There are just so few places today where you can see vast wildflower fields on the floor of the San Joaquin Valley. The one time when we can see something that looks a bit like these early descriptions is in the spring following a fall burn. It's pretty impressive. Burning the thatch bares the soil surface and exposes seeds to a lot more sun, a lot more of the first rains. If there's a huge biomass of non-native grasses lying on top of their seeds, they don't compete very well.
Fire gives the natives a competitive edge.
Fire is almost an analog of grazing, because grazing opens the thatch layer and makes it possible for annual forbs to compete against exotic grasses. If you look at a little game trail where squirrels or jackrabbits have run across the fields, they trample the thatch down and have, in a way, the same sort of impact as grazing or fire. They create an area where there's not much grass left. Sun and water get to that spot real quickly. So the following spring, those little squirrel trails often blossom with the best native flowers, which are not found in the neighboring thatch layer.