No abstract

Detailed statistical analysis of data obtained from five species of New Zealand native trees was used to develop an empirical model of pressure-volume curves. The model combines an exponential turgor function with a hyperbolic osmotic function and provides a very good fit to all data sets. The resultant function permits precise calculation of the bulk modulus of elasticity and turgor loss point from simple manipulation of the coefficients as well as providing the usual estimates of osmotic potential and free water content. The use of several water relations parameters, rather than one or two, each expressing different ecological aspects has permitted a better insight into the adaptability of the species.The data was obtained as part of an investigation into the causes of forest mortality to examine the tissue water relations of the principal species and to assess their ecological status. For all parameters highly significant differences were found between species. Silver beech (Nothofagus menziesii) and quintinia (Quintinia acutifolia) or tawari (Ixerba brexioides) are well separated and are consistently placed in opposing groups by the parameters. Osmotic potential proved to be of limited value for differentiating sites possibly because of the greater amplitude of diurnal variation compared with the site differences and also because of large within site variation. However variations in the pressure potential at the turgor loss point, which was closely correlated with osmotic potential, discriminated the levels of effective precipitation at the study sites, and the drought tolerance of the species.For one species, tawari, it proved possible to classify the sites on the basis of water content at the turgor loss point and osmotic potential using discriminant analysis. Variation in the water potential at the turgor loss point, which was closely correlated with osmotic potential, can be used to rank the species in order of drought adaptability and this was found to coincide with their order of permanence on the site. The bulk modulus of elasticity and the coefficient of elasticity are closely linked but their value in the ecological comparisons was limited by higher variability than found for other parameters. Ranking of the species using the variability of the parameters indicates the presence of to two distinct survival strategies; stress avoidance and stress tolerance. The two canopy species utilise opposing strategies and may be less at risk than the seral plants which combine both strategies. The data provides evidence for current stress on the unhealthy site which most stongly affects the seral species.

Osmotic potential (III,) was determined from freezing point depression measurement on leaf exudates. Similar values of 1jI, were found from pressure volume methods. Increased 1jI, before and during budbreak was followed by a recovery towards normal values. This pattern is similar to that usually reported for evergreen trees. IjIs was generally higher on sites of greater altitude reflecting the wetter environment. The results suggest that the seasonal variation in 1jI, is greater than betweensite differences.

Severe forest decline exists in upland areas of the Kaimai Range, North Island, New Zealand. Stand structure of major affected vegetation types is described. As stand dominants are well represented in induced seral vegetation, the overall species composition of the upland forests is not likely to change following decline. Within the decline zone a considerable range of forest damage is found. Characteristic damage types are described including effects on seedling vigour, root system development, and shoot phenology. The decline appears to result from a sequence of natural phenomena. High fog occurrence coincides with the decline zone producing soil waterlogging and generally poor growth conditions. This predisposes the forests to periodic drought damage.

Studies of aerial photographs show clear evidence of increased erosion, since 1943, concentrated at differing altitudes in different parts of the Kaimai Range, North Island, New Zealand. In the Te Rere area, the increase was greatest between 1943 and 1960, but in the other 2 areas (Mt Te Aroha and Te Hunga) the increase was greatest between 1960 and 1974. There is also a high normal erosion frequency at Te Hunga due to steep terrain. Landslides do not appear to be concentrated on slopes of any particular aspect, slope, soil type, or basement geology, but may be linked with areas of poor vegetation.A sample of landslides in each area was visited and mapped in detail. Measurements demonstrated a marked difference between upland and lowland landslides. At the upland sites, landslides occurred on slopes as low as 15°, and on slopes of over 35°b edrock was usually exposed, even in the track. Presence of allophane at the failure plane, and unhealthy vegetation at the landslide sites, markedly reduces slope stability and causes a lower natural angle of repose. At lowland sites, most landslides occurred on slopes steeper than 40°and bedrock was less frequently exposed. Examination of scars suggests that the landslides occurred under saturated soil conditions during a storm.In the scar, bedrock is usually exposed, minimising the risk of further erosion; but in the track, deep soils and slow revegetation may increase the risk of further erosion, particularly if browsing mammals are present.