The wind’s song at Irongate last weekend was lovely. The sound of the wind in the she-oaks is a quintessentially Australian sound, part of the atmosphere of the bush. This Irongate tree is a belah, one of the toughest and most drought-hardy of she-oak species.
The green parts of Casuarinas are neither leaves nor needles, but little green branchlets. You'll notice that they have small joints along their length. Break one apart by pulling both ends, and you will see tiny “teeth” on one of the broken ends. These are all that has been left after evolution has dealt with the “leaf problem”.
The problem, for a tree which wants to survive in a very climate, is that leaves lose water. She-oaks have adapted to drought by the simple expedient of getting rid of them.
They still need to photosynthesise, however. This job, making food out of the carbon in the air, is usually done by leaves, which are green because they contain chlorophyll, a substance essential for the process.
So the branchlets have taken over the chlorophyll, and the job and the job of making food for the tree.
Belahs are dioecious. In the season, the tiny male flowers encrust the male trees with old gold. The reddish female flowers (and these seed capsules) form only on female trees. They grow inside the canopy on the branches.
Belahs tolerate a wide soil pH range, and are the she-oak best suited to the alkaline black soils of the Darling Downs. They are among the faster-growing of our she-oaks, and make good windbreaks. When young, their foliage reaches to the ground, but for a long-term windbreak they should be planted with lower-growing bushy species, as they do eventually develop tall trunks.
Casuarinas and Allocasuarinas
These are all trees of a very ancient type.
Apart from the conifers, they are the only native Australian trees to be wind-pollinated. (Wind-pollination was the first kind of pollination to evolve. Fancy techniques using insects came later, during the age of the dinosaurs.)
These wonderful trees grow very well in poor soils because they can make their own fertilisers - and they do it better if you never fertilise them. Like legumes, they can develop nitrogen-fixing roots. Their ability to do it depends on their roots finding the right soil bacteria, so the plants grow better if the seedlings are planted in a little “mother soil”, from around the roots of established trees, which contains the right bacteria to get them started.
Casuarinas go one better than legumes, however. They can also access help from mycorrhizal fungi, which co-operate with the tree to grow a dense mat of extra root-like structures. These occur seasonally, and resemble the famous “proteoid roots” of Grevilleas and their relatives. They help the tree with the uptake of phosphorus and minerals - but can result in poisoning of a tree if it has these good fungal connections is then fed a high phosphorus fertiliser.
Bandicoots love to eat these particularly nutritious fungi, and fungus spores are spread via their digestive systems, to the great benefit of the soil’s health. Any sturdy plant community has a “wood wide web” of these fungi. It’s good reason for encouraging bandicoots in our gardens.
Casuarinas are all excellent fuelwood trees, being amongst the hottest-burning woods in the world. They were much used by bakers in the early days of white settlement, so tend to have been eliminated from around all our little towns.
Belahs are capable of reaching a trunk diameter of 1m, but trees this size are rarely seen nowadays.
Our local she-oak species are:
River she-oak, Casuarina cunninghamiana;
Forest she-oak, Allocasuarina torulosa; and the
Perhaps we can also include the bulloak, Allocasuarina luehmannii, as a “local” - though it rarely occurs on basalt soil, which is the topic of this blog. (Where we do see it on the basalt soil, there is always a strong admixture of sand.)
(For more on Casuarinas see my blog June 2008, and the article above.)