PROTECTIVE VALUE OF REGENERATING TEA TREE STANDS ON EROSION-PRONE HILL COUNTRY, EAST COAST, NORTH ISLAND, NEW ZEALAND

Abstract

The effect of pasture reversion to tea tree communities (comprising manuka (Leptospermum scoparium J.R. et G.Forst.) and/or kanuka (Kunzea ericoides var. ericoides (A.Rich.) J.Thompson) of varying ages) on landslide damage resulting from Cyclone Bola was studied in hill country on the East Coast of the North Island, New Zealand. Eighteen sites containing areas of both pasture and regenerating forest aged 6-79 years were selected from areas of steep, colluvial-covered slopes prone to shallow landslipping. Vegetation composition and stand dynamics were ascertained from sample plots, and aerial photographs were used to measure landslide damage. Manuka dominated younger stands but within 20-30 years of establishment, kanuka had become dominant. Mean canopy height stabilised at 13 m by age 40, basal area reached 40 m2/ha by age 30 years, and stand density reduced from 20 000 stems/ha at 10 years to 3000 stems/ha at 40 years. Landslide damage showed a rapid and highly significant reduction against increasing age of tea tree stands and was estimated to be 65 % less than pasture at 10 years and 90 % less at 20 years. These findings have implications for land-use options, including clearfelling of indigenous vegetation for planting of Pinus radiata D.Don.