Australia: The Land Where Time Began
Australian open forests are usually dominated by evergreen eucalypts having a foliage cover of 30-70 %. The trees are usually of forest form with flat crowns and boles usually of greater height than crown depth. This type of forest usually develops in high rainfall regions, usually above 65 mm/year. This type of forest is found in the wetter parts of the Kimberleys, across the northern part of the Northern Territory and in the coastal and highland regions of Cape York, south through New South Wales and across the southern half of Victoria. In South Australia this type of forest usually grows in the high rainfall areas of the South-East and the Mt Lofty Ranges. In the southwestern corner of Western Australia it is common and on the highland slopes, up to about 1000 m of much of Tasmania. Open forests cover a wide geographic range from the tropics to the temperate regions, and over this range there is a great variation in height of complexity.
Tall Open Forest - formally wet sclerophyll forest
An open forest with trees greater than 30 m in height, a dense understorey of small trees, large shrubs, tree-ferns etc. - equivalent to wet sclerophyll forest. Eucalypts often more the 60 m tall. Found in the wetter parts of Victoria and Tasmania and in favourable habitats in western New South Wales and southeastern Queensland. Slightly lower forests of this type are found in the southwest of Western Australia. Self-thinning and branch pruning, when branches drop when tall trees sway against other trees. Foliage cover among mature trees may be about 30 %. Enough light penetrates the canopy to allow a dense understorey of tall shrubs or small trees, such as Pomaderris, and Olearia to develop. Tree-ferns are also common in the understorey. A ground flora of grass, ground ferns and herbaceous plants also occur.
Many of the eucalypt dominants are fire-sensitive, so fires can result in widespread destruction of mature forests. Soon after the fire, seeds germinate to produce dense regeneration which is thinned gradually and a mature forest stage can be reached in 100-150 years. Stands of Mountain Ash (E, Regnans) more than 400 years old have been found in Tasmania, but the tall open forest rarely survives to this age. Occasionally a second fire will destroy the regenerated trees before they reach reproductive age, the result is a forest lacking the dominant tree stratum.
Trees 10-30 m tall - equivalent to dry sclerophyll forest. Includes grassy, shrubby, and layered open forest. In Australia, much of the forests have a tree height of 10-30 m. In some areas of favourable conditions the same Eucalyptus species and understorey components may form tall open forests. Usually the dominant tree species are different and the makeup of the understorey is different, associated with the decrease in tree height.
Two extreme understoreys may develop in open forest south of the Tropic of Capricorn. One is characteristic of the widespread very infertile soils in the area. It contains a dense assemblage of xeromorphic shrubs to 2 m tall and a ground layer that is poorly developed. The other, found on more fertile soils, is mostly herbaceous, grasses, sedges and herbs with a poorly-developed shrub layer that may be absent. Between areas of these extremes there are many places of intermediate soil type where the vegetation can be a mixture of the 2 extreme types. Marked changes in their understoreys have been caused by frequent fires.
Shrubs rare or scattered, well-developed understory. These become increasingly common in subcoastal regions of southeast Queensland;
Open forest with well-developed understorey of xeromorphic shrubs, herbaceous layer sparse. These are mostly confined to the infertile sandy soils found on the coastal plain and between Stanthorpe and Armidale on the granite belt. Repeating burning has produced many leguminous shrubs, e.g., Acacia spp. In Queensland, many open forests look like grassy open forests soon after a fire, but as the shrubby species grow in the understorey they become layered open forests.
An open forest with at least 2 substrata, shrub and herbaceous equally important. These occur across the monsoonal northern parts of Australia. There is a prominent ground cover of tropical grasses and herbs, often almost a metre tall. Scattered shrubs and small trees are prominent in the understorey. Among these are species with strong Indo-Malaysian affinities, giving a dominantly Australian flora a distinctive variance.
All these types of open forest are subject to periodic fires - both crown and ground fires. The eucalypts of the open forests are fire-insensitive, unlike the eucalypts of the tall open forests, which are fire-sensitive. After a fire passes a mass of shoots sprout from epicormic buds in the stem. Among the understorey species have underground root stocks which survive even when the above-ground parts are totally destroyed, allowing these plants to regenerate rapidly. There are fire-sensitive shrubs in the understorey. These species usually have hard woody fruits that are cracked by the fire, releasing the seeds when the fire has passed, or seeds on the ground that have hard seed coats. Whether hardened seed coat or hardened fruit, many seeds germinate after the fire.
Trees 5-10 m tall, understorey may be any of 3 types - grassy, shrubby or layered - common in the open forest subformation. Depauperate trees, 5-10 m tall, may be characteristic of the community, growing in marginal environments around the forest. As long as the foliage cover of the upper storey is 30-70 %, it is a called low open forest. The growing season of the community may be reduced in these sites by drought, waterlogging, or low subalpine temperatures for much of the year.
Depauperate forms of low forest community often contain the same species as are found in Open Forest, and any of the 3 extreme understoreys may be present.
At other places there may be different species able to survive the shorter growing season. Snow Gum (Eucalyptus pauciflora) is present in the low open forests of the subalpine regions of Victoria and New South Wales. Some Callitris species (native pines) may replace Eucalyptus in some drier areas. Species of Eucalyptus that normally grow in the multi-stemmed mallee form of growth in even drier areas may grow as single-stemmed trees in the low open forest.
Until recently Brigalow (Acacia harophylla) formed extensive areas of low open forest in central Queensland. In the wettest and driest extremes of its distribution it grades in to closed forest and low woodland.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|