Distribution of native forest in the upper Clutha district, Otago, New Zealand.
The upper Clutha district extends from steep, glaciated mountains along the South Island Main Divide to broad plateaux and terraced valleys further downstream, and covers an annual precipitation gradient from 4000 mm to 400 mm. Tall forest prevails from the valley floors to the subalpine tree limit in north-western headwaters, but southwards down the rainfall gradient becomes increasingly confined to small pockets. Nothofagus is overwhelmingly dominant, with N. menziesii prevailing in the northwest and N. solandri var. cliffortioides in drier country to the east. Pockets of forest in the south of the district can consist of either species. N. fusca co-dominates with the other Nothofagus species at low altitudes through part of the Matukituki catchment, and over a small area by Lake Hawea. Low-altitude N. menziesii forest in the Makarora Valley has a similar composition to forests west of the Main Divide, and includes several conifer species and a wide range of broadleaved trees, notably Weinmannia racemosa. The hardiest conifers, Podocarpus hallii and Phyllocladus alpinus, are widely distributed in the Nothofagus forests of the district and also dominate small stands of native trees on otherwise treeless mountains. In the moister pan of the district Kunzea ericoides and Leptospermum scoparium are successional to broadleaved forest, whereas in the drier parts they are self-perpetuating. Grey scrub, dominated mainly by Discaria toumatou, is widespread on slopes where rainfall is low, and on river flats under higher rainfall. Small trees of Sophora microphylla and Olearia spp. occur locally in this scrub.