Biological flora of New Zealand 16: Pittosporum kirkii Hook.f. ex Kirk, Kirk's kōhūhū, thick-leaved kohukohu. Issue 1 (2nd January 2021) - Myron, Kirsty J.
ABSTRACT: This review summarises the ecology and biology of the North Island New Zealand endemic evergreen epiphytic shrub Pittosporum kirkii Hook.f. ex Kirk (Pittosporaceae). While mainly found as an epiphyte associated with other nest epiphytes such as Astelia spp., it also exhibits rupestral and terrestrial lifestyles. Pittosporum kirkii is distinguished from other members of New Zealand Pittosporum by distinctive thick, fleshy, coriaceous leaves, which may assist survival in water-stressed habitats. Water availability is speculated to be the most limiting factor across all of the habitats that P. kirkii occupies and when under water stress, the species utilises a desiccation postponement strategy. Pittosporum kirkii has a restricted distribution with strong habitat preferences restricting it to upper lowland and lower montane mainly old-growth forest ecosystems of the upper North Island. The highest probabilities of P. kirkii occurrence lie within mean annual temperature between 7.8°C and 15.6°C, mean annual rainfall between 1172 and 4432 mm, mean minimum solar radiation in the coldest month between 4.8 and 6.6 MJ/m 2 /day, and vapour pressure deficits between 0.15 and 0.39 kPa. Pittosporum kirkii is currently listed as at risk–declining and qualified as data-poor and partial decline because of a lack of autecological information. Palatability to introduced brushtail possums and forest clearance have been suggested as the main agents of decline. Due to significant morphological differences between mainland New Zealand and Great Barrier Island/Aotea populations, further study is warranted to determine distinctness. Including P. kirkii in restoration plans and conservation monitoring is recommended in areas where it naturally occurs as this species is recognised as an indicator of overall forest health. Further detailed research on P. kirkii would be invaluable to fill the current gap in knowledge on its ongoing threats, sexual expression, pollination and seed dispersal.