Wildfire

When an ecosystem is disrupted by a wildfire, it can quickly suffer an intensive mortality of its dominant species, along with disruptions of its physical ecological structure and other damages. However, except in the case of rare, extremely intense fires, some plants survive the disturbance, and these can contribute to the post-fire regeneration that immediately begins.

Plant species vary greatly in the strategies they have evolved to survive wildfire, and to regenerate afterwards. Often, the below-ground tissues of certain plants can survive the fire even though the above-ground biomass was killed by combustion or scorching, and the regeneration may then occur through stump or root-sprouting. In North America, trembling aspen (Populus tremuloides), other woody plants, and many understorey herbs commonly display this sort of survival and regeneration strategy.

Other plants may survive the fire as long-lived seeds that are buried in the forest floor, and are stimulated to germinate by post-fire environmental conditions. Species such as pin cherry (Prunus pensylvanica) and red raspberry (Rubus strigosus) can regenerate vigorously from this so-called buried seedbank. A few conifers maintain their seedbank in persistent, aerial cones, which are stimulated to open by the heat of the burn, so that seeds are released to the fire-prepared seedbed immediately afterwards. These fire-adapted tree species often form even-aged stands after fire, as is the case of knobcone pine (Pinus attenuata) in the southwestern United States, and jack pine (Pinus banksiana) farther to the north.

In other cases, species may invade the burned site, by dispersing from unburned communities nearby. Species with light, windblown seeds are especially efficient at colonizing burned sites, as is the case for white birch (Betula papyrifera), fireweed (Epilobium angustifolium), and various species in the aster family, such as goldenrod (e.g., Solidago rugosa).

The post-fire ecological recovery that is manifest in the regenerating vegetation is a type of secondary succession. In the absence of another wildfire, or some other catastrophic disturbance of the stand, the post-fire secondary succession often restores an ecosystem similar to the one present prior to the fire.

If the return frequency of natural wildfire is shorter than the time required for a climax ecosystem to develop, then disturbance by fire may be important in maintaining the land in an earlier successional stage. For example, most of the region of North America that supported a tall-grass prairie was climatically suitable for the development of an oak (Quercus spp.) dominated forest. It was only the periodic burning of the prairie that prevented the encroachment of shrubs and trees, and maintained the natural prairie. Today, tall-grass prairie is an endangered ecosystem, because most of its original area has been converted to agricultural purposes. To maintain the ecological integrity of the few, small remnants of tall-grass prairie that remain in protected areas, these areas must be deliberately burned to prevent them from successionally turning into forest.

Even stands of the giant sequoia (Sequoiadendron giganteum) appear to require periodic ground fires of a particular intensity if that community type is to be sustained over the longer term. Fire helps to maintain stands of this species, by reducing fuel loads and thereby preventing catastrophic crown fires that could kill mature trees, and by optimizing recruitment of sequoia seedlings.