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One of the borders of my map is a forest that was burned so badly and for such a great distance that it is currently impassable for humans. There is a once-great city on the edge of the burned forest that was damaged and is being rebuilt.

The city is home to both humans and centaurs who haven't interacted greatly in the past, but are on good enough terms that they are working together to rebuild the city and restore the forest. The Centaurs want it cleaned and restored, because it is where they lived. The Humans want it restored, because it was the major source of lumber for their nation.

It is a pre-gunpowder era with cannons and flint just being developed/ coming into use. Humans are not populous enough to destroy the forest through logging and so the logging causes no strife with the Centaurs. The Centaurs are living with the Humans because they can no longer live in the forest and have also begun to work with the humans outside the city to protect their nation.

What is beyond the forest is unknown because humans have never successfully traversed it before the burning and the centaurs never had a reason to leave it before.

What I need is an event that would burn the forest so thoroughly that it would force the Centaurs out, make it impassable and spark the cooperation between the Centaurs and humans. I thought of potentially a meteor or some other form of heavenly fire, but something like a lightning strike seems like a little bit of a cop out.

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    $\begingroup$ Why would a burnt forest be less passable than it was before? (I mean unless it is still burning, of course). Slash-and-burn is a commonly used method to turn dense forests into open terrain, so I would assume that burning down a forest makes it easier to pass instead of harder. $\endgroup$ – Philipp Nov 20 '14 at 9:01
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    $\begingroup$ Also, I don't think that a forest would be a good habitat for centaurs. The legs of horses evolved for navigating open planes, not forests. Maybe if they were rather half-deer than half-horse? $\endgroup$ – Philipp Nov 20 '14 at 9:08
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    $\begingroup$ Notice that a lot of people seem to think that the question is good enough to answer, but not good enough to uvpote. +1 to make up for that, because it's a question that really made me think. $\endgroup$ – HDE 226868 Nov 20 '14 at 16:04
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    $\begingroup$ I don't see how making the forest "impassable" is required here. A burned out forest could be unusable for both men and centaurs and require them to move out to less workable areas. Perhaps the forest itself was a 'farm'. Another fact is that in older days, forests near farms were more open than modern ones. People went in to collect fallen wood for fires, underbrush was kept down by having pigs 'graze' in the forest for acorns. If the Centaurs were doing the same or using some tree product as a crop, this would be even more true. $\endgroup$ – Oldcat Nov 20 '14 at 17:19
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    $\begingroup$ Only one thing. $\endgroup$ – MDMoore313 Nov 20 '14 at 17:50

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I would suggest that perhaps the forest is impassable because it is still on fire.

At some point in history this forest was more of a wetland, over the years a thick layer of peat formed. The forest grew above this layer, and this peat layer stayed undisturbed for many years. Now some great forest fire rolled through destroying the forest and in the process ignited the underground peat layer. Burning peat creates clouds of toxic smoke, and leave the ground unstable causing mini sinkholes, making the forest extremely dangerous to cross.

Peat can burn for many years, and putting out peat fires is very difficult, thus you could have your forest remain impassable for an extended period of time.

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    $\begingroup$ This reminds me of the fire swamp form the princess bride. Perhaps there could be some rodents of unusual size. $\endgroup$ – ankh-morpork Nov 23 '14 at 21:59
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The problem with this scenario is that burning the forest actually makes it a lot easier to pass through. It clears out the underbrush and small stuff that normally slows down travel.

One option that might work though is if the terrain in question was a maze of rocks, holes, and small canyons. The larger canyons had wooden bridges constructed to cross them, the smaller ones had fallen trees covered in vines that made them passable.

The fire swept through long enough and hard enough to destroy all the bridges, both natural and artificial.

Crossing the land is now incredibly difficult because not only have all the bridges been destroyed, but it takes a lot of hunting around to find good enough quality wood to build new ones. You need to find particularly large trees with the trunk still standing, and it will be at least twenty years until the forest regenerates to the points where the trees are useful for this.

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    $\begingroup$ "humans have never successfully traversed it"~ not sure how many bridges there would be in such scenario. $\endgroup$ – David Mulder Nov 20 '14 at 19:26
  • $\begingroup$ @DavidMulder The centaur could have built the bridges for their own use and jumped over smaller gaps, meaning that they would be far less useful for humans. Additionally the artificial bridges would be in the centaur-controlled areas and not extend all the way to the human lands. $\endgroup$ – Tim B Nov 21 '14 at 9:43
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    $\begingroup$ It could be impossible to cross it if the forest was so huge, it would be like crossing a desert. $\endgroup$ – dyesdyes Nov 21 '14 at 11:17
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The woodland on the hill behind my house (Forestry Commission, managed forest, spruce and fir) suffered greatly in the big storm of 2005. I heard of an anemometer further north that recorded 146mph ... then blew away. About half the trees, in great swaths, in the more exposed places, blew over, with a domino effect whereby one tree going increased the pressure on its neighbours.

You'll cross the first fallen trunk - there's nowhere to go but the next, higher, and the next after that, until soon you're 30 feet in the air with no (forward) way down. Treetrunks criss-crossing, almost woven into impenetrable 30 foot fences. And the next stand is almost untouched, but the stand after that is flattened again.

Get down safely and you're back where you started.

The forest's well-nigh impassable.

But could fire do that? It could perhaps weaken the trees. Or, if hot enough, (encouraged by a good wind) it could build a firestorm as happened during the bombing of cities in WW2, generating high windspeeds to do the real damage. I don't want to link to descriptions of these firestorms, they are quite harrowing reading.

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  • $\begingroup$ This is a much better solution then a fire, because it leaves a tangle of trunks and branches that makes it impenetrable. $\endgroup$ – user3106 Nov 21 '14 at 13:02
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Nearby active volcano? Either employ actual lava burning a swathe through the forest and leaving essentially a wall of rock treacherous terrain with the ground cracking underfoot into huge volcanic caverns or magma tubes. Or have the volcano semi-active, poisoning water with sulphur compounds, lethal out-gassing, heat causing evaporation or even burning the trees from the roots up. Could even make the ground in some areas too hot to walk on for months or even years.

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  • $\begingroup$ This is good and practical $\endgroup$ – James Nov 20 '14 at 20:07
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    $\begingroup$ To add to this: Have gaseous buildups frequently sending dirt and lava flying like sparks, igniting what little regrowth is actually happenning. It's not uncommon for active volcanoes to "spit" every now and then as pressure builds up. $\endgroup$ – Robotnik Nov 21 '14 at 4:00
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There's probably only a single scenario I can think of that would make a forest impassable from any forest fire, regardless of the catalyst.

There's a real-life phenomena that occurs when a forest fire reaches a certain size and temperature it requires a massive amount of oxygen to keep feeding it.

What happens is that the amount of oxygen being gathered to feed the fire will actually uproot the trees due to extreme winds and leave them on their sides. Like a pile of matches or toothpicks.

Similar to what you would see if a bomb went off.

I can't remember what term is used to describe it.

That timber laying on the ground is similar to dead fall and is extremely difficult or impossible to traverse without great effort. The trees when thrown to the ground in this manner usually don't burn entirely and will mostly loose some of their exposed branches instead.

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Instead of a big meteor which would possible cause a Extinction Event, small meteors which would barely reach the surface could still develop enough destruction and would give an awesome light show of which storys can tell for decades.
Meteors can bring in lots of toxic materials. If the world passes through a stream of meteorites which carry lots of these materials (Sulfur, Arsenic ect.), they would be brought into the atmosphere and/or ground, causing acidic rain and similar events. Nice sideeffect is that this is not immediatly lethal, so some of the centaurs could escape. One could even imagine radioactive materials, though they would probably very longliving, otherwise they wouldn't have reached your world in the first place. Have it rain once or twice to bring the material underground and you have a toxic wasteland.

Depending on your intentions, this is extremly difficult to remove. If the ground is simply widespread acidic, humans and centaurs can bring out akalic materials to counter the effect. The cost-to-use-ratio would still be horrible, but if humans and centaurs are really determined...

Radioactive materials are another case though. While I'm no nuclear scientist, the actinoide group doesn't look too bad. They have a long enough half-life to reasonably reach your planet, but seem still active enough to pose a threat. The disadvantage is that you can do absolutly nothing against it.

Just in case you care: The small meteorites could be the remains of a (shattered) planet (for example after the impact of a moonsized planetoid), sorted out by several orbits around your local sun. Or only small meteorites managed to escape.

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  • $\begingroup$ "The disadvantage is that you can do absolutly nothing against it." In a world with centaurs there might be magic ;-) $\endgroup$ – David Mulder Nov 20 '14 at 19:27
  • $\begingroup$ @DavidMulder well, waving hands and producing octarine bands of magic always work, but if we get to this point, one could simply seek the local Archmage and everything's good. Besides, nobody knows whats killing the people. The go in, run around for a few hours and die. They go in for minutes, come back alive and everyone dies two years later. What can a wizard do against that (except healing cancer)? $\endgroup$ – J_F_B_M Nov 20 '14 at 23:48
  • $\begingroup$ Nah, I was mostly objecting to the idea that there is this idea around that something is either 'magical' or ' technological'. For all we know mages have used radioactivity since the beginning of times as their basic way of interacting with a lot of things. Not saying that magic can necessarily solve it, just that saying "one can not do anything against something" in a magical world is per definition short-sighted (though please ignore the negative connotation that word has). $\endgroup$ – David Mulder Nov 20 '14 at 23:50
  • $\begingroup$ Radio active elements are rare in meteorites and once the rain has washed them into the ground walking about should not be harmful. The bigest problem with uranium contamination is that it is a heavy metal as toxic as lead. (about 2g fatal dose) $\endgroup$ – Donald Hobson Jun 19 '16 at 15:27
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Burnt is burnt, the cause does not matter. And after a wildfire, a forest would be in fact way easier to pass through (undergrowth and many tree trunks would have just dissapeared).

The only way into your scenario would be logistics... lets say that, pre-wildfire, travellers could through the forest because they could get water from drinking the juice of the fruits of a tree common to the forest. Better yet, make the fruit (or some other) provide not only food but also liquids (water). Make it that inside the forest there are no other sources of water (streams, wells).

If the affected area is big enough, it will be almost(*) impossible to cross the burnt area because of a lack of water (they cannot carry enough water with them to do the travel).

(*) You would have to find a way to explain why they cannot use a pack animal or a cart to try to carry more water with them.

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  • $\begingroup$ The problem with the big area solution is that puts sharp limitations on what made the trip viable before the fire. The distance would be the same. $\endgroup$ – Taemyr Nov 20 '14 at 11:51
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    $\begingroup$ @Taemyr now we are used that you can travel from point to point on Earth in a day, but before that it was not unusual for travels to take months. The "limitations" was the ability to get food / water and unsafe (bandits) roads. The travel taking longer will just mean that, given that more time needs to be invested, you need a better reward at the end of it for it to make sense. But if someone is interested enough, there is no "limitation" at all (walk, eat and drink, sleep, walk.... and so on). $\endgroup$ – SJuan76 Nov 20 '14 at 12:38
  • $\begingroup$ Yes, we agree, for a big area solution to work the incentive to make the trip has to be something that is out of the ordinary. $\endgroup$ – Taemyr Nov 21 '14 at 8:27
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Perhaps the once mighty trees and their deep, living roots pierced down through the soil, forming a prison for some kind of evil. It is this same evil force the somehow compelled whatever evil faction you may have (or haven't) to initially set the forest ablaze.

With the majority of the trees dead, the evil is no longer contained, thus dooming any would be travelers to a horrible end.

This would certainly spark cooperation as the evil could threaten the humans anyway, and the centaurs are benefiting by getting their home back if the humans are successful.

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The basic fact is that a forest fire would leave a forest more passable, not less.Additionally, after a forest fire in the ancient world, you would not have to work to restore the forest.

Typically forests experienced small forest fires that destroyed dead leaves, underbrush, fallen timber, etc, and left the mature trees standing. These fires were quite common occurrences, both naturally and deliberately man made.

Originally at least in the US forestry services decided all forest fires were bad and tried to put them all out -this leads to a huge pile up of flammable debris that eventually, when a fire gets out of control, burns so high and hot that even mature trees that would normally be safe in a smaller forest fire get destroyed. But this would not have occurred in an ancient pre-industrial society. Nowadays forest fires are fought primarily to protect expensive homes nearby and not because it is needed for the health of the forest. Small forest fires are a normal, natural, healthy, and even necessary part of a forest lifecycle. Lately the importance of forest fires to maintaining forest health is starting to become more realized and we are backing away from our stance of stopping all fires.

If you do want something that would make the area impassable and potentially burn all the trees, look at some sort of volanic event. The lava would burn even mature trees, and the resulting rock, depending on type, could easily be completely impassable to land travel. Look at something like A'a lava as opposed to Pahoehoe lava. There wouldn't be much hope of 'restoring' the land, however, though you could work to build a road through it or something (perhaps to reach some remaining unburned forest on the farther side of the lava flow?

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  • $\begingroup$ That does depend on the climate. There's an oak forest here in Tullamore which I'm fairly confident has not seen a fire for many hundreds of years. Ireland is a slightly damp place, you may have heard. $\endgroup$ – TRiG Nov 23 '14 at 20:42
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If the ancient forest had been large enough, and had enough wood of the right type (hardwood broadleaf), it could have burned down, leaving behind massive piles of ash. If the climate was also very wet or marshy, the water could become so alkaline from the ash (essentially becoming a form of lye) it could then cause severe or even lethal chemical burns. Who knew wood ash could be so dangerous?

As for getting humans and centaurs to work together, if the humans built a very large city near by and burned lots of oil and coal, they could create acid rain storms (sulfuric acid) that could neutralize the alkaline marsh and allow the land to start to heal. Unfortunately this would only be a temporary solution since eventually the pH would balance out and further acid over-correct leading to an acid forest. And humans are not usually interested in changing their lifestyle once it has been established.

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A great deal of the forest was consisting of not only easily flammable material, but the plants also contained a great deal of oily substances that made the fire work like the whole forest was covered in napalm. It burned for so long and so hot that no seeds survived and no water/humidity was left.

Due to its sheer size, the remnants of it are not only currently transforming the land into an ash desert, they also contain lots of toxic materials (heavy metals, asbest like crystals etc.) that after a day or two walking through it you get severe and almost uncurable lung problems. It may not be impassable, but no sane person would even try it...

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The reckless and ignorant use of gunpowder shortly after its discovery started a fire that destroyed the forest. With no trees to block their view, the humans are now able to see a city in the distance that was previously unknown to them. Unfortunately, the forest was a natural barrier to strong winds or an underground miasma that now makes the area impassable until the forest is restored.

I hope you don't mind the incentive substitution, but it was difficult to get past the conflict of interest between the humans needing lumber and that lumber coming from the forest the centaurs call home.

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  • $\begingroup$ Of course, because I tried to answer this at work, it took me too long and now my answer sounds like a mix tape of all the other recent answers. $\endgroup$ – Bad Neighbor Nov 20 '14 at 14:36
  • $\begingroup$ The idea of the miasma could work. It could even be something that is released by the burnt wood as a consequence of burning it, harmless enough when it's a small cooking fire and everyone stays away from the smoke but dangerous otherwise.... ahh just read the other answers and I see what you mean. That's very similar to PlasmaHH's answer. $\endgroup$ – Tim B Nov 20 '14 at 15:15
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Considering that a fire would generally make it easier to pass, could I suggest a large coal fire?

Consider the Centralia mine fire:

The Centralia mine fire is a coal seam fire that has been burning underneath the borough of Centralia, Pennsylvania, United States since at least May 27, 1962. The cause of the fire is suspected to be from a trash burning that hit a coal strip in a cave.

The fire burns in underground coal mines at depths of up to 300 feet over an eight-mile stretch of 3,700 acres. As of 2014, the fire continues to burn. At its current rate, it could burn for over 250 more years.

The blaze has resulted in most of the town being abandoned. Population dwindled from 2,761 in 1980 to only 7 in 2013, and most of the buildings have been leveled.

If the fire were close enough to the surface that many holes and cracks would see fire and dangerous smoke coming out of them. This could create a near-endless fire for hundreds of years that would make it extremely dangerous to pass over the terrain. Additionally, toxins in the smoke would make the air very dangerous to breathe.

Other potential issues:

  • Someone or a group falling into a hole covered by debris/ash
  • Brittle ground giving way under a vehicle pulled by horses, dropping everyone involved into the fire
  • Acid rain!
  • Strong winds blowing toxic smoke towards the village in question
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As others have mentioned, there are some problems here

A forest is generally difficult to pass through. A burnt forest is easy to pass through. Forests are capable of coming back on their own with no input from humans.

As a possible solution to both of these:

There are many plants/trees where seeds lie dormant until after a fire, and then when everything else has been cleared out, they grow up in the free space.

You could easily create some sort of plant where the seeds sprout after fires, that grows very rapidly, will choke out trees and other plants, can quickly form an impassable barrier, possesses spikes with some variety of chemical irritant/poison on them, and that is very hard to kill.

Basically, Lantana.

http://en.wikipedia.org/wiki/Lantana

Caused by fire? Check.

Impassible? Check.

Nature cannot rebuild on its own? Check.

Humans/Centaurs can step in to help the forest return? Check.

Maybe also make it relatively fireproof so that humans can't even use it as kindling.

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Forget the fire. As many have said this leaves the area better to penetrate/cross. The answer of a lava flow might initially seem sane - cutting the trees off at the base - but lava contains so much heat that burning the entire trees is inevitable.
It looks like you cannot make a believable scenario with fire.

What makes a forest impenetrable?

  • A tangle of trunks and branches, and/or:
  • A biological/chemical hazard (that could include predators)

Brian Drummond's answer of a big storm is a good example of the first.

Another cause that meets both bullet points:

In general plants (trees) live in symbiosis with bacteria and fungae in the soil.
One fungus has mutated to a form where it kills the tree roots - that could be any kind of biological/chemical mechanism that blocks transport of nutrients or water.

This fungus is also poisonous to humans.
If it is not poisonous to centaurs, and you assume another powerful capability unique to humans (climbing trees?), you would have a powerful reason for both species to work together.

BTW Given my initial sentence, I suggest you remove the word 'burned' from your question title if the only thing you need is an impenetrable forest.

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The ancient trees are left standing, burnt right through, but still standing.

As people/centaurs step on and break the small roots which anchor these trees in place, they have a tendency to fall over, making travel through the forest incredibly dangerous.

Along with this, these centaurs are unable to find food or clean water in this forest (hey, it's gone!). This forces the centaurs into the closest or best known non-burnt place: Where the humans are.

Finally, after a catastrophe, people have a tendency to help out those in need. (Although this often wears off quickly).

A little more polishing could be done... but it's a start.

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    $\begingroup$ The breaking roots to knock over trees is unrealistic. If the weight of someone walking on it would break it's anchors then the wind shear from a slight breeze would also be enough to knock them over. $\endgroup$ – Myles Nov 20 '14 at 14:45
  • $\begingroup$ perhaps not the best explanation... falling trees (perhaps while camping for the night? treacherous/completely blocked paths?) If this forest is old enough, these things could be huge (either fantasy, or more realistic giant redwoods). I'm just saying that the trees themselves get in the way. $\endgroup$ – Kent Nov 20 '14 at 14:52
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Thanks to Builder_K for the inspiration. Ancient trees reproduce every few of centuries assisted by fire. Roots and bark burns off, seeds are dispersed. Trunks are protected by a fluid/fire resistant layer. When the trunks begin to decompose in the years following it covers the ground with a medium that is not conducive to other species that eventually activates the seeds. This ecology would leave the forest littered with tree trunks, fallen and half fallen for many years following the fire.

Sorry if this is a different direction from what OP is looking for but that is one way to make an impassable burned forest. If you don't want the whole forest completely impassible you could incorporate elvation changes which would alter the dominant ecology, which would create different zones within the burn.

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The forest was very large; it would have taken you perhaps months or years to cross it. You could not carry enough food and water to last you so long, so if you ever wanted to cross it you would need to hunt and gather along the way. Now that the forest is all burnt, there is nothing to hunt or gather, so you cannot cross it.

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Subterranean coal seam fire

Subterranean coal seam fire that fills the area with carbon dioxide and carbon monoxide.

Active volcano

Unpredictable streams of molten rock, sudden explosions of boiling hot water, invisible noxious and/or toxic fumes and gas clouds is no fun.

Natural nuclear reactors

No, I am not kidding, there is / was such a thing.

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