Factors affecting establishment, survival and growth of rainforest tree species before and after canopy closure in early successional rainforest communities were investigated in the wet tropical and subtropical regions of Queensland, Australia. The studies were undertaken in the context of ecological restoration. Several studies were undertaken using a range of rainforest tree species with varying attributes to evaluate the use of direct seeding and broaden the applicability of this methodology to a larger range of site conditions within the wet tropical region.
The studies simultaneously investigated factors which may potentially act as impediments to ecological recovery of degraded sites, including micro-site limitation, seed predation, seedling herbivory and competition with neighbouring species before and after canopy closure. These factors were investigated in relation to species characteristics in order to determine how these may affect the assembly sequence of early successional communities and subsequently the early-stage outcomes of restoration projects.
Seed size was found to be a significant factor affecting the success of seedling establishment and survival in the early stages of growth in direct seeding trials. It was found that seed size and its relation to micro-site condition significantly affected the re-establishment success of tree species at degraded sites. Large seeds showed higher rates of establishment over the experimental period, whereas small seeded species showed declines in establishment and poorer growth performance where weed growth was high. Trends were inconsistent between species with intermediate seed size.
Seed size also appeared to be an important factor affecting susceptibility to predation with seed survival increasing in response to increase in seed weight in some trials. However, larger seed size was also seen to increase predation risk in some habitats where foraging by larger predators was evident. Habitat type and canopy cover density were seen to exert an influence on seed survival, though trends were not consistent between north and south Queensland sites or between seasons. Larger seed size also enabled increased tolerance to re-shading with two larger seeded species showing higher acclimation in response to increasing light levels in their juvenile post-emergent stage. Survival under shaded conditions was also linked to shade tolerance adaptations of species owing to their successional status (typical light environment or regeneration niche). The growth performance of tree species at a later stage of growth (10 years) showed no relationship to seed size but was linked to other species attributes including height growth rate, shade tolerance and sensitivity to competition from neighbouring individuals.
Studies in this thesis demonstrated that species survival and growth performance in restoration is highly linked to species characteristics and the benefits these characteristics confer to growth and survival under various conditions. Consideration of species traits including seed size, germination phenology, growth rate, shade tolerance, sensitivity to competition, susceptibility to predator populations and tolerance limits (i.e. to changes in light condition and moisture availability) are important factors when selecting species for use in restoration projects. Species selection is particularly important for direct seeding where plants are established from seed and are therefore subjected to the full range of selection filters limiting recruitment, survival and growth of tree species in successional habitats.
Limitations to the direct seeding technique must be considered when aiming to achieve high success rates in restoration projects at degraded sites. If direct seeding projects are undertaken with sufficient planning, appropriate site selection and preparation, suitable species selection and appropriate weed control and monitoring, the risks associated with utilising the revegetation technique are likely to be substantially reduced. Direct seeding can therefore be considered a viable method for restoring degraded areas in wet tropical areas particularly as costs pose a constraint to using other methods of revegetation.
Results and recommendations arising from this work have application for similar environments globally. Local ecological conditions and species attributes must be evaluated when adapting direct seeding methods to other tropical or sub-tropical regions.