Australia: The Land Where Time Began
Dry Rainforest Last updated 21/10/2016
This rainforest type is distinguished by the scattered emergent species such as Hoop Pine, Teak (Flindersia australis) and Lacebark (Brachychiton discolor) trees in the upper canopy, and 10 to 30 species in the lower canopy. Buttressing and palms are not common, or absent. It is common to find a prickly shrub layer and very large vines such as 'Waita-a-while' and 'Lawyer Vine'. Ground cover consists mostly of leaf litter and some large epiphytes. This type of rainforest usually occurs on fertile eutrophic rock soils, in warm, sheltered areas with rainfall about 600-1100 mm/year with a marked dry spell. The transition between various types of rainforests and eucalypt forests can be abrupt. Examples of this type of rainforest can be seen in all the national parks in the Tweed (Wollumbin) Volcano Region. The Wilson Nature Reserve near Lismore contains a significant stand of Dry Rainforest.
'Dry rainforest' includes monsoon forest and dry scrubs or vine thickets that grow in regions with a pronounced seasonal climate - distinct wet and dry seasons. This dry type of forest are classed as rainforest mainly because of the closed canopy as well as genera composition that is largely similar to the that of 'normal' rainforest.
Rainforests in Australia have been divided into 4 broad groups based on floristic/geographic factors, hot dry in the north, hot moist in the northeast, and in the southeast, warm dry and cool moist. Both hot dry and warm dry grow in areas with pronounced seasonality that diminishes towards the cooler edges of the warm zone. The Indo-Melesian forests, the well-formed monsoon forests grow in the wetter extremes of the hot dry zone.
There are approximate divisions in the monsoon category into deciduous vine forests or deciduous vine thickets, mostly on nutrient-rich basaltic soils or calcareous beach sand, or levee systems associated with seasonal inundation. Vegetation with a higher proportion of small leaved evergreen plants grow on poorer soils that have been described as more skeletal (Meier & Figgis, 1985) and plants are mostly closely related to rainforest species from wetter areas with less pronounced seasonality on the east coast. It has been suggested that hot dry forests are species poor, though this is not always the case in some seasonal forests and thickets that are better formed, in eastern Australia, on better soils in particular.
Softwood scrubs are included among the hot dry rainforests, as well as bottle-tree scrubs and semi-evergreen vine thickets. At the start of the 20th century these are thought to have covered about 7 million hectares in the inland parts of Queensland and New South Wales. Unique Australian species such as Brigalow (Acacia harpophylla) and cypress pine (Callitris columellaris) were associated with these scrubs or thickets. Australian dry rainforests are extremely variable with regard to component species and structure, and their fragmentary distribution pattern makes them difficult to recognise on a small scale. A map of the distribution of this rainforest type shows the potential area they could occupy, and probably did in earlier times, based on bioclimatic suitability of an area, but burning and clearing for agriculture has removed them from much of their former range.
The vegetation of these rainforests produce characteristically diverse foliage in response to the seasonality of its region. In seasonal Australia there is a high incidence of evergreen, sclerophyllous (hard-leaved) plants, though there are also deciduous species in seasonal plant formations.
A wide range of adaptations has been evolved by the leaves of dry rainforest species to survive periods of water stress. The evergreen and deciduous species have adapted in different ways to cope with the same problem of water availability, for at least part of the year. The leaves of the deciduous species have adopted the low-cost, short-lived method, combined with a high rate of photosynthesis during the wet season, the part of the year when the plant does most of its growing, shedding its leaves when there is insufficient water to support photosynthesis.
Evergreen leaves have adopted the high-cost method of achieving the same goal, usually being much smaller than deciduous leaves. It is thought their smaller size and denser structure, and high reflectivity they can better withstand the extremes of heat and water stress, as well as desiccation than the leaves of deciduous plants that have a larger surface area and tend to be more membranous, and possibly less reflective. Some broad-leaved thicket plants, such as Buchanania and Gardenia, are found in eucalypt savanna as isolated individuals. When they occur in this open situation their leaves become thicker, and to some degree succulent, as a way of coping with heat stress, that is not the normal condition of their leaves when not in such exposed situations. The scrambling Sarcostemma and the hoya vine, that are succulent-like, are some of the few native Australian species that approach the level of succulence of exotic species such as the cacti.
Softwood scrubs, with trees such as the bottle trees (Brachychiton ruprestris), boab (Adansonia gregorii) and kapok (Bobmax ceiba), are characterised by tree succulents, among the softwood trees species, deciduous trees with stems and roots that conserve water.
Unlike the wetter rainforests in which the canopy is a mosaic of muted greens, there is a much greater range of colour in dry rainforest. Dry rainforests have a range of olive to silvery greens of the Acacias and melaleucas, and the kurrajongs (Brachychiton spp.) have deciduous leaves that are membranous and are bright green. The deciduous species, often flowering precociously in the dry season, just before the arrival of the wet, among the otherwise sparse, stark vegetation. The softwood scrubs provide food for many animals, especially as they flower and fruit prolifically.
There are many plants with high concentrations of alkaloids, and plants that are phytochemically rich. These plants are being investigated for their pharmaceutical potential. A 'twilight' drug has been found in 2 plants, the better known one being Duboisia hopwoodii, which is used to assist in childbirth. An extract of Tylophora, a vine in the family Asclepiadaceae, is used in the treatment of heart disease. Chemicals that are toxic to both mammals and invertebrates are present in many of the plants. Many species of plant on the edges of the rainforests and softwood scrubs are toxic, some of which are food trees for the larvae of a number of butterflies, leading to the speculation aboutt the possibly co-evolution between the plant and the insect. Milkweeds (Asclepiadaceae) are the main food for the wanderer butterfly (Danaus plexippus). Crow butterflies, close relatives (Euploea spp.) lay their eggs on Hoya australis, a common vine in softwood scrubs. Both plants contain cardiac glycosides. It is believed the plants evolved the toxins to deter their predators, such as the butterfly caterpillars, but in this case the insect has subverted the toxin to its own use, evolving a way of incorporating it into their own tissues without doing them any harm. Potential predators of the butterflies or their larvae learn to avoid them because their bodies are toxic.
There are also other species of butterfly that mimic the appearance of the toxic butterflies, so avoid predation because they look toxic. Dry rainforest and associated vegetation types are typified by such complex ecological relationships.
Many of the remaining patches of dry rainforest support unusually high numbers of birds and animals, forced together as their habitat is shrunk by clearing.
Author: M. H. Monroe Email: email@example.com
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