Further research shows this, from C.A. Browne, The Spontaneous Heating and Ignition of Hay and Other Agricultural Products, Science, 3 March 1933: Vol. 77. no. 1992, pp. 223 – 229, DOI: 10.1126/science.77.1992.223
In 1929 the author suggested as a possible solution of the problem of spontaneous ignition the formation, by micro-organisms under anaerobic conditions, of unsaturated unstable intermediary compounds which, in the sudden exposure of the interior of the fermenting mass of hay to the air, absorb atmospheric oxygen with so much avidity that the temperature is rapidly raised not only above the death point of the micro-organisms but even to the point of ignition of the hay. This spontaneous ignition may take place almost instantly, as James, Bidwell and McKinney, of the Department of Agriculture, have observed in the case of heating horse manure, or it may take place more slowly, according to the rapidity with which the outside air gains access to the hot pocket in the interior of the hay. (emphasis added)
Maybe Old Lady Leary’s cow didn’t kick over the lantern, after all, but merely…
Which raises the interesting question of how common such ignitions might be, whether they could occur under natural conditions and how much they migtht increase with rising temperatures. Were any known to have occurred in south Texas this summer?
From EurekAlert:
Plants’ response to fire tested
“A team from the National Institute for Agricultural and Food Research and Technology has developed a new method for identifying the flammability of plant species by using a device that measures how construction materials react to fire. The technique, which is being presented this week at the Fifth Spanish Forestry Congress, can be used to improve fire risk maps.”
No mention of spontaneous combustion, but the potential in brush piles seems apparent.
Woody plants adapted to past climate change more slowly than herbs
“Can we predict which species will be most vulnerable to climate change by studying how they responded in the past? A new study of flowering plants provides a clue. An analysis of more than 5,000 species reveals that woody plants adapted to past climate change more slowly than herbaceous plants did. If the past is any indicator of the future, woody plants may have a harder time than other plants keeping pace with global warming.”
I suspect vulnerability to loss via burning is a big factor in this.