THE ECOLOGY OF THE CENTRAL COASTAL AREA OF NEW SOUTH WALES. II. PLANT SUCCESSION ON THE HAWKESBURY SANDSTONE. By Ilma M. Pidgeon, M.Sc, Linnean Macleay Fellow of the Society in Botany.* (Plates i-iv; one Text-figure.) [Read 30th March, 1938.] Factors of the Habitat. Plant Succession: (1) Primary Succession, (a) Xeric Lithosere (Initial serai phases — Scrub — Tree-scrub — Low scrub-forest — Tall scrub-forest — High forest) ; (6) Moist Lithosere (Various types — Swamps). (2) Secondary Succession. An account is given of plant succession and the plant communities on the Hawkesbury Sandstone area. Most of the detailed field observations were made on the Hornsby Plateau, but the sequence of communities is similar throughout the sandstone uplands of the central coastal area of New South Wales. Specific variations occur in different localities; for instance, there is a marked variation in the tree species controlled by altitude and latitude, but the differences in the floristic composition of the early serai communities are not marked. Unless other-wise stated, the species quoted are those typical of the Hornsby Plateau. Previous accounts of the sandstone flora of the Sydney district have been floristic, e.g., Hamilton (1912, 1923, 1932) and Hamilton (1918). Osborn (1930) commented on the general features of the sandstone flora and reviewed and summarized our knowledge of the so-called xerophytic features of the sclerophylls (see also Wood, 1934). The sclerophyllous vegetation of the Hawkesbury Sand-stone is characterized by hard, tough, dry leaves, and has developed under conditions of bright sunlight, exposure and ready drainage through a shallow soil of poor water-retaining capacity. Thick cuticle, numerous stomata, frequent veins and excess carbohydrate formation, leading to the development of thick-walled cells and a consequent hard internal skeleton of fibres, are signs of intense vegeta-tive activity. The external conditions also favour a vigorous transpiration rate so long as water is available. The significance of the structure of the sclerophylls lies in the fact that high lignification prevents or delays any evidence of wilting. In this feature lies the chief difference between sclerophylls and mesophytes. Factors of the Habitat. The general physiographic, edaphic and climatic features of the Hornsby Plateau have already been referred to (Pidgeon, 1937). Here the habitat factors will be discussed in more detail. Topography. — The topographical features of the Hornsby Plateau exert a marked influence on the vegetation. There are two extreme topographical habitats, the plateau surface and the gully. The plateau is much dissected; the slopes (PL iii, fig. 21) and gullies (PL iii, fig. 20) present more favourable habitats than the plateau surface. The angle of slope varies with the depth and extent of the * The writer commenced this work in 1935 as Caird Scholar in Botany, and continued it in 1936 as Science Research Scholar of the University of Sydney.
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