Abstract:
Under climate change scenarios predicted for New Zealand, seasonal drought is forecast to increase in intensity and frequency, increasing the occurrence and severity of soil moisture deficits across the country, particularly in Auckland and Northland. Soil moisture is an important factor in the determination of plant community structure and particularly for seedling recruitment, as the limited root systems of seedlings make them highly susceptible to drought stress. Furthermore, drought survival strategies and hydrology preference have been shown to influence species distribution patterns. Thus, impending climate change may alter plant community recruitment patterns and subsequent plant distributions but there is very little information available on the relative drought sensitivity of native plants in New Zealand. I used a functional traits approach to assess the physiological responses and subsequent sensitivity to drought of 18 plant species commonly found in native forests of the Auckland region. Selected species display a range of soil moisture tolerances across different habitats ranging from gully to ridgetops, and represent both coloniser and enrichment plants used in restoration projects. Seedlings were subjected to a 12-week manipulative dry-down experiment and assessed at four-week intervals for alterations in leaf gas exchange, and water potentials. Plant vigour was assessed with a wilting index throughout the experiment and harvested samples were assessed for foliar nitrogen content and ratios of above and below ground biomass. The 18 species expressed three different drought response strategies including: drought avoidance/postponement, drought tolerance, and drought intolerance. The experimental species loosely defined positions along a continuum between two contrasting plant growth strategies: fastgrowing species with acquisitive traits and low drought survival; and slower-growing species with conservative traits and drought tolerance. Stem water potential was the best indicator of drought tolerance. Drought-habitat interactions demonstrated a broad pattern of drought-resistance mechanisms associated with plant species acclimated to drier ridgetop/disturbance habitats, and drought-intolerant species associated with comparatively wetter habitats. Species recommended as colonisers in restoration plantings were fast-growing, drought-intolerant species with the exception of Kunzea robusta, Hebe stricta, Hoheria populnea, and Pittosporum crassifolium. Findings from this study will contribute to literature focused on seedling recruitment in drought scenarios, as well as indicate which plant species commonly recommended in catchment rehabilitation planting are more drought resistant. Ideally, results will improve the success of rehabilitation and restoration projects during dry periods.