New Zealand Ultramafics

Abstract

Ultramafic rocks comprise less than 0.1% of the New Zealand land surface but their occurrence in areas of contrasting climate and glacial history has produced a wide range of plant habitats. Compared with other New Zealand soils, ultramafic soils, ranging from deeply weathered laterites to youthful skeletal regoliths, have low concentrations of most major nutrients, a wide Mg/Ca quotient, high concentrations of nickel, chromium and cobalt, and a high pH. Ultramafic vegetation, while often strikingly different from that on adjoining rock types, has close floristic and physiognomic similarities with communities on other edaphically extreme soils. Forest on ultramafic soils is restricted to lowland and montane sites in areas of higher rainfall, while in drier areas an open mixed shrubland with isolated stunted trees appears to be the maximum vegetation development. With increasing elevation and declining soil conditions, the vegetation becomes more open, stunted and sparse with a greater proportion of low-growing, microphyllous shrubs and several small grasses. Most of the major plant families and genera in New Zealand have representatives on ultramafic soil. At present 34 taxa are considered to be ultramafic endemics with distributions limited to outcrops which escaped direct effects of Pleistocene glaciations. Studies of ecotypic differentiation, element accumulation and species response to soil amendment suggest that plant species adopt diverse strategies to achieve ultramafic tolerance.