Photo: C & D Frith
Australia's Wet Tropics Rainforest Life
- The canopy can be described as the seal on the rainforest. If there is no seal, then there is no rainforest, only sclerophyll forest.
- It is the search for light that creates the canopy, and the rainforest depends largely on moisture and control of moisture by the canopy in its survival.
- The rainforest canopy may be as high as 45 m above the floor. Its height is controlled by competition. If there are a few high trees, then others will be forced to grow taller too.
- It may be even or uneven with several layers of trees, and if a gap appears in the top layer, wait-a-while and adjoining trees will be fast to plug up the holes in their competition for light.
- No matter how high, it plays a vital controlling role in rainforest ecology with no canopy there is no rainforest as nutrients will be lost to the clouds. It also creates a darker atmosphere as it is closed.
- There are many physical factors that affect the composition of the rainforest. These include altitude and its effect on rainfall, prevailing winds (these bring the most rainfall to the south and east of the Tablelands), soil types (parent bedrock), fire frequency and intensity to name a few.
- The canopy provides an enormous food source for the many insects, birds, possums, and other creatures, as well as a habitat for some tree-dwelling species such as the snake. The large quantities of epiphytes provide more habitats for lizards, birds and other small animals.
- Animals living in the canopy play an important role in the rainforest ecology too by knocking fruit down from the trees, they allow the terrestrial animals to feed for a longer period of time as they do not have to wait for the fruit to fall by itself in its own time.
- In young rainforests there are generally many small trees and in old rainforests there are generally fewer but larger trees. As one tree dies or is removed, others will grow up and fight for the available light.
- As the rain falls to the ground it helps leaf mould develop, which allows nutrients to be brought up through the soil to the plants. Condensation in the atmosphere leads to rain again, so a nutrient cycle is successfully completed.
- Giant emergent trees often project well above the canopy and reach even more impressive heights, spreading their crown over it. Leaves are of great variety, differing in shape, size, texture and colour. These sometimes provide food for species like the ringtail possum and tree-kangaroo.
- Fruits are also an important food source for many fruit and seed-eating canopy-dwellers (who in turn help with dispersal). The pericarp (outer fruit case) may be soft and succulent or hard and woody. It may enclose one or many seeds. Fruit-eating pigeons, bowerbirds, and flying foxes eat succulent fruits whereas parrots eat harder woody nuts and seeds. Indigestible seeds voided by the birds will germinate in a tree crevice or hole.
- Most tree seedlings have to start their struggle for survival on the forest floor. Others, such as the strangler fig, begin life as an epiphyte in the canopy, slowly growing and putting out long cable-like roots that travel down the trunk of the host tree to the soil beneath. This allows it to readily absorb nutrients and water so it can flourish.
- Mistletoes, partial parasites that also start life up in the canopy, have developed a fascinating way of ensuring the dispersal of their fruits. Mistletoebirds and other frugivorous birds eat its soft berries. These berries have sticky seeds inside that when voided adhere to a branch and germinate.
- Flowers in the canopy often provide a contrast to the colours of the dense green background. Their size, shape, colour and scents are considerably different, and are designed to attract nectar-feeding tree-dwellers to ensure their pollination. This may be from butterflies, bees, lorikeets, and other species during the day and from moths, blossom bats, flying foxes and others during the night.
- One significant plant family in the Atherton region is the Proteacea. They are represented by about 50 species, of which approximately 70 percent are peculiar to the area. Some examples of this family include silky oaks, tree waratahs and macadamias. Some endemic species of the Wet Tropics are the Northern Silky Oak, Brown Silky Oak and Ivory Curl Flower.
Additional Information: Courtesy of Damon Ramsey
The canopy of the rainforest
The rain forest consists of a multi-layered canopy. The main canopy consists of an undulating ‘sea’ of tree crowns with heights of between 25 and 45 metres in lowland tropical rain forest (Whitmore 1999).
Visitors are often surprised that the trees in tropical rain forests are not taller. While it is true that many of the bigger trees in the Daintree were taken out by loggers for their timber, in fact many of the taller tree forests are not found in the tropics at all. In Australia the really tall forests (in fact the biggest flowering trees in the world) are found in the quite temperate Eucalyptus open forests in the south-east and south-west of the continent.
Every now and then, taller trees poke out above all else. These trees are known as ‘emergents’ (Johns 1999). In the swampier lowlands of the wet tropics these emergents are commonly straight tall palms such as Black Palms and Alexandra Palms.
Below the main canopy may be many different levels or strata of trees. These may be at different stages of growth, depending on local patches of disturbance.
On the coastal lowlands, disturbance is often the result of cyclones. Trees that are knocked down bring down a mass of other plants, through the tangle of connected vines and epiphytes. Huge holes in the canopy grow over in a matter of months, or there is an explosion of tangled plants taking advantage of the new light. Areas along the coast that are repeatedly battered over the years have distinctive plants smothering them, such as Wait-a-whiles and the appropriately named 'Cyclone Vine'.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide