Factors affecting survival of tree seedlings in North Queensland rainforests

Seedlings 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.