Softwoods

From Tropical Topics newsletter, No. 71, December 2001, produced by Stella Martin at the Queensland Environmental Protection Agency. Download the PDF to read the whole issue

Many woodland trees, such as eucalypts, are hardwoods, but there are softwoods among them, notably the kurrajongs. They are often found on rocky outcrops. They have links to rainforest trees and most have ancient Gondwanan origins.

boab trees

Boab trees (Adansonia gregorii ) are distinctive features of the Kimberley region of Western Australia (left). They are also found in northwest Northern Territory. The immense, swollen trunks on older trees can measure over 15 metres around. Radio-carbon dating of a related species in Africa indicated an age of 1000 years. Given their slow growth rate and the immense size of some trunks, large Australian boabs are also likely to be very old. The large white flowers, which are pollinated by hawkmoths, appear on the spreading branches of the tree when it has dropped its leaves. The name boab is a shortened version of the African 'baobab'.

bottle tree

Boabs are a puzzle. In spite of their trunk shapes, they are not related to Australian 'bottle trees' (Brachychiton sp.). Their closest rellies are six species in Madagascar and one on the African mainland. It has been suggested that seeds, or even entire trees, arrived in Australian after floating across the Indian Ocean, but this theory runs into trouble because there are no closely related species in Madagascar, unless they have since become extinct. Alternatively, these trees may have been in Australia since before the break-up of Gondwana. If that is the case, however, why have they not spread further, since they are well adapted to drought conditions? Possibly the superior root systems of eucalypts have given them a competitive edge. Fire, however, may play a part. Studies have shown that although young boabs can resprout successfully from the roots after a fire, they bear very few fruits. On the other hand, in areas where overgrazing has led to a reduction of fires, boabs are spreading rapidly. To see a recent list of research findings on boabs click here .

Kurrajongs

Most (29 of 31) of the trees belonging to the Brachychiton family are endemic to Australia. They are found growing in a variety of situations from rainforest to woodlands but all seem to have evolved to cope with drought conditions. They tend to drop their leaves in the dry season and several species have swollen stems for water storage. Many of the species which grow in tropical woodlands are commonly known as kurrajongs.

The northern kurrajong (B. diversifolius) is found across the north. Its leaves vary enormously as it grows. Simple leaves on young seedlings go through about six distinct changes, becoming more lobed as the tree grows. This tree spends much of its early life developing a huge, deep tap root.

kurrajong flower

The red-flowered kurrajong (B. paradoxus) (left) has, as the name suggests, beautiful red flowers which form clusters along the branches when the tree is leafless. Some of the flowers are male and some are female. This tree occurs in northern Northern Territory and Queensland.

kapok seeds

Three species of kapok trees and bushes (Cochlospermum spp) are found across the north, in open-woodland. They grow particularly well on rocky hill slopes and ridges. These trees drop all their leaves during the dry season but become covered at this time with bright yellow flowers. These result in large, green, barrel-shaped fruits up to 8 cm long which, as they mature, become brown and split to release masses of silky hairs, embedded with seeds (right). Each seed has a little parachute and can be dispersed by the wind. The kapok has been used as cottonwool for stuffing cushions and in Aboriginal body decoration.

Serious chemicals

erythrophleum chlorostachys

With their spreading crowns of bright green leaves, groups of Cooktown ironwood (Erythrophleum chlorostachys), right, stand out from the crowd. In spite of its name, this tree is common throughout northern Australia, not just around Cooktown, growing on well drained soils in association with bloodwoods, stringybarks and other eucalypts. It is a highly poisonous tree, known in parts of Western Australia as 'camel poison'. All parts of the tree are toxic, even dry leaves, although it seems that little corellas can eat the seeds. Suckering shoots are particularly dangerous. Domestic animals have been poisoned by leaves accidentally trapped in the side of trucks which were transporting them; 50 grams of leaves is enough to kill. Do not tie your horse to this tree!

On the plus side, the hard timber, which is one of the densest of any Australian tree, is resistant to termite attack. It was widely used by early settlers for railway sleepers and fence posts. Traditionally, Aboriginal people have made good use of the hard timber. Infusions of the leaves have been used to cure scabies while burning green leaves repel mosquitoes and sand flies. To see a recent list of research findings on E. cholorstachys click here .

The tree produces creamy green flowers in spikes which eventually turn into flat, brown, woody, seed pods up to 14 cm long.

Heart-leaf poison bush (Gastrolobium grandiflorum) which is found across northern Australia, contains the poison 1080 (monofluoroacetic acid). This is extremely poisonous to many introduced animals. There are 27 species of Gastrolobium sp in Australia. All are thought to be poisonous and 18 are known to contain toxic amounts of 1080. All except heart-leaf poison bush are found in south-western Western Australia (where the seven species of Oxylobium plants containing 1080 also occur). It seems that native mammals and birds in this region are not killed by the toxin, having evolved alongside these plants. For that reason 1080 is often used for fox-baits in Western Australia because it is considered safe for native animals.

Heart-leaf poison bush is a straggly, multi-stemmed bush which grows to about 2 metres. Leaves are grey-green, often with a notch at the tip. The large flowers are red and 'pea-shaped'; a shrub in full flower makes a spectacular display. 

Matchwood, or turpentine (Erythroxylum ellipticum ) is a fairly unremarkable small tree found across the north but it has some notorious cousins — related species in South America are the source of cocaine. None of the Australian species produce this, but do contain some interesting compounds. The timber is very durable, burns when wet and yields a beautiful red heartwood which is popular with wood turners.

Fire!

Along with drought, fire is one of the greatest trials facing vegetation in tropical woodlands, and trees living in savanna lands had to evolve strategies for surviving it long before humans entered the equation with firesticks and matches. The thick barks of many trees are a good defence, while smooth ones give little for the fire to catch on to. Most sclerophyll trees have special epicormic buds protected deep below the bark which will sprout if the main foliage is badly damaged. Woody swellings around the roots, lignotubers, can also shoot if necessary and some species (such as Darwin stringybark) produce extensive root suckers. Fire-sensitive trees may avoid the problem by creating dense stands and dropping abundant leaves to suppress the growth of fuel (grass).

Many woodland species have woody pods which protect the seeds from fire, although the heat often stimulates them to open and release their seeds, at the appropriate moment, into a nutritious ash bed. Some hard seeds, such as acacias, need heat to stimulate germination. However, recent experiments have shown that it may be the smoke (possibly the ammonia in the smoke rather than the heat) which triggers germination in some species.