Forest Patterns and Possum Densities within Podocarp / Mixed Hardwood Forests on Mt Bryan O'lynn, Westland

The podocarp/mixed hardwood forests on Mt Bryan O'Lynn, Central Westland, are described from 304 plots surveyed during 1975-77. Eight major plant associations are identified, using patterns of forest structure, physionomic dominance, diversity and distribution. Each association possesses distinct physiographic requirements, which ultimately results in a broad altitudinal stratification of the forest. Thus, the forests on low-altitude cutover terrace lands are dominated by Weinmannia racemosa and Cyathea smithii on partially cleared areas and by scrub hardwood species on clear-felled sites; the unmilled mid-altitudinal slopes are dominated by W. racemosa and Quintinia acutifolia; the ridges by Metrosideros umbellata and W. racemosa; the high altitude forests by Griselinia littoralis and Libocedrus bidwillii; and the alpine scrublands by scrub hardwoods, e.g., Olearia spp. and Dracophyllum spp. Forest mortality is altitudinally stratified also. Few dead stems occur in the low-altitude and alpine scrub forests. Conversely, in mid and high-altitude forests 9.6-15.5% of all standing stems are dead. The latter forests are structurally simple, are highly palatable to browsing animals, particularly possums, and appear to have been extensively modified by this marsupial. Possum densities throughout the forest profile are derived from a comprehensive extinctiontrapping programme and verified by faecal pellet surveys. Mean density is 10.7/ha, ranging from 25.4/ha to 1.9/ha in the low and high-altitude forests respectively. Current densities correlate with a gross palatibility index calculated for each association, but do not reflect present patterns of forest mortality as possum densities within the highly modified mid-altitude forests are moderate to low.