The ecology of lowland totara in South Island, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Historically, lowland totara was a common forest tree throughout much of lowland New Zealand but is now confined to small, mainly protected, areas usually as the dominant species in mixed podocarp forest with matai and kahikatea. Lowland podocarp forest has suffered the most extensive reduction of all New Zealand forest types, with only circa 1% of the original forest cover remaining. The lowland totara dominated stands remaining therefore represent an highly fragmented population with relatively few individuals, making the collection of large data sets difficult. Sample sizes are small in some stands due to the limited area of to tara dominated forest (circa 1 hectare). The present study aims to describe the natural extent of lowland totara forest, document its reduction and current status, and to provide for the long-term survival of this species in the now fragmented forest environment. Literature relating to the ecology of Podocarpus totara is reviewed with respect to vegetative morphology and growth, reproductive biology, timber properties and· utilisation, and population dynamics. The ecological niche of lowland totara is defined, examining site characteristics and soil type. The distribution of lowland totara dominated stands is a function of several interlinked site characteristics such as climate, landform, drainage, disturbance regime, soil type, and altitude. Lowland totara appears to be confined to semi-fertile or fertile soils which are the result of landform and disturbance e.g. flood events on a river plain. Stand descriptions identify lowland totara forest as ranging from pure totara, to totara - matai, to totara - matai - kahikatea, to dense mixedpodocarps. The relative proportions of totara, matai, and kahikatea appear to be due to edaphic features such as fertility and drainage. In dense mixed podocarp forest, totara is confined in small populations to optimal sites such as terraces and ridges. The podocarps have differing nutrient, light, and disturbance regime requirements and these factors work together to partition the environment into optimal areas for the regeneration and persistence of each species. Totara dominated stands require high light to initiate and capture a high fertility site, and relatively free-draining ABSTRACT 3 fertile soils to remain dominant. Sub-optimal totara sites will see the dominance of other species such as rimu. Totara and kahlkatea dominate in catastrophically disturbed areas, miro responds to medium levels of disturbance such as tree fall, and rimu dominates in areas subject to continual disturbance such as canopy collapse. Regeneration of lowland totara dominated forest is seen to be a consequence of seedling dispersal to preferred microsites, and disturbance history. The broad scale forest pattern is detennined by the level of disturbance and the finer scale pattern of tree distribution is a result of light levels, parent and perch trees, and edaphic properties. There is no evidence that totara dominated stands are older than first generation, indicating that periodic catastrophic regeneration is required to reinitiate stands. The alteration of the natural disturbance regime by humans may pose problems for the future of lowland totara. There is no evidence that totara forest regenerates under a senescing totara canopy. Presently protected areas are therefore likely to undergo floristic composition change away from totara/matai/kahlkatea in the absence of disturbance. A landscape management plan is proposed as a mechanism for appropriate land use using ecological principles at catchment scale and to provide adequate areas for the long-term survival of totara dominated forest in a fragmented landscape. New Zealand's natural environment has been radically altered with the invasion of humans, introduced weeds and pests, and alteration of the natural disturbance regime. Restoration and revegetation are therefore options for the future survival of lowland totara in a fragmented landscape. Silvicultural plantations can provide timber (to tara heartwood being highly prized), income, and seed sources for regeneration. Land values such as water and soil quality will also benefit from artificial plantations. Stand spacing and age structure is described and can be used in the future when revegetation projects have the aim of approximating natural stands.