Fire had previously acted as a disturbance agent, stirring up the ecosystem and setting the stage for change. As the ecosystem shifts, conditions may favor new species. Perhaps a severe fire destroys a leafy canopy, reducing a lush forest to a brush field. And the returning trees may or may not be the same species that burned. Fire effects are very diverse, and this diversity promotes biological diversity.
Many trees depend on fire for their survival.
Some types of conifer have developed cones that need the heat from fires to release their seeds. In areas of Tahoe where pine dominates, frequent, light surface fires previously killed most of the seedlings and saplings in the forest understory. Those that survived grew large and old without the pressure of competition from neighboring trees. Without fire checking their encroachment, new species, such as the shade-tolerant firs, are free to invade the pines' territory. The composition of the forest has changed significantly since the 1800s.