13th April - Succession

Suppose there is a stable mature forest somewhere in southern Australia. This could be for example a mountain ash forest in Victoria with trees a few thousand years old. Further suppose that this is before European settlement so there are no introduced plants. Now suppose it is destroyed by a high intensity fire.

If nothing else happens to interfere with its development, over the next thousand years or so the forest will regrow and return to what it was before the fire. Of course, if it happened today it would regrow into something quite different because of all the introduced plants but let us leave that for the moment.

However it wouldn't do that the same as an individual grows. It would grow through a series of quite different habitats. The bare ground caused by the fire would trigger a whole series of plants called “pioneer species” to grow. The seeds of these species have been lying dormant waiting for just such an event to occur.

It may be that first of all a whole series of grasses grew. Then a series of shrubs may grow. Etc.

Now it may not be a large scale fire causing this. It could be a small scale fire caused by lightning. It could be a mighty mountain ash falling over and thus leaving a huge clearing or it could be a mob of kangaroos. The important thing to understand is that there are different degrees of succession ranging from complete forests to just where a wombat dug a hole.

What we have therefore in what I would call a balanced forest are many events occurring throughout the forest in a fairly random way. This means that the different species needing different habitats to grow, all get their chance to grow, flower and replenish their stock of seeds in the soil to wait for the next suitable event to trigger their germination.

Therefore in a real forest you do not get succession continuing through to the end result. You get different stages of succession happening all the time in different parts of the forest. Unfortunately today we do tend to get succession more than we should because we now do whatever we can to stop events happening as they should.

The forest animals are all gone. Once we would have had mobs of kangaroos ranging widely, as the water cycle allowed. They would have grazed an area heavily and moved on. We do not allow fires in our forests. If we do we have them when it suits us, in the cooler months, not as they once occurred.

We now have the wildfire every 20 to 50 years which destroys all and then we see succession. However the mini events are all but gone. Many areas of our forest never see an event of any sort.

It is important to point out that wildfire is a symptom of a badly managed forest. If evolution was allowed to proceed without our interference then animals would graze the forest so they never became a fire hazard.