Introduced conifer spread in the South Island high country, New Zealand

Abstract

Throughout the high country grasslands introduced conifers have shown a tendency to develop wildling populations in surrounding landscapes. Investigations were carried out on various aspects of the ecology and the importance of land management on introduced conifer spread in the South Island high country. This was carried out through a literature search, and a case study of conifer spread on a high country run.

"The Hossack", a high country run adjacent to Hanmer forest in North Canterbury was chosen as a study site because of the variety of conifer species present, the apparent large natural variations in soil fertility and vegetation cover, and a change in 1974 from a purely sheep (Ovis aries) oriented land management scheme to dominantly cattle (Bos taurus).

The spread was first assessed by mapping the pre and post 1974 distributions of conifers in relation to the broad vegetation patterns through field observations. Areas appearing reasonably uniform in their vegetation and landform then had their ground cover, soil fertility, and conifer age distributions quantitatively sampled. The FORTRAN computer programs DECORANA and TWINS PAN were used to clarify the relationships between the introduced conifers, vegetation, and other environmental variables.

Conifer establishment prior to 1974 was found to be restricted to areas of Kanuka (Kunzea ericoides) / Manuka (Leptospermum scoparium) scrub. Since 1974 conifer establishment has become widespread throughout most of the adjacent grassland areas. Conifer population densities in the grassland areas ranged from 0 stems per hectare on the ridge tops and valley heads to 2100 ± 660 stems per hectare on the hill faces.

Climatic factors and seed fall have been suitable for conifer establishment since the 1940's and it is concluded that the land management prior to 1974 had prevented conifer establishment in the grassland areas through the grazing of wildlings by sheep. Conifer establishment during this period was restricted to the areas of Kanuka/Manuka scrub where the grazing intensity would have been significantly less.

Slope was found to be the most important underlying factor in the present conifer establishment account for this:

Three mechanisms were identified to account for this:

1. Less steep, flatter areas have higher nutrient levels promoting more vigorous ground cover vegetation which out compete the germinating conifers. 2. Less steep flatter areas possess a greater number of species that have been identified as important in a sheep's diet. Flatter areas such as ridge tops and gully bottoms are also easier for stock feeding and movement. This would result in higher localised grazing pressure of these sites. 3. Steep slopes are less stable and more prone to disturbance from erosive events and stock movements reducing vascular cover and initiating early successional vegetation less attractive to sheep.

Introduced conifers were observed growing well on recently depositedand uncovered alluvium. Good mycorrhizal relationships are thought to aid conifer establishment on these sites. Future potential for further spread at Hanmer and some land management options are discussed. A strategy for the control of unwanted conifer wildlings is proposed and recommendations made for future high country plantings.