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Related to my previous question, I figured that the method of hunting a dragon by burning the forest it was in has to be sustainable to make it reliable. The fire must have minimal impact on the environment and the economy (a few villages caught in the fire is deemed acceptable loss), which means controlling the fire is a must. So I was wondering if it's possible to have any controlled forest fire at all in a fantasy medieval setting?

Parameters:

  • The forest is a rainforest like the ones in Southeast Asia.
  • The fire has to be contained to an area of about 100 square kilometer (I'm not sure about this number, a T.rex sized animal might have a much bigger territory than that. But let's go with 100 square kilometer.)
  • The fire has to be intense enough to flush a T.rex sized animal out of hiding.
  • There are thousands of men working together to do the burning (basically an army).
  • They have weeks to prepare for the burning.
  • The setting is low fantasy, so no magic or stuff like that, but stretching reality a bit is OK.

I imagine the combined forces of thousands of men could chop down some trees to create a buffer zone between the target forest and everything around it, although I'm not sure about the feasibility.

Also, what kind of effect a fire of this scale will have on the surrounding area, assuming it's contained? And how long would it take for the forest to recover, either by itself or with the help of people?

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    $\begingroup$ controlled forest fire is a technique that exist even during stone age, it even use by aborigine to kill giant komodo "dragon" in australia to extinction. $\endgroup$ – Li Jun Dec 21 '20 at 8:05
  • $\begingroup$ Successfully hunting animals with a far longer lifetime than humans is never sustainable. ;) $\endgroup$ – Karl Dec 21 '20 at 8:29
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A couple of details to help set the scene.

There isn't really an economy as we'd now think of it in the area you're describing. There may be people living there but they'll probably be hunter gatherers, or at best subsistence farmers. You're describing wild forest, which isn't specifically economically productive, it becomes woodland once it's managed and productive, otherwise it's just a place peasants go to hunt and cut/gather wood.

What this means in practice is that burning it to the ground doesn't cause any sort of economic damage.

However once it's burned to the ground and largely cleared, it does then become useful farmland. Slash and burn being one of the major issues related to loss of rainforests. You'll also cause some environmental damage and may cause droughts years down the line, but that can "not be a thing" as you say in your comments, or it can be the basis of the sequel.

The key here is to stop thinking like a modern person and think like you're on the ground in the period.

In short, wait for the wind to be blowing away from the village and burn it all to the ground. It'll take a few hundred years to recover, but by then your expanding population will have turned it all into farmland anyway.

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the combined forces of thousands of men could chop down some trees to create a buffer zone

creating a buffer zone is a sensible choice when trying to control a fire. However...

  • 100 square km is a square with 10 km side, so 40 km perimeter. 35 km if it's a circle.
  • chopping down trees in about 10 meters of the perimeter to create the buffer means cleaning about 3500000 to 400000 square meters of forest
  • with the limited logistic capabilities of the middle age it will take a lot of time, plus you will also need logistic to supply all those thousands of men with food, water and tools

I am not saying it is impossible, but I find hardly believable that a beast would stay quiet and put while few thousands men chop down trees all around it.

Another option, which is still labor intensive, is to divert a nearby river to encircle the area you want to set on fire with water. But this requires to have the right orography.

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You can have a controlled forest burn, it wont be intense enough, but it might work for other reasons.

I'll link you to a document outlining how a culture millennia ago was able to tame the Australian Bush using fire. They were able to shape the landscape using controlled burns before they were able to create fire on their own.

However those fires are cool. You can walk up to them. They dont reach the high foliage. It wont smoke the dragon out. I mentioned this on my previous answer in more detail.

However, by burning the undergrowth using this technique, you'll get significantly more visibility in the forest. There will be less places to hide and you can see further.

A controlled, cool burn could give you the upper hand.


Can I just make fire breaks and burn the forest to the ground in segments?

NO.

The reason this wont work is threefold:

  • Big fires contained in a tight region can build their own weather systems. This can cause storms to form, triggering distant lighting strikes, which start new fires. This happened a lot in Australia earlier this year and renders fire breaks meaningless.
  • Ember attack. When you have a big fire burning embers get swept up in the wind and launched incredible distances in advance of the main fire front. We've seen ember attack in Australia start fires 25km in front of a main fire front. When a bushfire surrounds a city random buildings several km in from the fire can ignite from embers getting in.
  • Radiant heat. A big bushfire like some of the Australian ones this year can heat a tree to auto ignition temperate at a range of hundreds of meters. This allows the fires to cross highways and other fire breaks. It can also kill you without you even seeing the fire.

For these reasons your primitive teams cutting fire breaks are not going to be able to divide the land up and keep the fires segmented. We barely managed to keep the fire behind fire breaks with the military and all our modern tech and a hundred thousand volunteer fire fighters. Medieval tech, thousands of men? No chance.


Surely the lessons of Australian bushfires aren't applicable to a rainforest fire?

The Australian rainforests burnt to a crisp too. Wet, damp rainforests burnt during the bushfires too. The difference is a few extra minutes of steam as the moisture boils away in the heat, then they burn like dry wood.

The damp may give you a little bit of extra safety, but if you get it hot enough to burn down, it will be a disaster regardless of the little bit of moisture that was present at the start.

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  • $\begingroup$ He's asking about a rainforest rather than a dry forest like Australia at this point.Australian forests are far more flammable. $\endgroup$ – Separatrix Dec 21 '20 at 8:28
  • $\begingroup$ @Separatrix the Australian rainforest also burnt. npr.org/2020/03/28/820294861/… $\endgroup$ – Ash Dec 21 '20 at 8:33
  • $\begingroup$ That's a very useful article for this question. Large swaths of old unbroken rainforest are not very susceptible to wildfire, "except in perhaps tens of thousands of year cycles," says Robert Kooyman, a botanist at Macquarie University who's worked in the Amazon and Australia's rainforests. But when humans break the continuity of the forest, fire finds footholds in the fractures left behind. $\endgroup$ – Separatrix Dec 21 '20 at 8:46
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    $\begingroup$ There's a news article from 1972 that states that the US military was going to burn Vietnam forest, but didn't proceed with it because the forest just would not burn. nytimes.com/1972/07/21/archives/… So I think not all forests are created equally. Not to mention recent forest fires are worsened by climate change, and in my fantasy medieval world, climate change can simply be not a thing. $\endgroup$ – M Arif Rahman Winandar Dec 21 '20 at 9:56

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