The dynamics of Nothofagus cunninghamii rainforest associations in Tasmania : an ecophysiological approach

Nothofagus cunninghamii is a widespread component of cool temperate rainforest in Tasmania. It dominates the canopy on fertile soils, with dominance decreasing with the soil quality. Elsewhere in the southern hemisphere, Nothofagus appears either to be restricted to sites away from the moist, mild, fertile optimum, or to be seral. This thesis examines regeneration characteristics and comparative physiology of the major canopy species of Tasmanian cool temperate rainforest in order to determine the mechanisms affecting species composition. Atherosperma moschatum is the most shade-tolerant canopy species in the Tasmanian rainforest. It regenerates continuously, primarily by vegetative reproduction (stem sprouts) and therefore is typically self-replacing. Its ability to capture the larger gaps created by the death of old trees of other species is limited by the infrequency of establishment away from the parent stem. The reproductive characteristics of A. moschatum where it grows in rainforest, and the absence of other shade-tolerant canopy species, explain the dominance of old undisturbed rainforest by relatively shade-intolerant species in Tasmania. Nothofagus cunninghamii regenerates continuously on most sites, primarily by seedling establishment in canopy gaps created by the death of an old tree. Eucryphia lucida regenerates continuously by seedling establishment in canopy gaps, and vegetatively by sprouts from the base of larger stems. N. cunninghamii and E. lucida have similar light requirements and are able to regenerate under denser canopies than Athrotaxis selaginoides and Phyllocladus aspleniifolius. On the most fertile soils, N. cunninghamii has the highest growth rate of all the canopy species and dominates the canopy by establishing in natural canopy gaps and pre-empting resources (primarily light). The greater canopy diversity on poor soils appears to be due to the loss of the competitive advantage in terms of potential size and growth rate of N. cunninghamii. On the poorest soils, P. aspleniifolius has the highest growth rate of the co-occurring rainforest canopy species under exposed conditions. However P. aspleniifolius is generally unable to regenerate continuously in closed forest due probably to its shade-intolerance and appears to rely on the higher frequency of fire on poor soils for its regeneration on these sites. A. selaqinoides shows more variation in its regeneration patterns. On sites with a relatively closed canopy, regeneration is insufficient to replace the older trees following their death. Where the canopy is more open, regeneration is occurring. The results of this study indicate that community composition in Tasmanian rainforest is primarily determined by a combination of shade-tolerance, growth rates and reproductive strategies. The growth rates of the species are the sum of responses to environmental features such as light, nutrients, temperature, water availability, and tissue losses due to browsing, frost damage and drought damage. On the more extreme sites (e.g. low temperatures and water shortage), tolerance rather than high growth rates may be of more importance. The dynamics of the Nothofaqus-dominated rainforests in Tasmania are compared with closely related forests in New South Wales and Chile.