Fire
Fire Return Interval (FRI) (in years) in different ecosystems, globally estimated from remotely sensed burned area. Grassy systems burn very frequently. Currently it is not possible to resolve FRIs of longer than 50 years, but charcoal analysis and tree rings tell us that boreal forests have FRIs as long as 200 or 300 years
Source: Archibald, S. et al., 2013.
Changes in fire frequency and timing can transform vegetation
Fire is a natural part of all ecosystems.
Wildfires have been burning vegetation and shaping landscapes far longer than people have been on Earth. However, changes in patterns of fire can result in degradation if the vegetation is not adapted to the new fire regimes. In this context it is possible to have both too much, and too little, fire in a landscape.
Patterns of fire vary across the globe: boreal forests burn infrequently (every few hundred years) but when they burn, fires are intense – consuming entire forests with flame heights of 20 m and more. Grassland ecosystems may burn every few years, but these fires are less intense, and seldom damage mature trees.
Organisms are adapted to the particular fire regime that they evolved under, but patterns of fire can quickly change with changing climates, human activities (ignition/suppression) and land cover. Two key factors that people change with fire is the frequency (or “return period”), and the season or time of year that fires occur. As fire frequency and season change, they can also affect fire intensity, with important implications for plants, animals, and people living in these regions, potentially causing long-term damage to land biomass components affecting soil structure, nutrients and water cycling.
Five major global “pyromes” or fire regimes, can be defined in terms of their size, frequency and intensity. Frequent Intense Large (FIL) and Frequent Cool Small (FCS) fires occur largely in grassy systems – those systems with more people tend to have smaller cooler fires. Rare Intense Large (RIL) fires are associated with crown fire regimes where an entire forest canopy can burn with very high intensity and the forest takes time to regenerate. Rare Cool Small (RCS) fires occur in wet/cool parts of the world where conditions are not often flammable. Intermediate Cool Small (ICS) fires are also in wet parts of the world, but strongly associated with people, who increase fire frequency. Switches from one pyrome to another are often associated with degradation of the ecosystems because organisms are adapted to particular fire regimes.
Source: Archibald, S. et al., 2013
South America has two neighbouring ecosystems – tropical forests and savanna (Cerrado) – with very different historical fire regimes. Degradation takes two forms here: too much fire in tropical forests and too little fire in the Cerrado which complicates the development of effective fire-management policies. Attempts to ban fire in order to protect tropical forest are having devastating impacts on the indigenous Cerrado vegetation, which evolved with fire to maintain its function and diversity.
In the Cerrado, reduced fire frequencies have caused encroachment of weedy forest species and the elimination of a range of endemic grassland species, which have additional impacts on water yields.
By contrast, tropical forest trees are very sensitive to fire, and when fires penetrate these forests (due to drought or runaway crop fires) it can destroy the canopy trees. Logging opens up canopies, dries the understory and makes the leaf litter more flammable. When this litter ignites, fires can be very destructive: they are usually small fires moving slowly through the understory, but because the tropical forest trees with thin bark are not adapted to fire, they are often killed. This opens up more gaps in the canopy and a fire cycle begins that can result in the degradation of tropical forest.
Centro de Monitoramento Ambiental e Manejo do Fogo, Universidade Federal do Tocantins.
In many ecosystems, invasion by non-native species can have big impacts on fire regimes and result in rapid degradation of native vegetation. Often this takes the form of alien grasses invading arid shrublands and increasing fire frequency and severity: e.g. Buffelgrass (Cenchrus ciliaris) in Australia16 and cheat grass (Bromus tectorum) in the USA. Invasions of flammable shrubs and pines can also increase fire intensity by increasing available fuels – as in the Fynbos region of the south-western South Africa.
Source: Cifor image.
In the western USA, humans suppressed natural fires for many years. Hence, when fires eventually occurred, they were much more intense and damaging than the vegetation or people were adapted to. More regular fires, which are less intense, result in less degradation of these systems.
Source: Credit Kari Greer, National Interagency Fire Center
In Mediterranean shrublands high rates of human ignitions mean that fires can occur more frequently than their natural cycles – reducing the return time from ~30 to ~10 years in some instances. Some species in these systems require a fire to germinate (“reseeders”) but they also need enough time between fires to complete their lifecycles and set seed. Thus, although they need fire to produce offspring, if return periods are too short some species (such as Pinus halipensis) may be eradicated from the system.
Source: Prof Juli Pausas.
Peatlands accumulate carbon over thousands of years, but there is a recent global trend of agricultural expansion resulting in the drainage of peatlands. For instance, in South-East Asia land transformation for palm oil plantations drains the water table and dries out ancient peatlands. This makes them more flammable and when they ignite they can burn for months, releasing large quantities of carbon to the atmosphere and affecting human health in the region. This is one of the most damaging changes in fire regimes globally today.
Source: Aulia Erlangga/CIFOR. Flickr.com
