Complementary resource use by tree species in a rain forest tree plantation

Mixed-species tree plantations, composed of high-value native rain forest timbers, are potential forestry systems for the subtropics and tropics that can provide ecological and production benefits. Choices of rain forest tree species for mixtures are generally based on the concept that assemblages of fast-growing and light-demanding species are less productive than assemblages of species with different shade tolerances. We examined the hypothesis that mixtures of two fast-growing species compete for resources, while mixtures of shade-tolerant and shade-intolerant species are complementary. Ecophysiological characteristics of young trees were determined and analyzed with a physiology-based canopy model (MAESTRA) to test species interactions. Contrary to predictions, there was evidence for complementary interactions between two fast-growing species with respect to nutrient uptake, nutrient use efficiency, and nutrient cycling. Fast-growing Elaeocarpus angustifolius had maximum demand for soil nutrients in summer, the most efficient internal recycling of N, and low P use efficiency at the leaf and whole-plant level and produced a large amount of nutrient-rich litter. In contrast, fast-growing Grevillea robusta had maximum demand for soil nutrients in spring and highest leaf nutrient use efficiency for N and P and produced low-nutrient litter. Thus, mixtures of fast-growing G. robusta and E. angustifolius or G. robusta and slow-growing, shade-tolerant Castanospermum australe may have similar or even greater productivity than monocultures, as light requirement is just one of several factors affecting performance of mixed-species plantations. We conclude that the knowledge gained here will be useful for designing large-scale experimental mixtures and commercial forestry systems in subtropical Australia and elsewhere

Photosynthetic characteristics of Flindersia brayleyana and Castanospermum australe from tropical lowland and upland sites

Photosynthetic responses to temperature, light and carbon dioxide partial pressure were studied in two-year-old Flindersia brayleyana F. Muell. and Castanospermum australe Cunn. and C. Fraser ex Hook. growing on coastal lowland and upland rainforest sites in tropical Queensland, Australia. Climatic conditions ranged from moist and cool (17-19 °C) to dry and warm (22-24 °C). The optimum temperature for photosynthesis was 23.7-25.6 °C for C. australe and 21.2-24.6 °C for F. brayleyana. Mean maximum rate of electron transport for each species did not differ between sites but was higher (60-62 μmol m s) in F. brayleyana than in C. australe (42-44 μmol m s). Ribulose-bisphosphate carboxylation rate did not differ significantly between sites or species. Maximum rates of photosynthesis at 1000 μPa Pa CO did not differ significantly between sites for each species, but did differ significantly between species. At 350 μPa Pa CO, photosynthetic light use efficiencies of F. brayleyana and C. australe were 0.05 and 0.015, respectively, at the upland site, and the corresponding values at the lowland site were 0.025 and 0.05. In C. australe, these differences were reflected in significantly greater maximum rates of photosynthesis at 350 μPa Pa CO at the lowland site than at the upland site (5.2 versus 3.3 μmol m s)