Discussion with Year 10 Walford Students
Mount Barker December 10, 2010

Bush Dynamics

Q1: What is bush?
Q2: What is it trying to do? Where does it want to go?
Q3: What would your backyard do if you fenced it off and didn't touch it?

This is called “succession”. Forest Succession was first talked about by a French naturalist, Adolphe Dureau de la Malle, in the 14th century. He was talking about what happened after a forest was clear-felled. It goes something like this:

0-5 years:        Small plants like grass, rush, sedge, annuals, orchids, lilies, etc. called Forbs;
6-25 years:      Middle sized plants called Shrubs; then
26-300 years:  Trees.

The important point here, is that you don't just get tiny tree seedlings gradually becoming large trees. For the first 5 years you get a whole variety of small plants. Then for the next 20 years there is a whole set of new species of plants appearing while the first set disappear and then the trees take over from the shrubs. Etc.

UNLESS! There is some sort of intervention.

Q1: Examples of interventions?
Q2: Do you weed your garden? Isn't this an intervention?
Q3: Do you mow your lawn? Isn't this an intervention?
Q4: Give me examples of natural interventions?

  1. Rainfall or lack of it – Goyder's Line. Succession stopped at shrub stage.
  2. Grazer – Grass Partnership. Succession stopped at the Forb stage.
  3. Drought? Fire? Flood? Tree falling over? Wombat digging a hole? Etc.

I call these interventions “Events”.

One problem with Succession is as follows: If I clear fell a forest then I get forbs for five years and then shrubs for 20 years and then forest for 300 years. Then I clear fell it again and I get forbs and shrubs again.

Q1: Where did they come from?

So! Back to your garden. If its anything like my garden, the biggest problem is weeds.

Q1: Where do these garden weeds come from?
Q2: Why do you get more weeds after you remove the first lot?

There is an old gardening saying, “One years seeding – seven years weeding”. It is actually worse than that.

The woody weed story:
Q1: You have a patch of mature woody weeds (say broom) why are there no seedlings underneath?
Q2: You remove the mature woody weeds and millions of seedlings appear. How?
Q3: You remove those seedlings and more appear. What's going on?

“Pheromones”, “ Genetic Wisdom”. Seeds “know” when to germinate and when not to.
Not all seeds germinate when conditions are right.

Since I am a mathematician, let me do some mathematics.


  1. That 20% of viable seeds (in the soil) become non-viable each year;
  2. That when conditions are right, 40% of viable seeds germinate; and
  3. We start with 1,000,000 seeds. Then (you can do this as an exercise on your computer):

Firstly suppose that conditions are not right for germination (for example there is a tree cover).
Year                            Number of viable seeds
0                                  1,000,000
D                                 N
D+1                             0.8*N
63                                1
67                                0
So! once you have broom seeding in your backyard you cannot assume that you have got rid of them for 67 years.

Next suppose that we make conditions right for germination each year by puuling out all the seedlings each autumn.
Year    Number Viable           Number Germ.            Number Left
0          1,000,000                    0                                  1,000,000
D         N1                               N2=0.4*N1                 N3=N1-N2=0.6*N1
D+1     0.8*N3=0.8*0.6*N1 = 0.48*N1
20        1
21        0
So! Once you have broom seeding in your backyard you cannot get rid of them in less than 20 years.


Of course The figures I have given are very approximate and vary from species to species. However they give an idea of what goes on. The point is that for plants to survive, their seeds must remain viable, in the soil, for a very long time. A good example of this was given by the recent wet weather in outback Queensland where plants germinated which had not been seen for thirty years. Some of these they thought were extinct.

Let us now confine ourselves to the Adelaide Hills. To me the most exciting part of the concept of succession and the grouping of plants into the groups forbs, shrubs and trees is the number of species in each. If we look we see:

Group Number of Species
Forbs >80% (about 350 in the Aldgate Valley)
Shrubs >80% of remainder (about 50 in the Aldgate Valley)
Trees 2% (about 10 in the Aldgate Valley)

Q1: How do we manage our bush to maintain its diversity?
Q2: Is it possible to manage bushland, in the Adelaide Hills, So that all possible species flourish?
Q3: Do we do that now? National Parks?
Q4: What about “Bushcare”?

To try to answer some of these questions I commenced the “Wirrapunga” project 14 years ago.

Basically, I used the theory I have just outlined to you. In particular that:

  1. Seeds stay viable for over 50 years in the soil and just need the right event to stimulate them into germination; and
  2. That although a lot of rubbish germinates with careful weeding the gems can be saved.

At its commencement there were about 40 species of indigenous plants growing there. Today there are about 300 including some very rare species. Next year it will be the first indigenous garden to be part of the “Australian Open Garden Scheme” (by invitation). Perhaps one day I can show it to you.