Defoliation was used to test the hypothesis that young southern rata trees, because of a greater ratio of green to non-green tissue, may be better able to withstand browsing than more mature trees. Three factors emerged as being important: (1) the timing and (2) extent of leaf loss, and (3) the age of the tree. Partial defoliation at all stages of the growing season depressed growth more in old than in young trees. Total defoliation before budbreak markedly depressed growth in old and young trees, but did not result in the death of shoots. All shoots totally defoliated immediately after budbreak died over winter. Conversely, shoots totally defoliated in late autumn survived. Only those of one young tree however put on any measureable growth the next season. The role of possums (Trichosurus vulpecula Kerr) in mortality of southern rata in Westland coniferous-broadleaf forests is discussed in the light of these results.