Abstract:
In New Zealand the peri-urban land use transition from pasture to urban provides an opportunity to improve biodiversity through the rehabilitation of forests and wetlands within a matrix (Meurk and Swaffield, 2000; Meurk and Hall, 2006) of connected ecosystem patches and corridors of variable sizes. Some such patches and corridors will be riparian ecosystems within Low Impact Urban Design and Development (LIUDD; van Roon and van Roon, 2009) style developments. The objective in forest rehabilitation sites is to provide the conditions and seed stock that will enable permanent forest cover. Only by this means can the objectives of the New Zealand Biodiversity Strategy, LIUDD, and the urban forest matrix (Meurk and Swaffield, 2000) be achieved. This research investigated planted tree communities within gullies in Manukau, New Zealand. The changes in the composition and structure were examined in relation to planting age, site and landowner maintenance. The composition elements examined included planted trees, weed and grass species, and secondary tree species. The aim was to determine the likelihood under the current conditions of these plantings developing into secondary forest and what actions if any need to be taken to improve these chances. Adequate and steady tree growth is occurring, pasture grasses take up to 17 years to be displaced, weed and pest management is very variable across landowners, and natural recruitment of future canopy native species is very low. The scarcity of secondary native species settlement brings into question the species balance of the initial plantings and whether or not there were sufficient canopy tree seedlings. Further supplementary planting of secondary species, and weed eradication on most sites, needs to be carried out to ensure that these plant communities mature into permanent native forest stands that will provide both terrestrial and instream biodiversity returns. The conclusions reached will assist future decision-making in the creation and maintenance of other re-vegetation sites in terms of initial planting composition, supplementary planting and maintenance.
