Seed biology, establishment ecology and comparison of community assembly models for rehabilitation of coastal dry seasonal rainforest near Darwin, Australia

Abstract

Dry seasonal rainforest (DSR) is one of the planet’s diverse biomes and once
occupied nearly half of the Earth’s tropical and subtropical land area. However, these forests are also among the most threatened vegetation types as a result of human activities and other abiotic influences, and have substantially decreased in size globally. Rehabilitation provides an important solution to restore the species diversity in these forests. Near Darwin, Australia, the DSR has been severely affected by urban development and cyclones. However, rehabilitation of DSR near Darwin has been ongoing with stands aged from 6 to 42 years since planting. During rehabilitation, only seedlings of pioneer species are planted, while climax species regeneration relies on recruitment from nearby remnant forest. Regeneration of climax species representing the floristic composition of the remnant forest is the goal of rehabilitation programs and their regeneration is an indicator of success.

Assessing restoration efforts requires understanding the dynamics of the planted pioneer and climax species. In species rich communities, understanding the factors affecting the coexistence of species is important. Different species may tolerate a similar range of resource levels and chance establishment may maintain biodiversity, as described by the Neutral Theory. Conversely, different species may have different optimum resource levels and thus, resource partitioning may maintain the diversity of species. The aim of this thesis was to determine successional dynamics between pioneer and climax species in the regeneration of DSR and whether community assembly in DSR is maintained through resource partitioning as described by Niche Theory or by the random establishment of species as indicated by Hubbell's Neutral Theory (2005).

The dynamics of pioneer and climax species diversity were assessed in the seed rain, dormant soil seed bank, seedling and sapling banks, and in the standing vegetation, for a chronosequence of rehabilitation patches and in a remnant DSR site. Demographic changes between pioneer and climax species in different diameter at breast height (DBH) classes were assessed to identify successional changes. Seed ecology was related to seasonal and successional changes in environmental conditions. Growth rates of climax and pioneer seedlings in response to the seasonal variation and successional development of the forest were assessed. The variation in light reaching microsites occupied by seedlings of different species was measured. Light levels were related to the relative seedling abundance of different species to determine whether the coexistence of different species in the remnant forest was maintained by resource partitioning or by chance establishment.

Successional convergence with the remnant, as indicated by climax species diversity, had not occurred within 42 years of planting. Planted pioneer species dominated the overstorey of all rehabilitation areas and climax species were under represented. This was mainly because, climax species had limited dispersal of seeds and lower seedling growth rates, resulting in a lower diversity of climax species even after 42 years in rehabilitation areas. Site conditions in the rehabilitated stands favour the growth of the pioneer seedlings and thus pioneer species were dominating all DBH classes across the chronosequence of sites. Dispersal limitation of climax species can be overcome by taking strategies to invite more birds and bats to drop seeds. This can be done through planting tree islands and using artificial perches. Abiotic conditions can be manipulated to accelerate the growth rates of the climax seedlings in rehabilitated forests. This can be achieved by addition of nutrients, and by removing more canopy trees to increase the light penetration. Within both the climax and pioneer species groups, different species had equal tolerances to survive under a range of light levels. Therefore, chance establishment maintains the coexistence of different species, supporting the Neutral Theory proposed by Hubble rather than Niche Theory.

Date of Award	2020
Original language	English
Supervisor	Sean Bellairs (Supervisor)