Utilisation of pressure-volume techniques and non-linear least squares analysis to investigate site induced stresses in evergreen trees

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.