Andrew W. Graham

Influence of seed size and seedling ecological attributes on shade-tolerance of rain-forest tree species in northern Queensland

Growth, leaf anatomy and stomatal responses were examined in seedlings of 12 rainforest tree species in northern Queensland. Plants were kept in a shade house for a period of 15 months at 37%, 10% and 2.5% photosynthetically active radiation (PAR), roughly corresponding to light environments in large gaps, small gaps, and forest understorey, respectively. The species were chosen to represent a wide array of taxa, ecological, and morphological characteristics, and were assigned regeneration niche (RN) index values based on observations of life history and spatial distribution of their seedlings and saplings in various forest microsites. Growth, biomass allocation patterns, leaf anatomy and stomatal responses varied appreciably among species, as well as in response to shading (...)

The Species Composition of Soil Seed Banks Beneath Lowland Tropical Rainforests in North Queensland, Australia

Twelve surface soil samples from each of four rainforest types on different soils of the huLmid, tropical lowlands of north Queensland were removed to germination-house conditions, Seed germination was monitored over a period of 14 weeks between AugLst and November. Seedling numbers at the four sites (mean densities ranged from 588 m-2 to 1068 m-2) were comparable to numbers recorded elsewhere by similar techniques in tropical rainforest soils. The numbers of germinated seeds were significantly higher at the site with coarse granitic sands than at the three sites with finer-textured soils. This was attribuLted to the ease with which seeds could be incorporated into the coarse-textured soil. Most of the seedlings produced at all sites were secondary species or weeds of the forest margins, many being common to all sites. The characteristic abundance of some secondary species at one or two sites could be explained usually in terms of the prevalence of those species in the standing rainforest on the sites. Only a few largeseeded primary forest species were germinated and most of these were fruiting at the time of sampling. The absence of many primary forest trees from the seed banks can be explained by the obligately quick-germinating, short-lived natuLre of their seeds. The large size of the potentially longer-lived seeds of some primary forest species was considered to be important in preventing their incorporation in the soil profile and, hence, affecting their susceptibility to predation and longevity on the forest floor. THE COMPOSITION OF VIABLE SEED in soils beneath a plant community is one indicator of the regrowth potential of that community. Most studies of soil seed storage have been conducted in temperate agricultural and pastoral situations where the information has been used to control weeds and predict compositional changes under various intensities of management (Milton 1943, 1948; Champness and Morris 1948; Douglas 1965; Suckling and Charlton 1978). The results of soil seed studies have been useful in predicting successional changes and interpreting past plant communities which occupied an area (Oosting and Humphreys 1940, Howard 1974), predicting and interpreting the results of various intensities of disturbance (Olmsted and Curtis 1947, Floyd 1966, Howard and Ashton 1967, Thompson 1978), and interpreting the pathways of natural regeneration and the strategies of establishment of individual species (Liew 1973, Baskin and Baskin 1978). However, few soil seed studies of tropical rainforests have been conducted (Symington 1933, Keay 1960, Guevara and Gomez-Pompa 1972, Liew 1973, Cheke et al. 1979) and more are urgently required (Whitmore 1978). This paper reports the results of a study designed to define the compositions of the soil seed banks of four Australian tropical rainforest sites containing different soils. It forms part of a larger study of secondary succession which is being carried out in the same area.

Factors affecting survival of tree seedlings in North Queensland rainforests

SummarySeedlings of six species of rainforest trees with widely constrasting ecology and seed morphology were transplanted at 3 weeks of age into tree-fall gaps and the shaded understoreys at two rainforest sites (Curtain Fig and Lamins Hill) on the Atherton Tableland, North Queensland, Australia. In each forest habitat, half of the transplanted seedlings were protected from vertebrates by means of wire cages, and survival was monitored over 16 months. The main objective was to estimate the extent to which independent variables (forest, habitat, protection from vertebrates, and species) contribute to explaining survival differences among the seedlings. Significant differences existed in the mortality among species, whether caged or uncaged. Seedlings unprotected from vertebrates suffered greater mortality (i.e. averaged over all species and forests, mortality > 75%), compared to the caged seedlings (mortality < 40%). Seedling of most species survived better in forest gaps, though the effects varied significantly among the six species. Survival of caged seedlings in Curtain Fig was significantly greater than in Lamins Hill, a pattern attributed to the partial deciduousness of the Curtain Fig forest. Because only few of the uncaged seedlings survived to 16 months, we focussed on the caged seedlings to examine the role of light at the forest habitats on survival. Relationship between seedling mortality and increasing light availability vary significantly among the six species. However across species, seedling mortality decreased linearly as photosynthetically active radiation (PAR) rose from 0.48 to 2.0%, after which an asymptote was reached and further increase in light (up to 8% PAR) did not influence survival significantly. Overall, temporal, species and the various environmental variables and their interactions explained about 60% of the variations in the seedling mortality data, with protection and species differences making the greatest contributions.

Growth of tree seedlings in tropical rain forests of North Queensland, Australia

The effects of forest habitat, canopy light condition, vertebrate herbivory and species mean seed size on growth of tree seedlings were evaluated for six widely different species of North Queensland tropical rain forests. Two forest localities differing in size and rainfall intensity were used for the trial. In each forest, seedlings were transplanted three weeks after germination into small to medium-sized canopy gaps and into the forest interiors, with half protected by cages and the other half unprotected. Growth measurements were made over a period of 16 months. All growth parameters examined differed significantly between the six species. At the end of the study period, for most species, forest site and protection from vertebrates did not affect seedling biomass (...)