Structure and dynamics of lowland podocarp-broadleaved forest in the Central North Island, New Zealand : the effects of different silvicultural regimes on stand structure and tree species composition.

Abstract

Podocarp-broadleaved forests and their alliances form one of the largest forest groups in New Zealand and cover an area of approximately 2.9 million hectares. Their vast exploitation and deterioration in the past 150 years called for sustainable forest management as an imperative need. As a consequence, commercial harvesting of these forests is restricted to private land only and is strictly controlled by the Ministry of Primary Industries under the Forests Act 1949 as amended in 1993. The Act demands sustainable management of podocarp-broadleaved forest in a form of selective harvesting of podocarps that is limited to single trees or small groups. But there is concern that selective harvesting of podocarps in this forest type is not sustainable due to the complex structure of those forests.

This thesis explores the regeneration dynamics in an old-growth podocarp-broadleaved forest in the Central North Island and how past harvesting has altered its structure. Tree core analysis is an important tool to obtain information on tree age and growth. As Beilschmiedia tawa is the dominant tree species in this forest and as little is known about the suitability of tree cores of this species in age and increment studies, their prospects and limitations are elaborated first. The second study examines the structure of old-growth podocarp-broadleaved forests and the natural processes that drive recruitment success and failure. In the third study, the impact of different sizes of harvesting gaps on tree species regeneration and subsequent growth is investigated. The final chapter evaluates harvesting prescriptions in the Forests Act on the basis of key findings of the above studies.

The results demonstrate that false and missing rings are common features in B. tawa tree cores but that individual errors in the form of overestimation and underestimation of calculated increments cancel each other out on a stand basis, making stand level predictions reliable. They further illustrate that the scarcity of large-scale disturbances in old-growth podocarp-broadleaved forest is responsible for a regeneration gap of the podocarps Dacrydium cupressinum, Dacrycarpus dacrydioides, Prumnopitys ferruginea, Prumnopitys taxifolia and Podocarpus totara but that harvesting can improve their growth conditions significantly if the gaps created are large enough. This is, however, not in accordance with the low-impact harvesting prescriptions in the Forests Act, while the results presented here show that selective harvesting of emergent podocarps is not sustainable and accelerates the decline of those species in this forest type.