The kahikatea (Dacrycarpus dacrydioides) forest of South Westland.
- Botany Division, DSIR, Christchurch.
Dacrycarpus dacrydioides forests in South Westland between the Waitangitaona and Saltwater rivers occur on low terraces of alluvial silt. Their relationship to other river flat and swamp communities on post-glacial surfaces is described. Recently abandoned river beds are colonised by pioneer species, which give way to vegetation dominated by grasses, sedges and rushes, which in turn are invaded by woody plants, especially Coprosma propinqua and Podocarpus totara var. waihoensis. Seedlings of several tree species grow up under the shrub thickets, leading to young forests dominated by Pennantia corymbosa and various other species on the heavier silts and by Podocarpus totara on stony ground. These forests form a nursery for Dacrycarpus and other podocarps.
Mature stands occur on surfaces less than 1,000 years old. They range from those with closely spaced Dacrycarpus trees and correspondingly sparse lower storeys, which are most extensive at the seaward ends of the main valleys, to those with scattered large trees of Dacrycarpus and other podocarps and a dense canopy of small hardwoods, which occur mainly between the meanders of the Waitangiroto and Saltwater rivers. Partial destruction and rejuvenation of Dacrycarpus forest during changes in river courses are discussed.
Stands of Dacrycarpus bordering swamps are wetter and include dense colonies of Astelia gramlis, Blechnum capense and, near the coast, Freycinetia banksii and Gahnia xanthocarpa. There are transitions to "fertile" swamps dominated by species such as Phormium tenax, to "infertile" swamps on the older post-glacial surfaces with Baumea spp., Calorophus minor, Leptospermum scoparium, small species of Dacrydium, etc. and to coastal swamps bordering tidal lagoons. In some places, Dacrydium cupressinum forest is interpolated between Dacrycarpus forest and "infertile" swamp.
The extent of mature Dacrycarpus forest seems disproportionately large in relation either to the frequency of young trees within the mature stands or to the extent of seral forest containing young Dacrycarpus. The apparent imbalance resembles that which has been previously described for hill stands of podocarps, although the condition of the stands near the coast may be related to tectonic uplift of the coast line.
Dacrycarpus forms a highly distinctive forest, meriting effective conservation, but the best existing reserves are in areas threatened by changing river courses. Reserves should therefore include the seral stages leading towards mature stands