Rehabilitation of native forest species after mining in Westland

Abstract

Rehabilitation techniques for native forest were investigated at an opencast coal mine site in cut-over beech (Nothofagus) forest on alluvial river terraces at Giles Creek near Reefton, Westland. Annual precipitation at the site was 2900 mm and soils were dominantly Allophanic and Acid Brown Soils. The survival and growth of 11 nursery-raised native woody species, and natural regeneration, were examined in three covering treatments consisting of (1) overburden gravel, (2) mixed-horizon forest soil, and (3) layered-horizon forest soil. Underlying gravels were either not ripped or ripped to a depth of 80 cm. Survival of bare-root and container-grown plants 4.5 years after planting was better in overburden gravel than in both mixed and layered soil, largely because of poor survival of beech species in soil. Plant height growth in overburden gravel, however, was minimal because of nitrogen deficiency. Height growth after 4.5 years in layered soil was nearly twice that in mixed soil, the better growth in layered soil being due to improved drainage and improved nitrogen nutrition. Poor survival of the beeches in forest soil is attributed to root-rot pathogens. Ripping of underlying overburden gravel had no influence on plant survival or growth in any of the three covering materials. In the fifth year of the trial, ground cover in the two soil treatments was 38-40%, but cover in overburden gravel never exceeded 1%. Tall-growing adventive rushes (Juncus sp.) dominated in mixed soil, reflecting poor drainage, whereas native and adventive herb species dominated in layered soil. Few native species, especially woody species, were introduced from the original forest through soil replacement. Research is required to determine optimum timing of earthmoving operations in relation to natural seed dispersal.