FIRE AND THE ANGIOSPERM REVOLUTIONS

Fires have been burning plants since the early Devonian and were common in the Mesozoic, before angiosperms became prominent. Fires were common in the Cretaceous, when angiosperms first spread to cover significant areas of the world. We suggest that this spread was facilitated by intrinsic angiosperm properties that altered fire regimes in two fundamental ways. The first was to promote more frequent fires than the slower-growing gymnosperms that they replaced. The high productivity of angiosperms would have fueled more frequent fires, while reproductive innovations allowed for more rapid recovery, particularly in upland areas too dry for ferns. The evolution of flammable grasses is the latest expression of angiosperm innovation, generating even more frequent fires capable of carving into forests, particularly after the spread of C-4 grasses from; similar to 8 Ma. The second major impact of angiosperms on fire was the development of forests. Closed broad-leaved forests fundamentally changed fire regimes by producing a vegetation structure that intrinsically excludes fires. No conifer analogues for fire-excluding forests seem to exist today or to have existed in the evolutionary past. Both fire-promoting and fire-excluding ecosystems are a product of angiosperm innovations. While these novel ecosystems may have caused the extinction of several ancient lineages, ferns and conifers still persist within them. One conifer lineage, the genus Pinus, has shown remarkable adaptability to highly flammable ecosystems, including frequently burned C-4 grasslands.