Comparison of Forest Structure and Regeneration on Bench and Stewart Islands, New Zealand

The absence of introduced browsing animals from Bench Island, located 5 km east of Stewart Island, provides the opportunity to compare forest structure and regeneration in these two areas of similar vegetation with and without browsing animals. The population structures of the main canopy tree species, rata (Metrosideros umbellata), kamahi (Weinmannia racemosa), and rimu (Dacrydium cupressinum), are similar in the areas compared. However, on Stewart Island, browsing animals, mainly white-tailed deer (Odocoileus virginianus) and the Australian brush-tailed possum (Trichosurus vulpecula), have drastically reduced the abundance of subcanopy trees and shrubs such as Pseudopanax simplex, P. crassifolius, Fucshia excorticata, Myrsine australis, Schefflera digitata, Coprosma lucida and C. foetidissima; ferns such as Phymatosorus diversifolius, Asplenium bulbiferum and herbs like Kirkophytum lyallii have likewise been reduced. The regeneration of the tree fern Dicksonia squarrosa is also severely impeded by browsing, and the generally sparse character of the understorey vegetation on Stewart Island contrasts sharply with the nearly continuous cover on Bench Island. On Stewart Island, the more open main tree canopy and greatly reduced abundance of plants in the 15-140cm tier results in greater species frequencies in the 0-15 em tier, but browsing pressure impedes their further development. Introduced browsing animals have already significantly altered the structure and floristic composition of the rata-kamahi-rimu forests on Stewart Island and their effects are likely to be more severe in the future unless animal numbers are reduced.