Environmental correlates of species richness at Waipoua Forest Sanctuary, New Zealand

Descriptions of 247 forest stands at Waipoua Forest, Northland, were used to explore relationships between species richness (alpha-diversity) of the vascular flora and stand environmental characteristics, both in terms of total flora and within a number of the component synusiae. The Waipoua forests, with an average of 52 species per forest stand, are comparatively species-rich compared to other New Zealand forests. An ordination score (interpreted as expressing variation in soil fertility related to soil catenary sequences), altitude, and canopy height correlated most strongly with indices of species richness and species diversity, although different synusiae showed different trends. Models using these factors predicted that greatest total species richness and tree species richness would occur in tall forest on moderately infertile sites, but at high altitude for total richness and low altitude for tree richness. Epiphyte richness was highly correlated with altitude, with greatest richness in tall forest on sites at high altitude. Observations of highest total and tree richness at intermediate positions on some environmental gradients are consistent with the intermediate stress/disturbance hypothesis.