Green, T.G. AllanJane, Graeme Travis2025-10-012025-10-011983https://hdl.handle.net/10289/17681Vegetation mortality has been evident in the Kaimai Ranges, North Island, New Zealand for at least 40 years and affects a wide range of species and forest types. The mortality occurs above 550 m altitude and on each prominence has a lower limit which corresponds closely with the predominant cloud zone in each area. The severity of the damage ranges from complete mortality of canopy species to a light mosaic of killed trees. On individual trees damage may be evident as leader dieback reduced leaf size, and other symptoms suggestive of waterlogging. Mortality appears to be episodic and several age classes can be distinguished by vegetation height. Recent mortality can be reliably dated to droughts in 1972 and 1946 but forest age class structure suggests a number of earlier dates for similar episodes including 1656, 1804, 1861 and 1914. Many species do not survive for more than a portion of their potential life spans. Seral species do not reach their potential stature and minor droughts appear to trigger mortality releasing the more permanent species. The distinct topographic and altitudinal bounds of the mortality and dendroclimatic analyses of mortality episodes suggested a strong direct drought effect. It was proposed that since the fog zone soil is frequently waterlogged, root systems would be restricted, and plants poorly able to respond to water stress. These proposals led to a series of investigations into plant water relations. An empirical model of pressure-volume curves was developed. Simple manipulation of the coefficients of the resultant function, provides estimates of osmotic potential and symplastic water content, and permits precise calculation of the bulk modulus of elasticity and turgor loss point. Osmotic potential was determined independently from freezing point depression measurement on leaf exudates and similar values were found. The bulk modulus of elasticity and the coefficient of elasticity are closely linked and variations in the pressure potential at the turgor loss point, was closely correlated with osmotic potential. During the summer 1981/2 water potential and stomatal aperture were monitored at a number of localities inside and outside the fog zone. Within the fog zone dawn water potentials remained high and were close to full turgor when overnight fog was present. Midday values generally compare with those published for plants of streamsides and wet habitats. Afternoon recovery was normally evident in all species except Quintinia acutifolia which was the least healthy of the species studied. Stomatal conductances in the seral species were high and those for the canopy species near usual values found in hardwood trees. Diurnal stomatal conductance patterns changed as the season progressed and late in the season closure often began before midday. Rapid closure at moderate temperatures appeared to be related to high vapour pressure deficit and generally minimised internal water stress. Fog lowers temperatures and light levels, for long intervals during the spring before and during budbreak slowing leaf development and maturation, and impairing stomatal development causing leaf scorch and leaf fall in fine weather. This results in part, from increased osmotic potential. Ranking of the species using the variability of the tissue water relations parameters indicates the presence of two distinct survival strategies; stress avoidance and stress tolerance. Similarly the effects of waterlogging could be avoided or tolerated and two canopy species (tawari and silver beech) appear to adopt opposing physiological and morphological strategies whereas the seral species maximise growth and are poorly adapted and intolerant of site induced stress. Knowledge of the water relations of the cloud forest species provides a new perspective that allows a better understanding of the structure and function of the forests. The evidence suggests that in spite of the very wet environment, features of the cloud forest plants result from water stress. Evidence of adaptation to water stress supports the general hypothesis that drought is a significant factor in forest mortality within the Kaimai Ranges.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis