Defoliation as a Means of Assessing Browsing Tolerance in Southern Rata (Metrosideros umbellata Cav.)

Old and young trees of southern rata (Metrosideros umbel/ata) were artificially defoliated to examine their response to foliage loss. Partial (50%) or total defoliation before budbreak depressed shoot growth more markedly in old than in young trees. Fifty percent leaf loss shortly after budbreak or at the end of the growing season did not significantly impair shoot growth in either age class. By contrast, total postbudbreak defoliation resulted in the death of most shoots in both old and young trees. While the terminal buds of both old and young trees totally defoliated in late autumn showed good overwinter survival, only in the young trees were there sufficient energy reserves to enable some of these shoots to flush the following season. Young trees proved to be more resilient to foliage loss, suggesting a more positive carbon balance (energy surplus) in young trees than in older trees