Secondary succession, which occurs more rapidly than primary succession, may take place if soil is already present and there are surviving species in the vicinity. In fact, survivors strongly condition subsequent succession. Many secondary successions follow human disturbances (e.g., land that was cleared when timber was harvested or land converted to farmland). Other secondary successions follow fires. Grasses and other herbaceous plants become established on burned or logged land. These usually are followed by trees and shrubs that have widely dispersed seeds (e.g., aspen and sumac in the Midwest and East, and chaparral plants, such as chamise and gooseberries, in the West). After going through fewer stages than are typical of primary succession, the climax vegetation takes over, often within 100 years.
Natural fires, started primarily by lightning and the activities of prehistoric humans, have occurred for thousands of years in North America and other continents. Trees, such as the giant redwoods and ponderosa and lodgepole pines, although scarred by certain types of fires, often survive; the dates of fires can be determined by the proximity of the scars to specific annual rings (see Fig. 6.7). In the West, growth rings of ponderosa pines show that in the past, forests of such pines used to burn on an average of every 6 to 7 years. These fires and the climate, topography, and soil combined to have a profound effect on various biomes.
As humans cultures developed, major and largely successful efforts to control fires were made, and this, in turn, significantly altered vegetational patterns. As knowledge of the role of fires in the maintenance of ecosystems has accumulated, it has become apparent that trying to eliminate fires, at least in certain areas, disrupts natural habitats more in the long run than allowing them to occur, and agencies such as the U.S. National Park Service may allow fires at higher elevations to run their natural course.
Fires also play a role in the composition of forests. In the mountains of east-central California, gooseberry and deerbrush appear in abundance after a fire, but their numbers stabilize within 15 to 30 years when larger trees return to the area. Ponderosa, jack and southern longleaf pines, and Douglas firs (which do not tolerate shade) are among the species that repeatedly replace themselves after fires; seeds of some species rarely germinate until they have been exposed to fire. The majority of chaparral species, both woody and herbaceous, are so adapted to fire that their seeds also will not germinate, as a rule, until fires remove accumulated litter and toxic wastes produced by the plants during growth.
Fires actually benefit grasslands, chaparral, and forests by converting accumulated dead organic material to mineral-rich ash, whose nutrients are recycled within the ecosystem. If the soil has been subjected to fire, some of its nutrients and organic matter may have been lost, and the composition of microorganisms originally present is likely to have been altered. Losses are offset, however, by the fact that soil bacteria, including cyanobacteria, which are capable of fixing nitrogen from the air, increase in numbers after a fire, and there is a decrease in fungi that cause plant diseases.
In some areas, such as the prairie states of the Midwest, grasses are better adapted to fire than woody plants, producing seeds within a year or two after germination. Perennial grass buds that are at the tips of rhizomes close to or beneath the surface, where they escape the most intense heat of fires, usually survive, producing new growth the first season after a fire. Accordingly, a fire destroys only one season's growth of grass, often after growth has been completed. Shrubs, however, have much of their living tissue above ground, and a fire may destroy several years' growth. In addition, woody plants often
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do not produce seeds until several years after a seed germinates. Many shrubs do sprout from burned stumps, particularly in chaparral areas (Fig. 25.13), but repeated burning keeps them small, thereby favoring grasses. There is evidence that at least some of the North American grasslands originated and were maintained because of fire. Since grassland fires have largely been controlled, many of the areas have now been invaded by shrubs that were once confined to watercourses.
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