Phyllocladus aspleniifolius (Labill.) Hook. f. and Anodopetalum biglandulosum (A. Cunn.) ex Endl. : a comparative autecology of coexisting wet forest trees in Tasmania

This thesis describes and compares the autecology of two coexisting wet forest trees. It demonstrates temporal, spatial and biological differentiation of the reproduction, regeneration and growth characteristics which allow these taxa to coexist in superficially similar environments. The distributions of the species and the environmental correlates of the communities in which they exist are compared and contrasted. The climatic range of P. aspleniifolius is greater than A. biglandulosum but this may be attributed to the greater vagility of P. asplennfolius allowing it to occupy areas that are dimatically susceptible to repeated disturbance by fire. It is determined that both are most important in poorly structured and diverse rainforest communities which are often associated with low fertility substrates. The physiological basis for this correlation was tested experimentally and by field measurements. A. biglandulosum performed best under add conditions while P. aspleniifolius was relatively insensitive to fertility. These responses are manifest in the absence of both species from the most fertile sites which support floristically simple tall Nothofagus cunninghamii dominated forests. The preference of A. biglandulosum for acid sites and the insensitivity of P. aspleniifolius allow these species to take advantage of the decline in the performance of the important canopy dominant, Nothofagus cunninghamii, and hence increase their abundance on such sites. Other than possible exclusion from pure Nothofagus cunninghamii forest their geographic range and phytosociological associations are not limited by edaphic conditions. Field performance of both A. biglandulosum and P. aspleniifolius does, however, reflect the physiological growth response measured under experimental conditions. The photosynthetic responses of A. biglandulosum and P. aspleniifolius were investigated in the contrasting micro-environments of the canopy, a gap and the understorey of a rainforest in Tasmania. Measurements were made using a portable infra-red gas analyser. Both species displayed the highest light saturation levels at the canopy but both produced the highest rates of photosynthesis in the gap environment. Diffuse light on cloudy days was important in the understorey environment. The rate of photosynthesis was affected considerably by low levels of stomatal conductance two days after rain. The results indicate that both species are \sun\" plants and that their dependance upon high levels of light to maintain competitive growth rates would influence their regeneration opportunities and may induce competition for sites. The phenological and demographic strategies of both species were compared. The timing of foliar and sexual development of Phyllocladus asplenhfolius and Anodopetalum biglandulosum were recorded at Mt Field in Tasmania. Seed production seed predation germination and survival were monitored in different regions of Tasmania. The wind dispersed A. biglandulosum suffered an extremely high level of fruit predation by a moth and by birds. The resultant low seed production is reflected in the poor seed regeneration in this species. The timing of pollen release and female cone development in P. asplenhfolius indicates that outcrossing between populations is likely. Outcrossing may provide the species with a high degree of genetic fitness but may not necessarily explain its wide ecological amplitude as this situation contrasts with the poorly dispersed and possibly genetically limited A. biglandulosum which also has a broad ecological range. P. asplenhfolius produced spatially variable amounts of seed and is a mast seeder. The number of germinants two springs after seed fall reflected the variation in annual seed production and suggests that pulses in regeneration are likely although soilstored seed and immigrant seed may offset this effect. The consequences of the phenological differentiation on regeneration opportunities was explored. The contrasting strategies required different approaches as A. biglandulosum dependsiargely upon vegetative regeneration and proliferation while P. asplenhfolius is an obligate seed regenerator. A growth-form/disturbance typology of A. biglandulosum based on data from 48 sites was produced. A multivariate data set from each site summarizes a stands' growth-form as stem types densities and diameters. A canonical correlation indicates a significant relationship between the growth-form data set and community floristics. The basic architecture associated with a particular forest is modified by proliferation in response to exogenic disturbances such as fire and heavy snow-falls or as a result of community processes such as crown senescence and tree-fall. This study aimed to determine the range of the P. asplenhfolius regeneration niche in terms of differential microsite occupation population structure and dispersion pattern at 52 forest sites. The distribution and sizes of stems were mapped on belt transects to within 0.5 m and the microsites of seedlings and saplings were recorded. A variety of population structures exist and are related to community floristics and structure. Community variation provides different microsite opportunities. P. asplenhfolius showed a preference for elevated sites including logs buttresses and old stump mounds. Aggregation of seedlings is common and determined by microsite distribution and dispersal characteristics. It is concluded that niche differentiation between P. aspleniifolius and A. biglandulosum is demonstrated in edaphic responses and reproductive biology and niche overlap is evident in their use of the light environment. Although their coexistence is theoretically dependent upon niche differentiation stochastic events producing opportunities for regeneration appear to be more important and therefore the results conform with the theory of non-equilibrium coexistence of species. The broad and common phytosociological associations may reflect Tertiary species extinctions which have produced an environment of low competition with few specialists. The extant species experience niche broadening via more opportunities for regeneration and hence greater levels of coexistence which are expressed as low beta diversity in Tasmanian wet forests."