Environmental and floristic gradients in Westland, New Zealand, and the discontinuous distribution ofNothofagus

Abstract

Latitudinal gradients of biologically significant climatic factors and floristic diversity of the woody forest flora in the western South Island are investigated employing cluster analysis. The most important environmental discontinuities are associated with the southern end of the Paparoa Rangenear Greymouth and coincide with the regional southern limit of 15 woody, mainly northern and eastern floristic elements. The increasingly superhumid climate further south creates optimum conditions for the resident podocarp-broadleaved forest, resulting in the total exclusion of Nothofagus in central Westland. Although latitudinal gradients of chmate and floristics are negligible between Pukekura and Haast, Nothofagus menziesii re-appears in the Paringa district. The more extensive stands occur in inland valleys west of the main divide; smaller and discontinuous stands west of the Alpine Faultare mostly associated with infertile and resistant parent rock of the coastal hills. Since comparable habitats do not occur in central Westland, further northward spread of N. menziesii is not expected. Previous studies on the distribution and dynamics of Nothofagus outlier stands are reviewed and results are interpreted as indications of a relatively stable ecotone in north Westland. Published pollen profiles from the same region are confirmatory by implying long-term low-density presence of the N. fusca group, even in the southernmost part of its present range.