Australia: The Land Where Time Began
Woodlands of Australia
Woodlands are a category of vegetation differing from forests and rainforests by the height, spacing and crown cover of the component trees. A definition was given by Richard Hobbs 'ecosystems that contain widely spaced trees with their crowns not touching' (Lindenmeyer, Crane & Michael, 2005). "Projected Foliage Cover' is the percentage of the soil surface that is shaded by the tree crowns. In woodlands it is usually between 10 and 30 %. In areas where the trees are regenerating after clearing and there are a high proportion of saplings in stands that are denser than in the mature woodland, the ground cover can be higher. Trees in woodlands are sometimes less than 10 m tall, depending on environmental conditions such as rainfall, and the trunk is never longer then half the height of the crown.
In Australia woodlands are usually dominated by eucalypts, or occasionally by Callitris, Casuarina, Melaleuca, and Acacia, having a foliage cover of 10-30 %. In these woodlands the trees usually have rounded crowns and the bole length less than the crown depth.
This formation occurs in lower rainfall areas than the open forest, occurring in a broad arc inside the open forest areas from the Kimberleys in Western Australia to areas to the west of the Great Dividing Range in Queensland, New South Wales and Victoria. There are also some small areas in South Australia, southwest of Western Australia and in Tasmania.
A gradient in height occurs across the rainfall gradient, as soil moisture increases it approaches the height of a forest. At the wetter end of their range the trees can be up to 20-25 m tall, have more compact crowns and more extended boles, characteristic of the forest forms. In the middle portion of their range the trees are more widely spaced, with spreading crowns and shorter boles, more characteristic of the woodland form. Towards the drier end of their range they are progressively shorter, 5-10 m, and more widely spaced than in the middle part of their range.
Over-mature stands of tall forest, about 75 m tall, become progressively open with age until the coverage of the canopy falls below 30 %. These communities, small in area, could be called tall woodland.
Ground fires occur occasionally, but the dominant trees are rarely destroyed, most of the understorey species regenerate easily from both underground root stock and seeds.
Like in open forest, variations in the understorey occur.
These woodlands occur mainly west of the Great Dividing Range from southern Queensland to Tasmania and in the southwest of South Australia. On the coast these woodlands are usually found between taller, wetter forests in the coastal strip and the hotter and more arid grasslands of the interior. This is the area that has been largely cleared as it is what is now called the wheat-sheep belt. The similar form of temperate woodland is found in southwestern Western Australia that has also been largely cleared for farming and grazing.
The area now occupied by temperate woodlands was previously occupied by rainforest, when Australia was still part of Gondwana. At the time, the climate was not strongly seasonal, being warm and wet the year round. Temperate rainforest covered much of southeastern Australia, consisting to a large extent of southern beech (Nothofagus), similar to cool temperate rainforest presently found in southern Victoria and parts of Tasmania. There is evidence, fossil pollen, that the early eucalypts were already present and widespread in the Gondwana rainforest as far back as 60 Ma.
As the climate became drier between about 30 and 15 Ma the rainforests retreated, eventually being confined to the wetter coastal strip, leaving much of their former territory to the hardier eucalypts that had been pre-adapted for just such an event, resulting in the widespread eucalypt and Acacia forests and woodlands. It is believed that many of the plants and animals in woodlands had begun to evolve in the Mid to Late Miocene. The spread of open forests and woodlands opened up new niches that would have increased the diversification of plants and animals that lived in and on them them. It has been suggested that the greater separation of trees encouraged the evolution of the gliding possums.
A side effect of a drying climate was the increased incidence of fire, even before the Aboriginal People arrived with their fire-stick farming. As the climate became hotter and drier the eucalypts and wattles took advantage of their adaptations that allowed them to survive the increased incidence of fire, using it to their advantage to reduce the competition of other species that were less fire tolerant. The combination of a climate that was becoming increasingly dry over the last 100,000 years and the Arrival of the Aboriginal People accelerated the spread of open types of vegetation.
After the close of the last ice age, about 15,000 years ago, wet sclerophyll forests expanded out of the refugia they had survived in throughout the cold, dry times of the ice age, reclaiming some of the territory lost to the woodlands as they ice age intensified. The pollen cores from the sediments of swamps and lakes, such as Lake George, and from the now-submerged sediments of earlier lakes on the continental shelf, has given some idea of the vegetation changes that took place over time. The sediments of Lake George cover a time beginning about 350,000 years ago. This pollen record shows that woodland similar to that of the present, began to form about 130,000 years ago. About 40,000 years ago the amount of charcoal in the sediments increased, leading to the belief that it resulted from the burning by the early Aboriginal People. Pollen cores from off southern Victoria indicate that by 125,000 years ago there were eucalypt and she-oak (Casuarina) woodlands with an understory of grassy and eucalypts. From that time it is believed that in many parts of southeastern Australia there were woodland, heath, shrublands and dry sclerophyll forests, wet forests being restricted to the moister, more sheltered locations.
The origins of eucalypts, members of the Myrtaceae family, is not known. The Myrtaceae did not evolve in Australia, it is believed the early eucalypt ancestors arrived in Australia from South America and Antarctica at a time when all 3 continents were joined as parts of Gondwana. Fossil evidence has been found in Borneo, Africa, South America and Antarctica.
Though the origin of eucalypts is unknown, there must have been substantial evolutionary pressure to cause them to evolve so many species. It is believed the most likely reason for the great diversity found in present-day woodlands probably resulted from changing climatic regimes.
Woodlands are some of the must used and abused environments in Australia. Clearing them can lead to a number of adverse effects, such as increasing soil erosion and salinity.
One of the reasons for conserving woodlands, that is apparently unknown to most land holders, is that they can contribute to farm productivity. Birds and glider possums eat large quantities of insects that are a threat to farm production. When the woodland is completely cleared the benefits of pest control is lost.
Grassy Woodland - formerly savannah woodland
This type has a well-developed herbaceous layer in which grasses dominate. There are usually few or no shrubs, though they may be present if the areas is subjected to frequent fires. This subformation is extensive on fertile soils in the rainfall zone of about 38-88 mm/year. This type of community has been grazed by sheep and cattle in the southern half of Australia. Many of the native grasses and herbs have been replaced by introduced grasses, herbs and weeds from southern Europe or South Africa. Introduced shrubs have also invaded the grassy woodlands - Olea, Crataegus, Chrysanthemoides, Lavendula, Ulex, Rubus.
Shrubby Woodland -formerly sclerophyll shrub woodland
This has an understorey rich in xeromorphic shrubs. The herbaceous layer is poorly developed. It forms on infertile soils and is a continuation of the extensive areas of shrubby open forest.
This has a well-developed shrub layer and a herbaceous layer. In southern Australia, the shrub layer is probably fire-induced, especially if it is composed of Acacia spp. and possibly other legumes. Multi-layered woodland communities is much more common in northern Australia, often containing a shrub layer with a component of Indo-Malaysian species. As with the shrubby woodland, the shrub layer may be fire-induced.
Low woodland, trees 5-10 m, becomes more prominent at the drier limit of the formation, often being distinctly layered with frequent shrubs and an herbaceous layer. though the herbaceous layer if often composed of ephemeral species. The low woodland is dominated by a variety of deciduous Eucalyptus species, of these 10 species are known. In the semi-arid south of the continent small trees often replace the eucalypts, such as Callitris collumellaris, Casuarina cristata, Heterodenrdrum oleifolium, Acacia sowdenii and Myoporumplatycarpum, in the layered low woodland.
This is comprised of scattered low trees, usually less than 10 m. They may occur in clumps or singly. The foliage cover is less than 10 %. This type of formation occurs at the driest end of grassy woodland or on deep sandy soils in higher rainfall areas of eastern Australia. It can be found in association with the following understorey types.
Hummock grasses of Triodia (spinifex)
This occurs between Mt Isa in Queensland and Tennant Ck in the Northern Territory. Scattered Eucalyptus brevifolia, hummocks of Triodia a prominent component of the understorey.
Tussocks of Lomandra, Danthonia, etc,
On the eastern side of Mt Lofty Ranges, South Australia. It occurs in the rain shadow of the ranges. Casuarina stricta trees are scattered in a low open woodland. Tussocks of Lomandra, Danthonia and Stipa form a conspicuous ground layer.
Dense sclerophyllous shrubs
Banksia and Eucalyptus trees, up to 10 m. Occurs on deep sandy soils in the higher rainfall areas of eastern Australia. The understorey is composed of a dense assemblage of sclerophyllous shrubs forming a 'tree heath'.
Dominated by shrubs 2-4 m tall, canopy cover greater than 70 %. Understorey made up of shade tolerant plants. Closed Scrubs typically grow in Swamps, fresh or brackish water. In Victoria they can be of Melaleuca ericifolia, M. squarrosa, and Leprospermum lanigerum. In Queensland they can be M. nodosa-Leptospermum flavescens. Along the coastal sand dunes of south-east Australia, when protected from wind- and salt-pruning coastal tea-tree (Leptospermum laevigatum) can develop into closed scrubs.
Closed scrub is not characteristic of south-east Australian landscapes
This type of community is prominent over large areas of southern Australia in the 10-20 inch annual rain zone. Included in this formation are mallee-type Eucalyptus species-dominated communities, with multi-stemmed shrubs 2-8 m tall with 30-70 % foliage cover. The stands are dominated by several evergreen mallee eucalypt species. Calcareous, often solonized soils, are associated with certain eucalypt species. Some species are confined to infertile, solonized, sand soil. The soil type and level of aridity strongly influences the makeup of the understorey. Fire-tolerance is a characteristic of this type of community.
This formation has a number of subformations.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|