|dc.description.abstract||This research investigated the autecology of the New Zealand endemic shrub epiphyte Pittosporum kirkii (Pittosporaceae). Pittosporum kirkii most frequently displays an epiphytic lifestyle, perched amongst nest epiphytes in the canopies of emergent or canopy trees in old-growth forest. The main objective of this research was to enhance the understanding of Pittosporum kirkii, with a focus on ecological, morphological and physiological characteristics.
Ecological characteristics were investigated in field surveys of five key Pittosporum kirkii populations that encompass its known latitudinal extent. Survey results showed that P. kirkii is late successional epiphyte commonly found in the inner crown of large canopy and emergent hosts amongst nest epiphytes, it has a short flowering season and shows clinal patterns in leaf length with increasing latitude. Pittosporum kirkii meets the definition of a facultative epiphyte as it grows in rupestral, terrestrial and epiphytic lifestyles. The most common host species for P. kirkii is Dacrydium cupressinum.
Flowering was concentrated between November and December and individuals in the populations examined were functionally dioecious with a preponderance of males; flower pollinators and seed dispersers are uncertain.
Field data and supplementary data sets (e.g. herbarium records) were combined to develop a predicted environmental distribution map. Pittosporum kirkii has an abrupt distribution with strong habitat preferences restricting it to the upper North Island, in upper lowland and lower montane old-growth forest ecosystems. High vapour pressure deficits (>0.39 kPa) and low annual rainfall (<1172 mm) restrict environmental distribution.
Investigations into the physiological and morphological characteristics of Pittosporum kirkii were focussed on water relations because water availability is speculated to be the most limiting factor across all of the habitats that P. kirkii occupies. In a glasshouse experiment P. kirkii seedlings were subjected to three levels of desiccation stress alongside epiphytic congener P. cornifolium. In the context of international literature, the stress strategies of each species were identified from analysis of morphological adaptations and physiological responses. Pittosporum kirkii has small relative leaf area, thick coriaceous upper leaf cuticles, substantial hypodermal tissue, and under desiccation stress, exhibited rapid reductions in stomatal conductance and photosynthetic activity with stress (ceased function at mean pre-dawn water potentials of -0.8 MPa). This evidence aligns P. kirkii with a desiccation postponement strategy. In contrast P. cornifolium has thinner cuticle hypodermal tissue, larger relative leaf area and maintains photosynthetic function under greater stress (ceased function at mean pre-dawn water potentials of -1.7 MPa), and as a result, lost water potential faster than P. kirkii. Therefore P. cornifolium is concluded to align with a desiccation tolerance stress strategy. Both species recovered within 3 days after re-watering, an important response for the drought-prone epiphytic, terrestrial and rupestral environments that both species inhabit.
Under the current threat classification Pittosporum kirkii is listed as in decline and is considered to be a data poor species. The threats to this species are uncertain; although possum browsing and forest clearance have been proposed, all available evidence is anecdotal and circumstantial. The abundant presence of P. kirkii on possum-free Great Barrier Island suggests relief from possum browse but other browsers are also restricted. Regenerative failures were not evident in mainland or offshore island populations and possum herbivory was not detected in mainland populations, likely because numbers were controlled. However, there may be other factors that may be potentially causing the decline of P. kirkii which could be identified with further research into pollinators, seed dispersal and sexual expression.
Pittosporum kirkii then is a distinctive member of New Zealand’s small guild of endemic shrub epiphytes but overall has the narrowest distribution, narrowest environmental profile, greatest habitat specificity and is the least abundant.
To present the key findings of this research alongside existing information, one chapter is presented in the format of the New Zealand Journal of Botany Biological Flora Series.||