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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Spontaneous combustion is a little odd. It requires a moist core of organic material. Bacterial activity creates heat, but will disapate unless the heat is contained. Bacterial will not occur without moisture.. The outer organic material acts as an insulator containing the heat.. A similar event happens with rags soaked in organic oils (vegetable oil, linseed oil and the like) and is insulated when they are piled up or put in a trash can.
As humidity goes down and organic material dries out the ease of ignition increases. Autoignition temperatures are the ambient temperatures for a material to start burning without a spark. Gasoline has an autoignition temperature of 536° F. However it will catch at -45° F with a flame or spark.
A few other factors are, how thick or thin the wildland material is. I.e. it takes a lot of energy to catch a block of wood on fire. It does not take much energy to catch a piece of paper on fire. The substance of the organic material such as turpines in pine trees become more flammable as they get dryer.
A haystack, mulch pile or chip pile have started wildfires. Its rare, but it has happened. Those are all man-made and are what we see starting fires from spontaneous combustion. Its an uncommon energy source. Other energy sources are much more common, arson, lightning, accidental events like catalytic converters, camp fires, broken glass (acting like a magnifying glass) gender reveals and the like.
In other words, low moisture levels , easily ignited material and an energy source in contact with the material. Occasionally the energy source will be a pile of organic material spontaneously combusting, but rarely.
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