The major culprit in almost all of the big lower elevation fires in the interior West is cheatgrass, which has a nicer name of downy brome (Bromus tectorum), probably given for the way it is during the brief period is it growing, green, and not ripening.
Cheatgrass has taken over the West, greatly increasing fire danger, damage, and harming wildlife habitat, causing stream erosion, etc. Global warming or not, there will be more and more fires as cheat grass spreads because that is a major way it wipes out its native competitors — fire.
It is an annual grass, and it is the first grass to sprout with late summer rains. It usually gets a good start before winter and continues in the early spring. If the autumn is dry, it sprouts in early spring. It is very shallow rooted, sucking the up moisture and nitrogen that native seeds need to sprout as well as shading them out. Cheatgrass is a tremendous producer of seeds, far exceeding native bunchgrasses.
Once it starts to move in, almost always as the result of some disturbance that creates bare ground, it is almost inevitable that it will convert most of the area into dry brown grass for most of the year, replacing semi-arid country shrubs and native grasses with a dreary monoculture. This will happen slowly if there is no fire, but with a fire it is almost inevitable and the annual probability of a fire increases with the proportion that is cheatgrass.
I took this photo of a tall perennial bunchgrass (Great Basin wild rye) and cheatgrass near Pocatello, Idaho in early July. To the casual observer it might seem the wild rye is doing great (it’s tall and green even though it was been half eaten by cows) and the cheatgrass dry and dead, but the wild rye is doomed even without a fire. That is a terrible thing because wild rye has very deep roots to seek out water in mid-summer, and these roots serve to hold the soil in place. Cheatgrass is only slightly better than a slope of bare soil.
I can’t tell if the cheatgrass was grazed before it ripened or not because it goes to seed even if cattle or sheep are put in to eat it during its brief “downy phase.” It can’t be eaten when it is ripe like in the photo because the seeds are very hard and sharp (you’ve probably gotten it in your socks — itch!).
Perennial bunch grasses can’t stand heavy grazing. If the rest of that green wild rye is eaten by a cow, it will be so weak when it sprouts next year that it will probably be overrun by the cheatgrass (no fire necessary). The wild rye’s seeds don’t have much of a chance sprouting in the cheatgrass, and cows have already chomped off all but one seed stalk on this bunch anyway.
Cheatgrass begins to invade an area most often by dirt roads or vehicle tracks, usually in conjunction with cattle grazing. Heavy grazing leaves bare spots, and the first seeds to sprout on these bare spots will be cheat grass once an infestation is nearby. The cattle also spread the cheatgrass seeds and trample them in, for good measure.
Overgrazed range (sagebrush steppe) with all the native grass eaten. Bare ground, the perfect spot for cheatgrass to sprout and then soon burn, destroying the sagebrush which has already been weakened by cattle trampling. This photo in the Owyhee country (BLM) of SW Idaho was taken this year by Katie Fite. A fair portion of it that has been invaded by cheatgrass is currently on fire.
More information. Cheatgrass: the Invader the won the West.