Posts Tagged ‘gum tree’

On Being a Predator

May 25, 2009

I’ve been thinking about the implications of choosing Scribbly Gum as a title. You can check out what a Scribbly Gum is up there on the right. It’s a kind of eucalypt, the classic Australian tree. It turns out what sounds like a cute kind of tree is anything but. Eucalypts have some nasty habits.

The first thing that you notice about most eucalypts is how messy they are. They drop their bark each year. It comes off in long strips and hangs in the branches and litters up the ground. OK, lots of trees drop things. Deciduous trees drop their leaves. Fruit trees drop their fruit. The difference with leaves and fruit is that they rot down to compost. Eucalypt bark is tough and fibrous and it doesn’t break down easily. And whereas deciduous trees drop the leaves as the weather cools down, the eucalypt sheds its bark as the weather is heating up.

So what difference does that make? It makes the base of the tree a fire hazard. Here in Australia that is a real problem.

The next thing about eucalypts is that the leaves contain high levels of very volatile and flammable oil. Eucalyptus oil is sold the world over to remove stains from clothing. That’s good. But in the typical Australian bush fire that oil evaporates from the leaves and hangs in the air like petrol vapour. It can explode into flame hundreds of yards ahead of the firefront.

Now lets look at the eucalypt leaves. They are different from most other trees. They hang vertically and they have this trick of slowly turning through the day so they are always edge-on to the sun. Eucalypt leaves have sun-phobia. They don’t like getting hot. There is little shade to be had from a eucalypt. The ground heats up, and all that littered bark heats up. A stray lightning strike easily sets the whole lot on fire.

Because the soil heats up, there is lessĀ  humus and bacteria under a eucalypt. The soil is drier so other plants can’t grow. There is very little understory in a eucalypt forest. And the tree has another trick to add here. Eucalypts exude a substance that inhibits the growth of other plants. They like to keep the place to themselves. The result is a surviving eucalypt, but loss of biodiversity. The soil becomes carbon poor, and this in an age when we desperately need to increase carbon sequestration into the soil.

And just in case you thought I was finishing with this point, here’s something else. Because nothing much will grow around the base of a eucalypt, animals don’t feed there. This means they don’t leave their droppings there. No manure, no fertiliser, no enrichment of the soil. This is all part of the evil plan of the eucalypt for world domination! OK, a little bit heavy on the metaphor there, but you get the picture. This tree does not like visitors, whether plant or animal. And it doesn’t treat its soil very well either.

The eucalypt is so used to living in poor soil that it actually works at impoverishing it’s own growing region to defeat any competition. One of the results of life in a dry climate is that when it starts to suffer drought stress it sheds branches. Eucalypt branches are held on by duct tape or something. They can suddenly drop from the tree, snapping clean where they attach to the trunk. Remember that fire starter bark lying on the ground? Now its got some heavier fuel.

Now we have a real paradox. The tree works at impoverishing the soil so as to prevent other species from getting close. It heats up the ground and litters the hot ground with fire-starter bark. Then it releases vast amounts of volatile oil as fuel into the air. And when the fire goes through everything is burned. But not quite. Eucalypts have an extraordinary ability to recover from bush fire. they can sprout leaves from their trunk and then branches follow. And the other plants? Well, after the first rush of weeds there’s nothing much left to grow. So the eucalypt has the place to itself – just as it likes it.

And there you have it. Maybe being a scribbly gum is not so nice after all.

This is your friendly local predator signing out.