al. 2003, Rooke & Bergstrom 2007). The ontogenetic stage of the plant when damaged can also play a key role in determining both the pattern of damage and plant responses (Boege & Marquis 2005, Dirzo 1984). The patterns of herbivory and the resources available for plant responses vary significantly from the seedling to mature reproductive stages. In general, lower resource-acquisition capabilities (leaf and root surface area), limited resource storage, and resource-allocation constraints of juveniles compared with adults suggest that herbivory tolerance (re-growth capacity) of juveniles might be lower than adults. An accurate assessment of the selection imposed by herbivores on plants requires study of responses at all ontogenetic stages, but our understanding of plant responses to herbivory is currently limited as most studies involve experiments and observations at only the adult stage (Boege & Marquis 2005). Woody species of tropical savannas and woodlands typically experience both leaf and stem tissue damage