Aspects of the ecology and conservation of the threatened tree Olearia hectorii in New Zealand

Olearia hectorii Hook.f. is a low-growing deciduous tree confined to four widely separated districts in New Zealand: Rangitikei and southern Wairarapa in the North Island; and Nelson-Marlborough and Otago-Southland in the South Island. Topographically it occurs where mountains or hill country meet alluvial lowland on sites variously affected by flooding and siltation, debris avalanching, seasonal waterlogging and drought, and frost - environmental factors that reduce competition from taller trees. The species escapes competition from taller forest trees as a component of seral vegetation, occupying sites such as cut-off meanders and frosty depressions, which are marginal to forest trees, and exploiting light gaps in forest understoreys beneath lightly-canopied or deciduous broadleaved trees. Only three of the 24 extant populations of O. hectorii occur in self-maintaining habitats; the remainder have low numbers and, in many instances, habitat viability through agricultural fragmentation of the former forest ecosystem. Clearance of forest, competing ground cover of introduced grasses and herbs, and damage from browsing animals have led to a loss of at least eight populations this century, and induced a regeneration failure in many of the extant populations. However, the present regionally disjunct distribution in eastern lowland valleys and plains is probably similar to that of pre-human times.