SOIL AND VEGETATION CHARACTERISTICS OF SOME TREE WINDTHROW FEATURES IN A SOUTH WESTLAND RIMU FOREST

The effects of tree windthrow on soil properties and vegetation were studied in an area of rimu (Dacrydium cupressinum) forest on a dissected glacial outwash surface in Saltwater State Forest, South Westland, New Zealand. Tree windthrow and the resulting formation of surface pits and mounds caused a redirection of pedogenesis, particularly on relatively better-drained low ridges; there, the normal development of a gleyed yellow-brown earth profile was altered towards organic soils in the pits and podzolised yellow-brown earths on the windthrow mounds. In poorly drained backswamps, organic soils also formed in the pits, while gleying processes predominated in both the windthrow mound soil and the adjacent unmodified soil. The sequence of vegetation development was different on windthrow pits compared with mounds. Mounds were rapidly colonised by ground-cover plants, especially ferns and tree ferns. These were subsequently replaced by broadleaved trees, particularly kamahi (Weinmannia racemosa). Pits usually pond with water. Plant litter then fills the pits within 30 years. The pit sites are subsequently colonised by vascular plants, but the succession involved is not clear. In this high rainfall environment, pits and mounds become unrecognisable on the land surface within about 100 years. Windthrow contributes to the currently observed soil pattern in these forests.