Abstract
Inland Otago’s native woody vegetation persists near the climatic limits of forest development in Aotearoa New Zealand. Severe frost, summer drought, infertile soils, and a long history of burning and browsing have reduced forests and woodlands to scattered remnants dominated by a small suite of stress-tolerant species. This thesis investigated the mechanisms that enable woody plants to persist and regenerate in these climatically marginal landscapes. It integrated palaeoecological and biogeographic evidence, species distribution modelling, seed biology, and experimental assessments of freezing and drought tolerance, with particular emphasis on the montane conifer Podocarpus laetus and the semi-deciduous angiosperm Sophora microphylla.
Palaeoecological and biogeographic evidence shows that inland Otago’s vegetation reflects the combined effects of late-Quaternary climatic oscillations, orographic rain-shadowing, and anthropogenic disturbance. Repeated exposure to cold, moisture limitation, fire, and browsing favoured long-lived conifers and small-leaved angiosperms capable of surviving episodic stress but left many populations isolated in sheltered or fire-resistant refugia. Species distribution models developed across the region’s strong west–east climatic gradient showed that P. laetus was associated primarily with cooler, moister environments, whereas S. microphylla extended further into warm, dry intermontane basins. Modelled distributions were broadly consistent with palaeoecological evidence and identified areas where historical disturbance, rather than climate, now restricts occupancy. Future projections indicated that warming may reduce the importance of extreme cold as a distributional filter, while increasing water deficits impose stronger constraints, particularly in eastern inland Otago.
Seed morphology and germination experiments demonstrated that regeneration in New Zealand Podocarpus is constrained by underdeveloped embryos and physiological dormancy. Micro-computed tomography revealed substantial structural variation among species and populations, including distinctive seed-coat features in P. laetus. Germination trials showed that embryo development and dormancy release depend on prolonged temperature treatments that include cold stratification. Variation among provenances indicated geographic differentiation in germination cues. Slow and episodic recruitment in P. laetus therefore reflects complex temperature-dependent controls on embryo maturation and germination, in addition to seed availability and microsite conditions.
Freezing experiments revealed substantial inter-population variation in P. laetus, with foliage from colder provenances tolerating lower temperatures than foliage from milder sites. Sophora microphylla showed less consistent population differentiation, indicating a greater potential role for phenotypic plasticity. Drought experiments identified contrasting response strategies. Podocarpus laetus maintained photosynthetic function longer under moderate drought and increased allocation to roots but recovered poorly following severe stress. Sophora microphylla declined more rapidly and shed leaves under drought, but some seedlings recovered after rewatering. Variation among S. microphylla populations suggested that drought responses have also been shaped by local environmental conditions.
These results show that persistence in inland Otago’s woody flora depends on interactions among climatic tolerance, regeneration dynamics, life-history strategy, and disturbance. Podocarpus laetus represents a persistence-oriented strategy in which longevity, slow growth, and tolerance of chronic stress allow populations to survive despite infrequent recruitment. Sophora microphylla represents a more opportunistic renewal strategy based on plasticity, recovery, and regeneration during favourable periods. Their contrasting strategies contribute to resilience at the community level.
This thesis provides a mechanistic basis for anticipating vegetation responses to a warmer and drier climate. Restoration and conservation should account for increasing drought stress, protect buffered microsites, and maintain genetic and functional diversity across provenances. Combining long-lived, stress-tolerant species with taxa capable of recovery and recruitment during favourable periods is likely to improve the resilience of restored inland forests and woodlands under continued environmental change.