1. Hi, I’m Dr Adam Liedloff, a Research Scientist with CSIRO and the Tropical Savannas CRC.
Join me as I take you behind the scenes of the Flames model in Burning Issues and explain the finer details of how it all works.

2. I’m sure you’re familiar with simulation models and programs that represent reality in great graphical detail including simulation games such as SimCity and virtual online worlds like 2nd Life.
Flames is a scientific simulation model that represents the savannas – the wide open landscapes of trees and grasses that dominate northern Australia – in scientific detail. For those interested, Flames is written in Visual Basic, C++ and C# programming languages.
This model follows the growth and death of all the trees in a hectare stand of savanna. For this project we use the two dominant (main) savanna tree species; the Darwin woollybutt (Eucalyptus miniata, shown with the charateristic two-tone trunk in the visualisations provided) and Darwin stringybark (E. tetrodonta). If you live in the Top End, you may recognise these trees shown below.
We can expose these trees to a wide range of different scenarios and management practices and see how they are likely to respond over long periods of time and with other influences such as climate change.
Photo: Sam Setterfield
Photo: Sam Setterfield
Woollybutt flowers
Stringybark flowers

3. What do you think affects the growth of trees?
To develop a model like Flames involved the synthesis of years of scientific research about the savannas. To simulate trees in the savannas requires a good understanding of areas such as soil, hydrology, meteorology, trees and grasses physiology, population dynamics, grazing management, water processes (ecohydrology), fire behaviour and weeds.
What do you think affects the growth of trees?
Take a look at the next 7 slides. These slides (Figures 1, 2, 3, 4, 5) will provide you with some clues about some of the factors that affect trees in northern Australia - both their growth and death.

4. What do you think are some of the key factors that affect a tree’s growth?
We all know individual trees don’t move. Rather than decide where to live, tree stands are the result of the establishment and survival of species. They are at the mercy of the climate. The local climatic conditions and soils dictate which trees are able to survive and what the landscape ultimately looks like.
The wet-dry cycle of seasons, high temperatures, nutrient poor soils and strong sunlight all combine to influence the trees and grasses of northern Australia’s savannas.
Figure 1 – The climate is important for trees.

5. What affects trees?
Trees need both water and nutrients for growth and survival. The ultimate source of water is from rainfall. This water is then stored in the soil for trees to use through the dry season. All the processes such as infiltration into the soil and run-off to creeks are therefore important in determining how much water is available to trees.
Figure 2 – Water and the soil are important for tree growth.

6. What affects trees?
Trees, like all plants, take energy from the sun and through the process of photosynthesis convert carbon dioxide (CO2) from the atmosphere into sugars and plant tissues for growth. Current increases in CO2 concentrations from burning fossil fuels may therefore benefit trees! Scientists are finding some trees and grasses grow faster with more CO2. Growth is also influenced by the temperature (it’s always nice and warm in northern Australia and good for growth).
Figure 2 – Water and the soil are important for tree growth.

7. What affects trees?
Like all populations, a stand of trees is in a balance between recruitment and death of the various species present. Recruitment is through the reproductive cycle of flowering, fertilisation, seeds and germination. After a tree has germinated it must grow and face many life threatening forces through its lifespan which may be centuries.
Figure 3 – Recruitment and death of species define the tree populations.

8. What affects trees? Did you know?
Termites are everywhere in the savannas. They consume large amounts of grass and the wood of trees. They are the original creators of the didjeridu. Most trees happily live with termites until a storm causes damage to the weakened trunks and branches.
Figure 3 – Recruitment and death of species define the tree populations.

9. What affects trees?
As we have learned, the annual wet season in the savannas, with hot days and plenty of rain, allows lots of grasses to grow. Like clockwork, the dry season follows “the wet”.
During “the dry” water becomes scarce for plants and animals. Grasses die, or become dormant, leaving lots of dead material on the ground. Hot, dry days, strong winds, and lots of dead grass, plus people and lightning equals fire! Fire and smoke are a common sight in the savannas in the dry season and this is natural. Healthy savannas require some fire, but not too much.
Fire also affects trees. Young trees and very old trees with termite hollows are most susceptible. Also, the bigger the fire the more trees die. It’s not just how big, but how many fires that’s also important. As a tree you might survive a big fire every 10 years, but not even a small fire every year.
Frequent fires can limit recruitment, by scorching flowers and killing very young trees in the grass layer.
Figure 4 – Fire is a major factor in the savannas affecting trees.

10. What affects trees?
What’s another impact on trees we need to consider?
People. A lot of different people use the savannas for their food and living, recreation, and to make money. Having people going about their daily activities often affects the trees.
For example, people light fires. They also introduce and spread weeds that changes the composition of species. They also graze cattle where lots of grass or pasture for the cattle is the aim.
Recently, people have realised the value of the savannas for storing carbon in the biomass of trees and in the soil. It’s always been there, but now we are more aware of its value. This means management of the savannas may become part of the carbon economy. Even if only a small amount is stored in a hectare, there are millions of hectares in northern Australia, that’s a lot of Carbon!
But to understand this we need to understand fire, decomposition, termites, tree growth and the list goes on.
Figure 5 – People use the savannas and affect trees.

11. These figures illustrate that there are many factors affecting trees and we need to account for all these in our model, but one of the most significants ones is water – from rainfall.
As we have discussed trees and grasses need water to grow. This water comes from rainfall and is stored in the soil but must last for the dry season for trees to survive in the savannas.
Look at Figure 6.
Figure 6: Annual rainfall

12. As the rainfall decreases, what do you think happens to trees?
Can you see that the amount of rainfall declines (decreases) as you move south (and move further inland)?
As the rainfall decreases, what do you think happens to trees?
Not only does the total annual rainfall decrease as we head inland, but the variability between years increases. Let me explain …
The rainfall by the coast is dominated by monsoonal rainfall and is quite reliable. Inland areas rely on storms and cyclonic depression systems to bring rainfall. These are less reliable and may be absent for a number of years resulting in a lack of water for trees. Trees and grasses in these areas must be able to cope with these conditions and utilise many strategies to survive.
Figure 6: Annual rainfall

13. But, What is the affect of fire?
Our Flames model shows that with this decreasing rainfall, tree populations also decline (see Figure 7 below).
But, What is the affect of fire?
You will notice the populations from further inland are a lot less stable. This reflects the fluctuating rainfall in these areas. It shows years of tree growth followed by death in hard times.
Next time you drive around the savannas look at how variable the tree populations are.
Figure 7: Flames graph showing tree populations decline with decreasing rainfall.

14. We included 20 years of research from the CSIRO Kapalga fire experiment as part of the background understandings for the Flames model.
This fire research was conducted in Kakadu National Park and has helped inform the Flames model as well as other research results.
Two of the key understandings that our Flames model demonstrates are:
Trees are affected by fire in relation to their size and the intensity of the fire;
Repeated exposure to fire will also affect tree populations.

15. Now you have some basic ideas of how the Flames model was developed, let’s try to put these together and look at some simulations. This will help us explain what the Flames model predicts will happen when we have No Fire, Fire (of different frequency) and an introduced grass - Gamba Grass. Once again we will focus on the two most common tree species in northern Australia: the Darwin Woollybutt (Eucalyptus miniata) and the Darwin stringybark (Eucalyptus tetrodonta). The scenarios are:

16. What happens in the Flames simulation?
Situations where the savannas are without fire for long periods are rare. This is because fire is common either through land management by people or lightning fires late in the dry season. Some exceptions include areas that are protected from fire by topographic boundaries such as rivers and gorges and around springs. In these areas we often find the thickest stands of trees (and different species) because they are protected from fire and also usually have ample supplies of water throughout the year.

17. NO FIRE
You may notice that when there is no fire in the simulations the tree populations are very similar in total size and size structure over time. The upper limit to these tree stands is the result of resource limitation. In the case of the tropical savannas this is generally due to limits in water for trees to transpire and photosynthesise, but nutrients are also often scarce.
While we think of northern Australia having lots of water, we need to remember that in most areas trees must survive the entire dry season using only water stored in the soil, which can be six months. The soil type, soil depth and rainfall places limits on the number of trees for a given area. These all need to be included in the model.

18. Let’s look and see what happens as we change the fire regime from NO FIRE to MORE FREQUENT FIRES as seen around Darwin?
You can use the model to predict the result of different fire management on the trees. Just remember that the effect of management may not be instant and we need to look at the trees over quite a long period of time to really see the full effects of fire.

19. What is the short-term and long-term effect of different fire frequencies?
The effects of fire on the trees may not be immediately obvious. If we allow for enough time in the simulation, changes in the tree populations may become clearer. There are two ways fire will affect the trees; (1) fire kills individuals and reduces the number of trees present and (2) fire changes the structure of the tree population (i.e. the number of small, adult and old trees).
Many of the trees in the tropical savannas are able to re-sprout after the top of the tree has been killed by the fire. You may have seen this in areas a few weeks to months after a fire where there is plenty of new growth from the root reserves. This process may change the stand from a mature stand of trees to many small trees if fire is frequent.
Make sure you look at the population structure graphs for each fire management to ensure your actions are not changing the stand to something less desirable.

20. What do the graphs mean?
y-axis shows the total number of individual trees (as well as the number of trees for each species).
x-axis shows the average size of each tree (measured as the diameter of the trunk at 1.2m above the ground). Individuals are grouped into “classes”.

21. Total Basal Area Graph
Scientists use a value to measure the size of a tree stand called the total basal area (TBA) measured in metres squared (m2) per hectare. This is a tricky value to use as the same value can represent either a few very big trees or lots of small trees.
This value is calculated by looking at the cross-sectional area of each tree at 1.2 metres above the ground. So you can imagine if we got a chainsaw and chopped down every tree 1.2m above the ground, then calculated the area of each cut after measuring the diameter and added these values up for all trees, we’d have the total basal area for the stand.
In these simulations we have replaced the graph of total basal area with an image of the tree stand to make the effects of the management a little easier to understand.
is replaced with …
Tree Stand
(Eucalyptus miniata, E. tetrodonta)

22. What happens when exotic grasses are introduced into the ecosystem?
The results from running the Flames model with Gamba grass are very interesting. This simulation is nothing like the effects of fire through burning the native grass layers in the other simulations.
You may like to think about what aspects of Gamba grass might lead to such a different result and negative effect on the tree populations?

23. As a clue, the effect of fire on trees is the result of fire intensity, or how hot the fire is and how frequently the tree experiences fire.
The intensity of fire depends on many aspects such as the weather at the time of the fire. This includes temperature, relative humidity and wind speed which all vary between seasons and with the time of the day (i.e. morning and afternoon).
Intensity is also influenced by the amount of fuel (fuel load) and how dry (or cured) the fuel is such as this Gamba grass fire. Green grass doesn’t burn very well or carry a fire across the landscape.
The size of the tree is also important. Small trees exposed directly to the flames are often killed whereas big trees can survive most fires as trunks have fire tolerant bark. The biggest, hottest fires have the ability to burn right up into the tree canopies and may kill the tree.
Gamba Grass Facts
Gamba grass can produce 20 tonnes per hectare compared with 2-5 tonnes for native grasses.
Gamba grass remains green and won’t burn until late in the dry season when the weather conditions produce the biggest fires.
Most savanna fires do not have tall flames up into the trees, unlike Gamba grass fires.

24. How can the Flames model help us?
Researchers can use Flames to explore the processes operating on trees.
We can use the predictions to make better fire management decisions.
We can test situations that haven’t yet happened such as climate change.
We can run simulations over long time periods that wouldn’t be possible with field research.
We can run lots of runs of a single scenario to see how the outcome varies.