A direct gradient analysis was conducted on Mt Egmont, New Zealand, firstly to examine quantitatively changes in composition, physiognomy and structure of the vegetation along the altitudinal gradient in order to investigate the accepted notions of vegetation zonation, and secondly to relate such an analysis to other New Zealand gradient analyses and to the two views on vegetation description and analysis, the continuum view and the discrete community (association) view.
Vegetation data was collected at altitudinally defined sites on five major ridges on Mt Egmont; North Egmont (Blundell’s Track, Ngatoro Track and the Razorback), York Road Track Ridge, Stratford (Summit Road), Dawson Falls (Summit Track), and Lake Dive Track Ridge. The data collected on the York Road Track Ridge provided the basis for describing altitudinal change while the remainder was used to examine in detail the tree-shrub and shrub-tussock interfaces.
Sample size (quadrat and line transect) was determined after minimal area checks to ensure that a representative combination of species was sampled at each site. Data from a total of sixty plots was collected and preliminary analysis, including graphing of the major species distributions and calculation of similarity matrices to compare quadrats, revealed a tendency toward discontinuity in species distributions. This allowed a classificatory rather than ordinatory strategy of data analysis to be followed. Cluster analyses based on statistical tests of species association were used to determine "natural" groupings of positively associated species and qualitative and quantitative classifications of samples were used to define vegetation zones or groupings at specific similarity index levels.
Tree-shrub interfaces and tussock-shrub interfaces were compared on the basis of species complement, the degree of floristic, physiognomic and structural discontinuity in the vegetation, and differences in species interaction at each of the locations.
The effects of data amalgamation, altitudinal interval employed, altitudinal range encountered, and classificatory versus ordinatory techniques, on the conclusions drawn were examined.
Graphing of species distributions, cluster analyses, and classifications supported in part previously described zonation patterns but enabled a refinement of the generally accepted notions of vegetation zonation on Mt Egmont. In contrast to previous research the vegetation zones identified have a quantitative and objective basis.
Comparison of the results with those of other New Zealand gradient analyses enabled elucidation of several major species distributions and species interactions because of the unique species complement found on Mt Egmont.
Comparison of different data collection and manipulation techniques indicated that decisions made may cause data to conform to one or other of the two major approaches (continuum or discrete community types) to vegetation description and analysis.
The study of the tree-shrub interfaces and shrub-tussock interfaces showed that the vegetation on Mt Egmont exhibits a spectrum of species distributions ranging from continua through to discontinuities at different locations and specific examples of each are described. Possible explanations advanced include differences in species complement and species interaction, the non-equivalence of the altitudinal gradient at different locations, and differences in environmental factors including slope and aspect.
