Resource patchiness in space and time: phenology and reproductive traits of monsoon rainforests at Gunn Point, Northern Territory, Australia

Monsoon rainforests occur as mostly small scattered patches across northern Australia within a landscape dominated by eucalypt savannas. The whole rainforest archipelago in the Northern Territory comprises 15000 such patches, which range from coastal deciduous vine thickets to evergreen rainforests associated with permanent water. Monsoon rainforests are subject to various threats, including wildfires, feral animals, weeds and infrastructure development. Long term viability of the rainforest archipelago may depend on the conservation of the whole network of patches, and the maintenance of plant-frugivore interactions. This thesis formed part of a project investigating several aspects of plant-frugivore interactions in a set of rainforest patches near Darwin, with the ultimate aim to design a reserve system for monsoon forests and mobile species. An objective of this thesis was to determine temporal and spatial variability of phenological patterns and fruit resources for frugivores in a set of wet monsoon forest (WMF) and dry monsoon forest (DMF) patches. Phenology of more than 100 species was studied over 30 months. Phenological patterns of both WMF and DMF strongly responded to seasonal changes. Fruiting was concentrated in the wet season, but fruiting peaks in WMF and DMF were separated by 3-4 months. Fruit production was substantially higher in WMF (annual mean of 1780 kg fresh weight ha-1) compared to DMF (annual mean of 122 kg fresh weight ha-1), but also differed between sites, seasons and years. Some species and habitats, such as WMF margins, function as keystone resources during the dry season. The second objective was to investigate aspects of the reproductive cycle of rainforest species. Studies of fruit characteristics, fate of diaspores and germination were carried out for eight WMF and eight DMF species. The WMF species investigated were Carallia brachiata, Carpentaria acuminata, Ficus virens, Gmelina schlechteri, Maranthes corymbosa, Myristica insipida, Syzygium nervosum and Terminalia microcarpa. DMF species were Canarium australianum, Cupaniopsis anacardioides, Diospyros compacta, Drypetes deplanchei, Elaeocarpus arnhemicus, Miliusa brahei, Mimusops elengi and Strychnos Iucida. Fruit characteristics were diverse, with varying morphological, structural and nutritional features, and were largely independent of forest type in which species grow. The ability to successfully disperse seeds varied greatly between species, reflecting species-specific attributes such as fruit production, seed size and nutrient qualities. It is concluded that species with limited dispersal are particularly vulnerable to further patch fragmentation. Post-dispersal removal/predation of diaspores was highly variable between species, and was related to diaspore characteristics such as size and seed hardness. The major removers/predators of diaspores were ants in WMF and hermit crabs in DMF. Ants in particular may function as secondary dispersers for small-seeded species. Shadehouse studies showed that germination success was generally high, and that pulp removal significantly enhanced germination for most species. DMF species showed a wide range of germination patterns and dormancy characteristics, which I suggest are primarily determined by abiotic constraints, particularly soil moisture. In contrast, biotic constraints (e.g. pathogens and predators) may have selected for rapid germination in most WMF species, which in tum limits fruiting to be largely a wet season event. In conclusion, this study demonstrated that rainforest patches provide a mosaic of fruit resources in space and time. Rainforests provide major food resources for frugivores in the wet season, whilst adjacent savanna areas provide important flower resources during the dry season. The different vegetation types thus comprise a complex mosaic of food resources, and this has important implications for conservation of the rainforest network. In order to best conserve interdependent rainforest patches and frugivore populations, management planning has to be considered on a landscape scale.