The open-forest -\treeless\" plains boundary on Melville Island Northern Territory"
Creators: name: Wilson, BA
Publisher: name: University Of Tasmania
Year: 2023

description: The 'treeless plains' is a colloquial name given to low lying undulating areas on Melville and Bathurst Islands which support a variety of shrubland and low open-woodland plant communities. The major aim of this thesis is to describe the vegetation of the plains and elucidate which environmental factors are determining the vegetation patterning, particularly across the forest plains boundary. Numerical classification of vegetation survey data is used to define seven plant communities on the plains and adjacent forests. These communities differ floristically and/or structurally to other northern Australian plant communities, and show similarities to vegetation which is classified under the general headings of 'heath' and 'savanna'. The major environmental correlate differentiating the treeless plains communities from each other is wet season inundation. Secondary variation within the plains vegetation and between the vegetation of the plains and adjacent open-forests is associated with changes in several inter-correlated edaphic factors, including texture and moisture. Detailed descriptions of the vegetation patterning across forest plains boundary shows that there is an abrupt structural change, with a distinct 'ecotone' or 'transition' zone between the forest and the plains. This structural change is accompanied by a gradual or 'ecoclinal' shift in composition of the understorey. A parallel study of environmental factors showed a gradational change in conditions with the plains occurring on soils which are generally sandier, less fertile, drier and possessing higher water tables than adjacent forest areas, and other red earth soils supporting eucalypt forests and woodlands in northern Australia. Eucalypt seedling growth was significantly retarded when subjected to flooding in pots, but the flooding tolerances of species tested did not correspond to their relative ecological ranges observed in the field. It was concluded that the deep sandy soils on the plains drain excess water rapidly at the end of the wet season and thus do not maintain water tables at levels high enough to prevent tree growth even in very wet years. Field measurements showed that although surface soil moisture levels on the plains were severely depleted during the dry season, there appeared to be adequate moisture at depth to support tree growth throughout the year. Field plantings and associated pot trials supported this contention by showing that seedlings could grow under conditions of higher moisture stress that those recorded in the field at the end of the dry season. There were significant differences in levels of nutrients, particularly magnesium, calcium and aluminium, across the forest plains boundary. Pot trials showed that eucalypt seedlings grew slower in plains soils compared to forest soil and that the growth differential was not due to differences in mycorrhizal fauna. The growth of transplanted eucalypt seedlings appeared to be suppressed by the forest overstorey, possibly due to competition for moisture. Seedling growth rates increased with site moisture and nutrient status on the plains, although there was no evidence that additions of calcium and magnesium had a differential effect on seedling growth across the boundary. It is suggested that the deep sandy soils of the plains have been subjected to heavy leaching of nutrients, which is a primary factor associated with changes in vegetation across the forest plains boundary. Fire is implicated as a secondary agent, interacting with the differential growth rates of the seedlings and suckers across the boundary to retard tree development and cause the sharp structural boundary. The dominant eucalypts in the area possess several characteristics, such as poor seed dispersal, sporadic germination and seedling development and suckers with a low potential to develop into trees, which reinforce the fire-edaphic patterning. The vegetation and environmental patterning across the forest plains boundary is discussed in relation to various models that have been used to explain the relationship between tree and grass growth, particularly with respect to tropical savanna vegetation.