Botanizing in Tasmania
By David Jewell – 1984 Thornton-Smith Scholarship holder
Tasmania, the smallest of the Australian States is a heart shaped island lying some hundred miles off the south eastern corner of the mainland. Rather less than26,000 square miles it is mountainous even though peaks only reach 5,000 feet.
On reaching the west coast the moisture laden westerly winds are forced upwards by the mountain chains, depositing much of their moisture, and in summer having a drying effect on the eastern part of the State. A marked rainfall gradient reaches up to 200 inches in the west falling to the mid twenties on the east coast. The whole of the western half of Tasmania is consistently wetter than the eastern half. Rainfall coupled with the higher average summer temperatures in the east results in marked differences in vegetation from east to west.
At the present time about 2,000 species of flowering plants are known to occur in Tasmania, either native to the state or as naturalised introductions. More than 200 native species occur naturally only within the State; that are endemic.
The vegetation has two components, the antarctic or southern oceanic flora, and the Australian type characterised by Acacia spp. and Eucalyptus spp. which Tasmania shares with the rest of South East Australia. Many plants regarded as belonging to the oceanic flora are endemic and are often confined to cold wet areas, their nearest relations occurring in similar situations in South America or New Zealand. It is believed that ancestors of these plants existed while Australia, New Zealand and South America were still joined in a southern land mass which included what is now Antarctica. The Australian flora was shared by the eastern part of mainland Australia and Tasmania during the several times these were joined, but divergence has occurred during times of separation and is still occurring.
The vegetation is constantly changing and programmes of burning off, land clearance, draining of swamps, and establishment of certain types of forest are important factors in that change. Several types of vegetation can be recognised; temperate rainforest or myrtle forest, Sclerophyll or Eucalypt forest usually divided into wet and dry Sclerophyll, mountain vegetation, coastal heath and sedgeland.
In areas of high rainfall and suitable soils temperate rainforests are found from sea level to an altitude of 3,500 ft. The characteristic trees, NothofagusCunninghamii (myrtle) and Atherosperma moschatum (sassafras), cast a deep shade and undergrowth is often reduced to a surface cover of liverworts, mosses and lichens with scattered areas of ferns. While Nothofagus and Atherosperma are characteristic and wide-spread throughout Tasmania’s rainforests, other species are locally abundant. Athrotaxis Selaginoides (King Billy Pine) and Athrotaxis Cuppressoides (Pencil Pine) are trees of 50-100 feet in height. They may be associated with Northofagus or they may form pure stands on mountain slopes.
Where soils are acid and poor in mineral nutriments and the canopy of the rainforest becomes broken, other trees and also tall shrubs appear. Phyllocladus Aspleniifolius (celery top pine) is widespread and Eucryphia Lucida (leatherwood) locally abundant. The latter often grows to a height of 40 feet or more. In late summer their flowers may be up to 1½, inches in diameter and provide a spectacular display.
‘Laurels’, Waratahs and Heaths
The tall shrubs of these forests include a number of endemics, many characterised by showy flowers or by bright fleshy fruits. Anopteris glandulosus (native laurel, family escsllaniaceae) is a handsome shrub bearing large terminal racemes of white flowers. The proteaceae (Waratah family) and epaeridaceae (heath family) are well represented. From the latter family, two endemic species are of particular interest Richea Pandanifolia (pandani or giant grass tree) has leaves three to six feet long, hard, rigid and drooping, borne at the summit of a trunk which may be 20 to 30 feet high. Prionotes Cerinthoides (climbing heath) is a climber of epiphyte. It forms pendant sprays of small evergreen leaves and crimson bell-like flowers.
Locally in poor acid soils where the water table is at or very near the surface an almost impenetrable scrub develops, the density of which is notorious. About five species are mainly concerned. Leptosperum lanigerum (woolly tea tree) forms dense stands of trees having slender, very tough trunks up to 50 feet high. The Sedges Gahnia psittacorum and G. Sieberi appropriately called ‘cutting grass’, grown in clumps which are often more than six feet in height and breadth. Bavera rubioides (family Cunoniaceae) has innumerable thin, wiry inter led branches often spreading over other shrubs to a height of 12 feet or more. The most unusual growth form is that of Anodopetalum biglandulosum (horizontal), an endemic representative of the family cononiaceae. This is a small evergreen tree making a closely packed understorey in the forest or forming pure stands in gullies. The trees sometimes grow erect with trunks up to 45 feet high but, typically, slender saplings arch towards the ground and many erect branches arise from the almost horizontal trunks. The branches in turn bend over, interlacing with each other and with branches from adjoining trees. In this way, dense platforms develop at varying heights above the ground.
Montane vegetation occurs on plateaux, mountain slopes and summits. It is characterised by plants which can withstand cold conditions – severe frosts, seasonal snow and strong winds, Although cloudy weather is frequent, there is a high light intensity and occasional very hot days in summer. These conditions cause slow growth and water stress so that the plants are short often stunted with small hard leaves and tough celled wood. The exposure to cold winds cuts young growth so that shrubs are rounded, each shoot protecting the next, a habit which reaches its extreme in the cushion plants.
Dwarf mountain forest may contain conifers such as the prostrate strawberry pine, Microcaerys tetragona, or the small and erect cheshunt pine, Diselma archer, and microstrobus niphophylus, both growing to about six feet, or perhaps stands of the deciduous beech, Nothofagus gunnii, a 15 feet high tree on sheltered hillsides or a prostrate shrub clinging to the rocky faces of high slopes.
Mountain shrubberies are found in poor rocky soils in exposed situations. They are filled with diverse and interesting plants especially of the daisy heath and protea families which provide a colourful display in summer and early autumn.
An interesting community which may be termed a microshrubbery develops on mountain tops, on the margins of shallow pools and on gentle slopes where snow may lie for up to six months of the year. Six species of cushion plant are concerned. The Pterygopappus lawrencii with its square stem tips is easily identified, but the other five form dark cushions which are very similar in appearance when not in flower. They are shrubby plants, prostrate with many parallel erect shoots, tightly packed and laced together by roots to make a rounded mound so firm that it does not dent when walked on. Such plants increase and may coalesce to form larger mounds. They grow across small water courses, impeding drainage and slowing run off to prevent erosion.
Coastal Heath Vegetation
Coastal heath is most extensive in the far north west, north east and islands of Bass Strait. It occurs on sandy soils often developed from windblown sand. Such soil is low in minerals needed for plant nutrition. The characteristic heath vegetation consists of shrubs less than two metres high with hard or leathery leaves. The trees are often small and stunted by strong wind.
Heathland is maintained by burning; if no fire occurs the shrubs become taller forming a scrub forest or eventually an open woodland. Heath species show various adaptations which enable them to survive fire; very many send up new shoots from bulbs, rhizomes, or woody knobbed rootstocks buried beneath the damp soil; some like Banksia and Hakea produce woody fruits which protect the seeds during fires and need heat to dry and open them.
Button Grass Plains
Extensive areas throughout Tasmania carry sedge moors which are given the descriptive name ‘Button grass plains’. The characteristic plant is Gymnoschoenus sphaerocephalus (Cyperacene) which grows in tussocks consisting of hard, narrow leaves, three to six feet long, with long slender flower stalks terminating in spherical heads of flowers and fruits.
This plant community may usually be found on wet, peaty acid soils. Other monocotyledons, particularly representatives of the Restionaceae, are abundant and sometimes dominant. The yellow flowered species of Xyris (family –Xyriolaceae) and mauve flowered Patersonia fragilis (iridaceae) are widespread and between the tussocks small herbaceous plants are often frequent. Where the soil becomes better drained, woody shrubs appear Sprengelia incarnata (epacridaceae) is characteristic; others include species of Leptospermum and of Melaleuca, representatives of the Myrtaceae.
Wet and Dry Sclerophyll Forest
Wet Sclerophyll or Eucalypt forest occurs on deep fertile soils in areas of high rainfall. The chief trees are the valuable hardwood species of Eucalypt e.g. E. Regnans, E. Sieberi, E. Obliqua.
The open canopy allows the development of a shrubby understorey and in areas where humidity is high rainforest species will grow myrtle, sassafras, tree ferns, Waratah and tea tree.
With decreasing rainfall wet sclerophyll passes into a more open forest, the dry sclerophyll forest, the most widespread Eucalypt being the endemic black peppermint, E. Amygdalina. Other species are rather local, changing with the soil type.
Acacia, Banksia, Casuarina and Exocarpus form the understorey trees; there is often a lower shrub layer of pea flowers and heath species.