Fire as part of the system
In some ecosystems, fire is as normal as rain or frost. Over long periods, plants growing in places that burn regularly have been shaped by that pattern. Some tolerate fire, some recover quickly after it, and some have come to depend on it to reproduce or to keep competitors in check. The study of these relationships is fire ecology.
This article looks at the ecological role of fire. The causes of ignition, the hazards to people and the prevention side of the story belong to the broader topic of forest fires, and the effects of smoke on air are treated separately.
Fire regimes
Ecologists describe the typical pattern of fire in a place as its fire regime. A regime includes how often fires occur, how intense they are, how large they get and what season they happen in. Broadly, the pattern differs by forest type:
- Frequent, low-intensity fire. Some open pine forests, savannas and grassy woodlands have historically burned often, in many cases every few years to a few decades. Surface fires clear litter and small plants but leave large trees standing.
- Infrequent, high-intensity fire. Some dense conifer forests, especially in cold or dry climates, historically burned much less often, but when they did, fires could kill most of the trees across large areas, a pattern called stand-replacing fire.
- Mixed patterns. Many forests fall in between, with patches of light and severe burning.
Exact intervals vary widely and are debated for many regions, so it is best to treat any single number with caution.
How plants cope with fire
Several strategies recur around the world. A single species can use more than one.
Thick bark and self-pruning. Mature trees of some species have thick, insulating bark that protects the living layer beneath from heat. Some also shed their lower branches as they grow, which keeps flames on the ground from climbing into the crown.
Resprouting. Many shrubs and some trees can regrow from buds on the stem base, roots or underground woody structures after the above-ground parts are burned or killed. This is common in Mediterranean woodlands and many eucalypt forests, where resprouting from the trunk or from a swollen base is a typical feature.
Fire-stimulated seeds. Some plants release or germinate seeds in response to fire cues. Heat can crack hard seed coats, and chemicals in smoke can trigger germination in certain species. These seeds may lie dormant in the soil for years until a fire occurs.
Serotiny. Some conifers keep seeds sealed in cones with a resin that melts under high heat. The cones open after a fire and release seeds onto freshly cleared ground. Serotiny is well known in certain pines and cypresses, and it often occurs in populations that historically experienced stand-replacing fires.
Fast growth in open conditions. Species that need full sunlight can thrive on burned ground. Some also rely on the nutrient pulse from ash, though how much benefit is gained varies by soil.
Fire-resistant structure. Some savanna and grassland trees grow protective bark or store energy underground, allowing them to survive repeated burns while saplings are often cut back to the ground and have to recover between fires.
Fire and forest renewal
Fire also changes the structure of a forest. By killing some trees and opening gaps, it lets light reach the ground, which supports young trees and understory plants. It recycles nutrients tied up in dead material and can reduce competition from shade-tolerant species that would otherwise take over. This is related to the way canopy gaps allow forests to renew themselves, though fire creates gaps through a different process than a falling tree.
Fire also plays a part in forest succession, resetting some stands to an early stage and keeping a landscape as a mosaic of ages and conditions. That mosaic supports a wide range of wildlife. Some birds favor recently burned forests with standing dead trees, which also supply deadwood habitat, while other animals depend on unburned patches nearby.
Species and habitats that rely on periodic fire
Examples commonly cited include:
- Open pine woodlands in parts of North America, where grasses and wildflowers growing beneath the trees need regular burning to avoid being shaded out by woody plants.
- Fire-adapted shrublands and Mediterranean-type ecosystems in several regions, where many plants resprout or have fire-triggered seeds.
- Some boreal forests, where certain conifers regenerate heavily after stand-replacing fire.
- Tropical and subtropical savannas, where grasses and fire-tolerant trees coexist, and where fire helps limit tree cover.
The details vary a lot by region and species, and local ecological references are the best source for any particular place.
What changes when fire is excluded
When fire is removed from an ecosystem that evolved with it, several changes are commonly reported, although the extent depends on the site:
- Shade-tolerant species increase. Without periodic burns, young trees and shrubs that would have been killed can grow into the understory, making stands denser.
- Open habitat shrinks. The grassy or wildflower-rich ground layer may decline as shade increases, which can affect the insects, birds and plants dependent on it.
- Fuel accumulates. More woody debris and denser vegetation can build up, and when a fire eventually occurs it may burn more intensely than the historic pattern.
- Regeneration of fire-dependent species suffers. Seedlings of species that need bare mineral soil or high light may not establish.
- Tree stress can rise. Crowded trees compete for water, so they may be more vulnerable to insect outbreaks and to drought stress.
In some places, managers use prescribed burning to restore fire to such ecosystems. Mechanical thinning is sometimes used first to make burning safer. Neither approach is a perfect substitute for natural fire, and both involve trade-offs including smoke and risk to nearby communities.
Not every forest depends on fire
It is a mistake to assume all forests need fire. Wet temperate and tropical rainforests have rarely burned in the past, and their trees often have thin bark and few adaptations to heat. For these forests, fire, especially fire that follows logging or drought, can be a severe disturbance with slow recovery. The ecological lesson is that the same event can be natural in one forest and destructive in another, which is why local context matters.