Is Pittosporum patulum Hook. f. threatened by pest herbivory in eastern South Island, New Zealand?
- Science and Research Unit, Science, Technology and Information Services, Department of Conservation, P.O. Box 5244, Dunedin, New Zealand
- Landcare Research, Private Bag 1930, Dunedin, New Zealand
We examined the threat status of the low tree Pittosporum patulum throughout its range in eastern South Island, based on plot-based sampling of habitat, defoliation by mammalian herbivores, demographic and dieback characteristics. Using environmental modelling (Land Environments of New Zealand), we found no explanation for the ‘gap’ in its disjunct distribution from Nelson–Marlborough–north Canterbury to south Canterbury as a component of upper montane Nothofagus forest and non-Nothofagus subalpine scrub. Sizeclasses in some populations suggest pulses of recruitment that may be phenologically or disturbance engendered, whereas others have demographic evidence for more continuous recruitment. In forest, disturbance appears not as important as environmental stress in maintaining understorey light gaps that allow it to reach reproductive maturity. A range of introduced mammalian herbivores appear to defoliate P. patulum, although consistently high levels of defoliation on adult foliage above ungulate browse–height point to possums (Trichosurus vulpecula) as the main pest. Demographic data and herbarium records show adults are few in Nelson–Marlborough and north Canterbury, where the species’ viability is in question despite many juveniles. Alternatively, south Canterbury populations, although browsed, show less dieback, especially in subalpine scrub. Its variable demography may be related to the history of possum colonisation throughout its range. Evidential support is provided for its threat ranking of ‘nationally endangered’.