Year

1986

Degree Name

Doctor of Philosophy

Department

Department of Geography

Abstract

This study presents a detailed analysis of the structure, floristic pattern and environment of the Illawarra rainforests and also evaluates their relationships with rainforests elsewhere in Australia. The results presented here illustrate the fact that the district's rainforests can be viewed as a part of a continuum of rainforest vegetation throughout eastern Australia, with links to those well to the north and to the south. This research contributes to a national effort, over recent years, to more fully understand Australian rainforests.

The rainforest vegetation found in the Illawarra district represents one of five major areas of rainforest in New South Wales, The original extent of the Illawarra rainforests is shown to have been largely in the form of three major rainforest concentrations: the "Illawarra Brush' of 12.000 ha: the 'Yarrawa Brush' of 2450 ha: and the 'Berkeley Brush' of 1600 ha. The present area of rainforest in the district is only 5700 ha following an estimated reduction of 75 percent since European settlement.

Several 'distributional nodes' for rainforest plant species distribution, of which the Illawarra district is one, have been identified in southern New South Wales. At each of these nodes between 15 and 13 percent of the total species of trees, shrubs and vines reach their southern limit of distribution.

The major environmental factors determining rainforest distribution are identified as: soil nutrient status, mean temperature as indicated by altitude and location, moisture availability and probably the influence of fire, however, these are not listed in their order of significance. Soils supporting rainforest are found to differ significantly, particularly in relation to the amounts of available calcium and aluminium. Soils supporting subtropical rainforest (Complex Notophyll Vine Forest. CNVF) are high in Ca and other nutrients such as Mn. K and Na. Soils associated with warm temperate rainforest (Simple Notophyll Vine Fern Forest. SNVFF). dominated by Ceratopetalum apetalum. are very high in Al (an element accumulated by Capetalum ) but lower in other nutrients. Mixed subtropical-warm temperate rainforest (Mixed Notophyll Vine Fern Forest, MNVFF) are found on soils intermediate between these two.

Numerical classification is used to elucidate patterns in the variation of rainforest structural and floristic characteristics. Analysis of structural-physiognomic attributes of 50 rainforest stands identified three main groups, largely determined by soil type and altitude: 1) a mainly SNVFF group which occurs on sedimentary soils. 2) a group composed of CNVF and MNVF which largely occurs on volcanic soils, and 3) a group of simple, but each different, rainforest stands which occur on all soil types and altitudes.

Similar techniques are used to investigate patterns in the canopy. understorey, ground cover and vine species. Analysis of the canopy species produced two major groups: 1) high altitude sites on sedimentary soils and Tertiary volcanic soils are largely SNVFF (warm temperate) and dominated by Capetalum. and 2) lower altitude sites on Permian voicanics or sedimentary soils, which are either CNVF or MNVFF (subtropical).

The understorey data also produced two main groups: 1) a group on sedimentary soils at higher altitudes, and 2) a group occurring on Permian volcanics at lower altitudes. There is a group of understorey species, including the tree ferns, which distinguishes these site groups from those identified using the canopy species data.

Classification of ground cover data identified three main groups: 1) a group at higher, moister sites containing a well developed ground cover. 2) a group of sites on steeper topography which are drier and have a less well developed ground cover, and 3) a group which is largely on volcanic soils with well developed ground cover.

Analysis of the presence/absence of vine species identifies three species groups: 1) a group of widespread and common species, 2) species which are common at lower altitudes, particularly on volcanic soils, and 3) a group of species found mainly at higher altitudes.

An ecological model is developed to explain and describe the occurrence of rainforest in the Illawarra. This model is based on the structural and floristic character of these rainforests and their distribution in relation to major environmental factors operating. Regional studies of this kind are important for developing a wider understanding of the Australian rainforests.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.