A fire regime is the pattern, frequency, and intensity of the bushfires and wildfires that prevail in an area over long periods of time. It is an integral part of fire ecology, and renewal for certain types of ecosystems. If fires are too frequent, plants may be killed before they have matured, or before they have set sufficient seed to ensure population recovery. If fires are too infrequent, plants may mature, senesce, and die without ever releasing their seed. Fire is a type of disturbance regime that can define an ecosystem. Disturbance regimes like fire can change soil erosion, soil formation, nutrient cycles, energy flow, and other ecosystem characteristics. Disruption of an ecosystem can allow changes in species dominance and mutations in individual species. Fire regimes can change with the spatial and temporal variations in topography, climate, and fuel.
Fire regimes are characterized based on their frequency, intensity, extent, type, and seasonality. Fire frequency is the average time a fire burns in a given area. Fire intensity is the amount of heat released over time. Fire extent is the size and spatial similarities of the burning. Fire types or fire spread include ground fire, surface fire, and crown fire. Ground fires use glowing combustion to burn organic matter in the soil. Surface fires burn leaf litter, fallen branches, and ground plants. Crown fires burn through to the top layer of tree foliage. Seasonality is the period of time during the year that the fuels of a specific ecosystem can ignite.
Effects of Altered Fire Regime
Biota that are able to survive and adapt to their particular fire regimes can receive significant benefits: the ability to regrow stronger, greater protection against fire and disease, or new space to grow in formerly occupied locations. As fire regimes change the area, both current and future species may begin to suffer. Decreasing fire intervals negatively affect the ability of fire-killed species to recover to pre-disturbance levels, leading to longer recovery times. Some species, such as resprouters, are better able to withstand changing fire regimes through increased resistance and resilience. However, many fire-killed species may be unable to recover if shortened fire intervals persist over time.
Examples of Fire Regime
Bushfire is especially important in Australia, where much of the vegetation has evolved in the presence of regular fires caused by the Aboriginal practice of firestick farming. As result, components of the vegetation are adapted to and dependent upon a particular fire regime. Disruption of that fire regime can affect their survival. An example of fire regime dependent species is the Banksia species which is both fire-sensitive and serotinous. In Banksia species, fire also triggers the release of seed, ensuring population recovery. In an ideal fire regime, a plant would need to have sufficient time to mature and build an adequately large bank of seed before the next fire kills it and triggers seed release.
The California chaparral and woodlands ecoregion, covering a large portion of the U.S. state, is dependent on periodic natural wildfires for optimal health and renewal. A study showed that the increasing rural-urban fringe interface and wildfire suppression practices of the last century have resulted in an increased vulnerability to less frequent, more severe wildfires. The study claimed fire suppression increased fuel in coniferous forests.  Upon analysis of California Statewide Fire History Database from 1910-1999, it was actually found that fire frequency and the area burned have not declined, furthermore, fire size has not increased. Chaparral fire suppression, unlike fire suppression in coniferous forests, has not affected the natural fire regime, according to a study conducted by the United States Geological Survey. Furthermore, prescribed burning in the shrubland area was also proven ineffective at reducing the risk of wildfires, which are normally driven by high winds and unaffected by modern fire suppression.
Invasive Species Effect on Fire Regime
The presence of invasive species can cause changes in the fuel properties of an ecosystem resulting in changes in the fire regime characteristics. This can change the fire regime properties to make it more difficult for native plant species to recover. The loss of native species of plants can effect an entire ecosystem of animals as well. One example of an invasive species that changed fire regime in Western North America is Bromus tectorum, also known as cheat grass. Idaho's Snake River Plains at one time burned it's sagebrush every 60–110 years, but now due to the presence of cheat grass it now burns every 5 years. The cheat grass is a continuous source of fuel thus changing the fuel characteristics of the ecosystem. These frequent fire make it difficult to impossible for native vegetation to fully recover.
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