Mixed species stands of eucalypts as ecotones on a water supply gradient

Eucalyptus tenuiramis frequently forms mixed-species stands with E. obliqua. A statistical analysis of 2140 forest stands in south-eastern Tasmania indicates that the mixed-species stands of E. tenuiramis and E. obliqua occupy and environmental space intermediate to that of pure stands of either species. Detailed examination of one such mixed-species stand showed that local-scale variation in soil depth could make the environment similar to that of pure stands and that this soil depth variation was correlated with the abundance of each species in the local area. Examination of another mixed-species stand over the first 15 years of development demonstrated that local-scale variation in soil depth did not affect the probability of establishment of either species. However, within 2 or 3 years of establishment plants were large enough to explore the full soil volume and soil depth became a significant factor in species performance. Logistic regression modelling indicated that threshold values of drought stress for species changeover at the broad geographical scale, due to variation in mean annual climate, were the same as those correlated with local-scale patches of species that occurred as a result of variations in soil depth. Finally, the photosynthetic performance and leaf-area production of plants in mixed-species plantings and monoculture across artificially induced gradients of water supply were examined. Differences in species response to drought-stress provided a physiological explanation of the growth performance of each species at different levels of resource supply.