Ecological studies in the indigenous forests of north Westland, New Zealand

The geographical limits of the four Nothofagus species in north Westland are described. An examination is made of the stability of the existing beech forest boundary and of factors which may limit the expansion of the boundary. Reccent rates of migration on different growing sites are estimated. The slowest rate of migration, 0.1 m/yr, occurs on hill slopes. More rapid migration is possible along flood plains, across poorly-drained high terraces and in the upland zone of the southern Paparoa Range. Nothofagus migration appears to proceed largely by slow marginal spread from existing stands and occasionally, by the formation of outliers following the long-distance dispersal of seed over distances of up to 12 km. It is proposed that the Nothofagus species have spread slowly throughout the past 10 000 years from nine, local glacial-period refugia and that there are no barriers to further expansion into central Westland. The population structures and regeneration requirements of the seven major canopy tree species of the lowland forests in north Westland are compared. The species studies were Dacrydium cupressinum, Podocarpus ferrugineus, Metrosideros umbellate, Quintinia acutifolia, Weinmannia racemosa, Nothofagus truncate and N. fusa. Seedling establishment, growth and mortality were studied using permanent quadrats and tagged populations over a three year period. A deficiency of large seedlings, saplings, poles and small canopy trees is evident in D. cupressinum, P. ferrugineus, M. umbellate and, to a lesser extent, N. fusca populations. For D. cupressinum the rate of regeneration appears to have declined from the period 400-500 years ago up till about 100 years ago when an upsurge in regeneration began. This parallels in extent and timing a regeneration decline in P. ferrugineus but predates a decline in M. umbellate and N. fusca. It is postulated that long-term changes in regeneration rates resulted from catastrophic damage to the hardwood component of the forest canopy within the period 300-500 years ago.