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Any change in fire regime, whether it be increased or decreased
frequency and intensity, has pronounced effects on a forest, from the
level of individual trees to the entire ecosystem and biome. In the
instance of the Northern Hemisphere boreal forests, a drastic change
in fire regime may have global effects on climate, owing to the forest's role in
regulating the global carbon cycle. This web page provides a brief overview of the impacts of a changing boreal forest fire regime on population and ecosystem-scale biotic processes. |
Different plant species respond to disturbances in characteristic ways. Some species have adapted in the absence of frequent fire--these species have no mechanisms with which to cope with fire and fire damage, and will likely not survive the transition into a shorter fire cycle (more frequent fires) (6). Other species have evolved in areas of boreal forest that experience frequent fire, and therefore possess mechanisms to allow the species to persist through these disturbances. One functional type of tree is able to resprout almost immediately after a fire has passed through, taking advantage of nutrients that were released from the soil and are made biologically available (6). Other tree species store their seeds in the canopy, where they are safe from ground fires, but not necessarily from crown fires (6). Clearly, these two types of trees each possess an advantage in one type of fire regime over another (if a fire burns hot enough to get through the soil to the depth of a tree's roots, however, mortality is all but guaranteed). In general any plant type that possesses a mechanism to recolonize an area immediately after a forest fire will have a pronounced effect on the cycling and storage of carbon in a boreal forest ecosystem.
As fires become significantly more frequent and widespread in boreal forests, an increasing portion of old growth, late successional tree populations with no fire adaptations will be eradicated (2, 6). As soon as these individuals are gone, they will be replaced by the rapidly growing, resprouting and reseeding tree species that will take advantage of the open canopy and plentiful nutrients. With stand-clearing fires becoming more and more common, this will happen on a larger and larger scale; invading species will establish widespread populations that quickly reach reproductive maturity (6). Within a short period of time after the fire, the preexisting forest type will be replaced by new species comprising a larger living biomass component--one that is adapted to surviving in frequent-fire habitats. The larger biomass pool creates relatively more fuel for the next fire that comes through, which will consequently be able to burn hotter and spread further (6). This is the natural processes of succession and disturbance, except that the system is never allowed to reach the final, mature stages, and instead is continually reset by the positive biomass-fuel feedback instigated by the opportunist colonizing species. Carbon in boreal forests will be cycled more rapidly under warming climate and increasing fire regimes; a greater portion of the organic carbon pool will be oxidized through combustion and respiration and added to the atmospheric carbon dioxide pool. Climate change has potential to play an additional role in this process. As more and more forest area is opened up by fire and other disturbances, species migrating northward may also have the opportunity to colonize the newly formed gaps. Depending on their life cycles and reproductive strategies, these invasive plant species may integrate themselves into the preexisting boreal forest system. This may result in species assemblages that have never before existed in the boreal zone (2). Several additional interactions and feedback processes will connect climate change, fire, and boreal forests in the future. Large crown fires temporarily remove large areas of vegetation, exposing bare soils to incoming solar radiation. Soils are lighter in color than the coniferous vegetation; therefore, the albedo of portions of the boreal zone will decrease, resulting in more reflection and a decrease in radiative heat. The decrease in absorption of solar radiation will cause a drop in evapotranspiration, which will, in turn, affect the energy balance of northern high latitudes (4). Forest fires may also interact in space and time with other disturbances similarly affected by climate changes, such as insect and pathogen infestations and storm activity. Although there is large uncertainty associated with many of these processes, the possibility of synergistic disturbance interactions exists (4). For a final example, the release of tropospheric aerosols, including black organic carbon, as combustion products of forest fires into the atmosphere has the potential to affect the amount of incoming solar radiation reaching the Earth's surface (8).
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Introduction | Fire in Boreal Forests | Climate Change | Climate Change & Fire | Ecosystem Effects
Future of Boreal Forests | Works Cited | Biology Dept Home | Duke University Home
