Plant species richness under Pinus radiata stands on the central North Island Volcanic Plateau, New Zealand

Exotic pine plantations constitute a significant landscape feature in the North Island of New Zealand but their conservation value for native plant species is not often documented. Pine stem density, height and basal area of nine plantations of Pinus radiata ranging in age from 6 to 67 years in Kinleith Forest was determined. Pines reached heights of 60 m, and stand basal areas up to 183 ± 14 m(2)ha(-1). The abundance of woody shrubs, tree ferns and ground ferns was assessed in each stand. Understorey composition of shrubs and ferns was reflected on the first two axes of DCA ordinations and correlated with the age of the pines. Adventive shrubs predominated in stands < 20 years old. Light-demanding native shrubs with bird dispersed fruits predominated in older stands, with more shade-tolerant species in the oldest site. Species richness increased rapidly in the first 11 years, but thereafter more slowly. Twelve native shrub species and 22 ferns were recorded from the most diverse stands. Richness and species composition were related to stand age, and probably also to topographical heterogeneity and aspect. Tree ferns reached densities of 2000—2500 ha(-1) and basal areas of 20—30 m(2)ha(-1) in the older stands. Initially the tree fern population was strongly dominated by Dicksonia squarrosa, which comprised 84% of individuals overall. Five species were present by 29 years. The faster growing Cynthea medullaris and C. smithii achieved greater heights than the Dicksonia spp., and their relative biomass was greatest in the oldest stands.