Why Australia's giant trees might not return

There's a common assumption that if you chop down or burn down a giant tree then another one will grow back to take its place. But I don't agree with that. There's a bunch of reasons why Australia's remaining giant trees could be our last ones.

How to make a big tree Step 1: First plant it at least a couple of hundred years before you were born

How do you make a giant tree?

Eucalyptys regnans is a fast-growing gum tree native to high-rainfall areas in the Australian states of Tasmania and Victoria. It holds the record for the tallest hardwood in the world. Some of biggest regnans could give some of the big sequoias a run for their money in terms of size and could even beat the tall redwoods for height. But you need a lot of things to go right to make a giant regnans.

First, it has to be growing in an area with good soils and a tremendous amount of rainfall - the very kind of land watched with envy by plantation owners. In the first years of its life the young tree should be surrounded by tall trees, forcing it to race towards sunlight, in its fast-growth early years. The tree then has to survive hundreds of years of fires, pests, disease, storms, lightning strikes, and of course, logging. It also needs a permanent stand of trees around it to protect it from exposure to storms and erosion.

Run the slide show below to see some of the processes involved in producing a giant.

If you compare the sequence in the slide show above with what's happening now you'll realise that these perfect conditions are unlikely to happen again.

The few giant trees that are spared from logging have everything around them chopped down. This leaves them standing exposed and vulnerable to lightning, erosion and wind damage. The ground around the tree is no longer shaded and becomes drier and harder than it once was. With all these things working against it, the giant is doomed to die much earlier than it should have.

Decaying log

This ancient log (above) used to be the trunk of a giant tree. It's slowly decaying into the soil, providing nutrients for the next generation of trees.

But what new trees? Why can't they become giants?

There are lots of things stopping them:

First, you have to wait several hundred years. Call me impatient if you like, but I can't wait that long.
When an area is clear-felled there are no tall trees left behind. The sunlight can reach every new seedling. This means there is no competition for the sunlight - the fast-growers and the slow growers can reach maturity. The selection process that favours fast-growing giants is diminished.
Because the old trees were taken away, with the remaining timber burnt and napalmed, there will be no nutrients left to decay back into the soil. Those nutrients are needed to provide the tremendous hit of fertiliser that it takes to grow a giant tree. This will only get worse with each successive generation of trees taken away from the soil.
With the areas of best soils and highest rainfalls already cleared and claimed for plantations, Eucalyptus regnans is being pushed into places that are less favourable for growing giants.
During the late 1800s there was a competition to chop down the tallest tree. This removed most of the giants from the gene pool. The smaller, shorter-growing trees were left to produce seed.
It takes the right combination of perfect climate, altitude, rainfall, and soils, plus surviving lightning, wind, parasites, fire and other hazards for three or four hundred years to produce a few giant trees. Climate change is causing instability in the weather and more extreme droughts and flooding - that mature trees are less able to cope with. The extreme droughts also increase the number and severity of bush fires.

When you consider how difficult it is, and how long it takes to make a giant, it seems crazy to chop them all down for woodchips - a product which is basically the same as mulch. I visited Tasmania in the early 1990s to see the great forests while it was still possible. I'm glad I did, but I also feel sorry for people younger than me who missed the chance, because most of those spectacular forests I saw are already gone. I just wish I'd taken more - and better - photos.

The last great area I saw briefly, with some of the most gigantic trees I thought possible, was the Styx Valley. When I went there in the early 1990s I was told by a forestry official that legislation had ensured that the Styx Valley would never, ever be logged. I was even considered a fool and a bit of a troublemaker for suggesting that might change one day. Of course it is the new area planned for extensive logging.