ELi5 How does forest or grassland catch fire, does it always need a spark or can drying out cause spontaneous combustion?
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No, just the foliage or wood getting dry does not cause spontaneous combustion. The ignition point of dry leaves or wood is something like 300°C or higher. So you need a source of heat to reach that temperature.
“Spontaneous” combustion of organic material *can* happen due to fermentation. The fermentation reaction then provides the heat source. But, somewhat paradoxically, this actually *requires* moisture, since the microbes that do the fermenting need water to survive. Also, the only scenario that I’m aware of where this actually works is in hay piles. Hay has an incredibly low ignition point, around 55°C, which is a feasible temperature to reach with fermentation, under the right circumstances (compacted hay that is not too wet to ignite, but moist enough to support fermentation). This is why, to prevent fires, it is generally recommended not to bale and store hay until it is completely dry.
As far as I know, the conditions required for this type of fermentation-induced combustion are unlikely to happen “in the wild”, though. As I said, the organic material found in forests has a much higher ignition point. You’re never going to get up to 300°C through fermentation, as the microbes will be long dead at that point. Hay of course is basically dried grass, but you do need to store it quite compactly in order to get enough concentrated heat. A field of dry grass, if it ferments at all, will be too spread out for enough heat to build up.
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