COMPARATIVE-STUDIES IN NOTHOFAGUS (FAGACEAE) .1. LEAF CARBON ISOTOPE DISCRIMINATION

Twenty-two species of Nothofagus were collected from South America, New Zealand, Australia, New Guinea and New Caledonia and grown under common conditions in controlled environment growth cabinets. This enabled us to measure genetically determined differences in carbon isotope discrimination (DELTA) of leaves. Discrimination is a long-term measure of intercellular CO2 levels and estimate of water-use efficiency. It is known to be positively related to intercellular CO2 concentration and stomatal conductance and negatively related to CO2 assimilation rate and water-use efficiency. Genetic differences were then explored in relation to the native environments of the individual species. The species originating from tropical latitudes showed less discrimination (16.5-19.8 parts per thousand) than the species originating from higher latitudes (20.9-23.5 parts per thousand). There were no differences between the ranges of values recorded in species native to New Zealand, Australia or South America over a similar latitudinal range. When the DELTA-values of some species from Australia, New Guinea, New Caledonia and New Zealand (grown under common conditions) were regressed against climate parameters of their native environment, the highest correlation was obtained with mean rainfall from December to March (r2 = 0.81). It is hypothesized that the species at high latitudes, where the warmest months are usually the driest, have evolved physiological and/or morphological mechanisms which allow continued water uptake and high stomatal conductance during periods of mild water deficit. This would allow the continuation of photosynthesis and potentially higher net productivity under these conditions than in species which close their stomata in response to relatively small water deficits. In climates where water deficits are rare, stomatal sensitivity may allow some avoidance of mild water stress, without the larger costs imposed by other adaptations.