Plant succession and soil development, Wanganui River catchment, South Westland, New Zealand

A chronosequence of plants and soils have been investigated in the Wanganui river catchment of South Westland. The sequence comprises a series of river terraces and extends over approximately 7,000 years with sites well spaced throughout this age range. Plant succession is taking place on fine alluvial material and shows a progression from dense Olearia avicenniaefolia scrub, through low seral Weinmannia racemosa forest and Dacrycarpus dacrydioides forest to climax Dacrydium cupressinum, Weinmannia racemosa/Quintinia acutifolia forest within approximately 3,000 years. No retrogressive phase was present. The patterns of plant succession were considered to follow the established model for South Westland. Soil development appeared to closely parallel the plant succession with progressive weathering producing recent, yellow brown earth, gleyed and podzolised yellow brown earth, after approximately 3,000 years and finally gley podzol soils. The general pattern of soil development together with acid extractable and organic phosphorus transformations also closely followed the existing models. However, soil nitrogen accumulation, patterns were more difficult to interpret. Plant succession proceeded rapidly over the first 1,000 years, thereafter, vegetation composition changed relatively slowly. This later development phase was marked by the presence of dominants which are long-lived and able to maintain themselves for more than one generation on anyone site. Soil changes along the sequence were characterised by decreasing pH, increasing weights of soil organic matter, decreasing acid extractable phosphorus weights and declining drainage status. Ordination of the vegetation data showed that over the first 3,000 years of forest succession acid extractable phosphorus concentrations and soil drainage were most highly correlated with the vegetation changes. Over the remaining 4,000 years the correlations of these variables became insignificant and soil organic matter concentrations were found to be most highly correlated with the vegetation changes. Plant succession is perceived as a function of the interaction of these habitat factors with phenomena associated with the individual species present in the sequence. Habitat factors and plant phenomena change with time bringing about changes in species composition