Fire regime

• 1. Introduction

  ›

  Fire regime is the term scientists use to describe the pattern, frequency, and intensity of bushfires and wildfires that prevail in a given area over long stretches of time. At its core, it is a portrait of fire as a recurring force in the landscape rather than a one-off disaster. That framing opens up a surprising set of questions: what happens to a forest that burns too often? What happens to one that almost never burns at all? And what is at stake when the rhythm of fire that a whole ecosystem evolved around is disrupted?

  The answers turn out to be deeply consequential. If fires come too frequently, plants are killed before they have matured or set enough seed for their population to recover. If fires come too rarely, those same plants may grow old, weaken, and die without ever releasing their seed at all. Fire, in other words, is not simply destruction. For many landscapes, it is a necessary pulse.

  Understanding why that pulse matters, what shapes it, and what threatens it is the core mission of fire ecology. The fire regime sits at the heart of that inquiry, and the story of how researchers study, map, and model it reveals just how far-reaching a concept it truly is.

• 2. What Shapes a Fire Regime

  ›

  Vegetation composition, fuel structure, climate, weather, and topography all combine to define the particular fire regime of a place. Because no two landscapes share the same combination of these factors, there is no single standard classification that fits every fire regime on earth. Scientists instead assess a range of characteristics and stitch together a picture suited to a specific ecosystem.

  Fire severity describes the impact a fire has on the ecosystem, which may include the degree of vegetative mortality, how deep the burn reaches into the soil, or other site-specific outcomes. Fire interval counts the years between successive fires and varies significantly depending on the spatial scale being examined. Fire rotation captures a broader picture: the amount of time required for fire to burn across an area equal in size to the study area itself. That statistic is most useful when applied to large landscapes where historic fire events have been mapped.

  Fire type adds another layer of distinction. Ground fires use glowing combustion to consume organic matter in the soil. Surface fires burn leaf litter, fallen branches, and low-growing plants. Crown fires push through to the top layer of tree foliage. Fire-line intensity measures the energy released per unit of measurement per unit of time and typically describes flaming combustion. Seasonality pins down which part of the year the fuels of a specific ecosystem can actually ignite.

  Animals are a less obvious factor, but they influence fire regimes by modifying the amount, structure, or condition of fuel. Human ignition and lightning-caused ignition represent two distinct pathways for fire to start, and both feed into the ignition component of a fire regime's character.

• 3. Mapping the Burn

  ›

  The Canadian National Fire Database, the first nationwide database of its kind, holds records of large fire events since 1980. It includes point locations of all fires larger than 200 hectares from 1959 through 1999. Across the border, the United States operates the Monitoring Trends in Burn Severity Project, which uses satellite data to map fires from 1984 onward. That system provides a standard on fire perimeters and severity for all fires within the country.

  A separate American system called the Landscape Fire and Resource Management Planning Tools classification, known as LANDFIRE, goes further. It collects and analyzes vegetative, fire, and fuel characteristics of fire regimes across a variety of landscapes. What makes LANDFIRE distinctive is that it draws on both historic and current fire regimes to track how conditions have shifted between past and present.

  For events further back in time, satellite data is of no use. Scientists turn instead to fire scar analysis on trees, the age distribution of forest stands, charcoal samples buried in the soil, and vegetation changes that accumulated over centuries. These methods can extend the view of fire history by thousands of years rather than just decades. Studies using them have found strong correlations between past climate and both the frequency and extent of fire.

• 4. A Planet Sorted by Fire

  ›

  Hardesty and colleagues undertook a sweeping classification of the earth's ecoregions and major terrestrial habitat types, sorting them into three broad categories based on their relationship with fire. Some are fire-dependent or fire-influenced, some are fire-sensitive, and some are fire-independent. The researchers assessed a subset of the Global 200 ecoregions identified by the WWF as priorities for biodiversity conservation.

  Their findings placed 46 percent of those ecoregions in the fire-dependent or fire-influenced category, 36 percent in the fire-sensitive category, and 18 percent in the fire-independent category. The most striking result was how many ecoregions are at risk: 84 percent were found to be experiencing altered fire regimes, whether from too much fire, too little fire, or fire arriving in the wrong season or at the wrong intensity. Only 16 percent of the ecoregion land area assessed had fire regimes consistent with ecosystem health.

  Tropical moist broadleaf forests emerged as the most threatened habitat type. Composed mainly of plants and animals that generally lack adaptations to major fires, these ecosystems had 93 percent of their assessed land area experiencing altered fire regimes. Among fire-dependent ecoregions, 77 percent of the land area had already seen their fire regimes change. The breadth of that disruption puts the scale of the problem in sharp relief.

• 5. When Fire Regimes Break Down

  ›

  Biota that survive and adapt to their particular fire regime can benefit substantially from fire: the ability to regrow stronger, greater protection against disease, and new growing space in formerly occupied locations. When the regime shifts, those benefits erode and the costs mount.

  Shorter fire intervals hit fire-killed species especially hard. If fires return before a species has had time to recover to pre-disturbance levels, the window for population recovery keeps closing. Obligate seeders face the sharpest risk: if fires come again before the plants have produced enough seeds, entire populations may fail to regenerate. Resprouters, which can regrow directly from roots or stems, are better positioned to survive shortened intervals. But even they have limits.

  Lengthened intervals cut in the opposite direction. Fire-adapted species that depend on fire for reproduction are set back when burns become rare. In plant communities where stand-replacing crown fires are part of the natural cycle, recruitment into the population occurs in the first year following a fire event. Take that fire away, and the recruitment window never opens.

  Australia's Banksia species illustrates how tightly a plant can be tuned to a specific regime. Banksia is both fire-sensitive and serotinous, meaning fire triggers the release of its seeds, ensuring population recovery. An ideal fire regime for Banksia gives the plant enough time to mature and build an adequately large seed bank before the next fire kills it and triggers that release. Disruption of that timing in either direction can threaten the species.

• 6. Climate Change and the Shifting Pulse

  ›

  Climate change has already affected fire regimes globally. Models project higher fire frequencies and reduced plant growth as warmer and drier climates take hold. Fire-intolerant woody species face particularly severe consequences: reduced recruitment, slower growth, and higher mortality are predicted to shorten fire intervals within their landscapes to the point of driving local extirpation or extinction.

  Warmer climates are also projected to lengthen fire seasons globally and increase the annual number of extreme fire weather days. Rising temperatures, reduced relative humidity, higher wind speeds, and heavier dry fuel loads are expected to push fire intensities and severity upward. Shorter fire intervals will reduce the time plants have to accumulate seeds, and may select for more flammable species over time.

  Evidence from the field supports these projections. A study in southeast Australia found that prolonged wildfire seasons burned 87 percent of the range of mountain ash. Subsequent re-burns of immature mountain ash stands led to complete regeneration failure, with forest cover giving way to shrubs and grasslands. Similar patterns have appeared in the Mediterranean forests of western North American chaparral regions.

  The feedback is self-reinforcing. A decrease in precipitation drives an increase in drought-prone years, which reduces seed recruitment probability. That reduced recruitment is then made worse by increased fire severity. A model examining these interactions predicts that woody plant extinctions will increase, altering ecosystem structure, composition, and carbon storage.

• 7. Invasive Species and Altered Rhythms

  ›

  Bromus tectorum, commonly called cheatgrass, offers one of the clearest illustrations of what an invasive plant can do to a fire regime. In the Snake River Plain sagebrush ecosystem, historical fire return intervals ran between 60 and 110 years. With cheatgrass now established as a continuous source of fuel, that landscape burns roughly every 5 years. The result is a cycle that makes it difficult to impossible for native vegetation to fully recover.

  The Brazilian pepper tree, Schinus terebinthifolia, native to Brazil, Argentina, and Paraguay, presents a contrasting case. Introduced as an ornamental species, it has established populations in Australia, South Africa, the Mediterranean, southern Asia, and the southeastern United States. In South Florida near the Everglades National Park, its spread has been severe enough that some studies reported only 7 species within a series of 100 square meter plots.

  Brazilian pepper creates a sub-canopy layer that outcompetes grasses and ground cover, changing the density and structure of the landscape. Historically, the areas it invades experienced frequent, low-severity fires. By creating a shaded, humid understory and reducing fine fuel loads, Brazilian pepper actually increases the fire-return interval in those areas, cutting off the fire-adapted plant community from the burns it depends on.