Magnitude of Canopy Dieback and Implications for Conservation of Southern Rata-Kamahi (Metrosideros umbellata—Weinmannia racemosa) Forests, Central Westland, New Zealand

The amount of conspicuous canopy dieback in all central Westland southern rata-kamahi forests east of the Alpine Fault, between 500 m altitude and treeline, was assessed and mapped from aerial photographs taken in 1984-85 and verified by aerial reconnaissance of selected areas in 1988. At least 20% of all canopy trees, predominantly southern rata (Metrosideros umbellata) and Hall's totara (Podocarpus hallii), were dead in 1984-85. Major catchments with their headwaters east of the Alpine Fault comprised 78% of the study area and were worst affected (5%-44% canopy mortality). Because affected trees decay and eventually disappear from the canopy, the extent of visible dieback underestimated total canopy depletion, particularly where mortality occurred more than c. 15 years ago. Geographical variation in canopy dieback reflected the intensity and duration of browsing by the introduced brush- tailed possum (inferred from patterns of invasion from their liberation sites) and the influences of forest composition and timing of dieback. Although 29% of the forests showed light dieback (<10% mortality), only 11% had not experienced heavier past dieback and could be classified as having canopies largely unmodified by possums. Such widespread and continuing forest depletion, and declining possum control effort over the last decade, indicate the urgent need for a coordinated rata-kamahi forest conservation strategy, involving long-term possum control and monitoring in representative tracts.