A forest is best thought of as a constant dynamic balance. Old trees are brought down by windstorms, creating open spaces of partial sunlight where young seedlings can take root. Small, shade-tolerant seedlings grow into adult, sun-loving tree canopies. Leaf and branch litter falls on the forest floor and gradually decays, restoring nitrogen and other nutrients to the topsoil and encouraging the growth of low brush and new seeds.
In temperate climates with winter and summer seasons, decay happens more slowly than accumulation of new organic matter. When the litter and low brush accumulate to the point that seeds cannot reach the soil and overhead leaf canopies start to block off the sun completely, new growth cannot take root, and the animals dependent on that new growth for their food will starve. Without intervention, such a forest could never renew itself.
Lightning-sparked forest fires provide that intervention. Forest fires can be thought of as filling the role of a large-scale predator of the forests, culling the weakest to make room for strong new growth. Fires caused by lightning quickly sweep bare the choking brush and leaf litter as well as removing the most vulnerable trees. The animals of the forest flee or hide underground, but the weakest among them will also be killed by the fire. The most choked-off forests also have the tallest trees, which are also the most likely to be hit by lightning.
After the fire has gone through the forest, the brush and predator insects and the weakest of the trees will be gone, leaving behind large clear spaces filled with light, nutrient-rich ash perfect for new germination. The ash also raises the pH of the soil, which may have become too acidic through years of coniferous growth and exposure to naturally acidic rain. First to regrow will be the reserve roots of ash trees and smaller, fast-growing trees such as aspen and pine. Next will be maples and spruces, and finally, slowly, the oaks. This enables the forest to preserve its ecodiversity.
Some tree species require fire to germinate. The cones of the jack pine and other serotinous species require the intense heat of a forest fire in order to open and release their seeds.
Human activity often interrupts the natural cycle of forest fires. When the natural cycle of fires is interrupted by any means, any later fires will be much more destructive. The major factors in such interruption are attempted protection of property in fire-prone areas and diversion of water for irrigation and other human needs. As well, the deliberate reduction in biodiversity after replanting logged forests creates monocultures much more vulnerable to future disease and fire and much less capable of renewing themselves.
Forests where fires have been suppressed for a long time will have acquired so much deadwood and debris that the resulting fire will be much hotter and much more damaging than usual, possibly to the point of disrupting the forest’s own mechanisms for post-fire renewal. In drought-prone areas where natural water sources are diverted for human needs, the trees which also depend on that water will become stressed and more vulnerable both to disease and to fire. This in turn will increase the intensity of any fire, and the lack of water will make it that much more difficult to do anything other than burn itself out. With the increased intensity, it becomes much more likely that the fire will also destroy the root systems holding the fragile topsoil in place. On flat spaces, the resulting open spaces become vulnerable to wind erosion. On slopes, it creates the potential for damaging mudslides.
Fire does not have the same beneficial role in all forest ecosystems. In the rain forest decay of forest litter happens so quickly that the debris never has a chance to accumulate. Any nutrients from fallen leaves and other litter are taken up by growing plants almost as soon as they are released by bacteria, sometimes without even reaching the soil. This is why rain forests have very thin, nutrient-poor soil despite all the organic material, and why slash-and-burn farming destroys such forests forever. Fire is not the friend of the rain forest.