Page 660 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

624 part IV Soils, ecosystems, and Biomes
F cus Study 19.1 Natural Hazards Wildfire and Fire Ecology
Fire is one of earth’s significant ecosys- tem processes, and in some areas of the world has become an economic burden. in 2006 and 2007, and again in 2012, over 3.6 million hectares were burned in wild- fires in the United States. in australia, the “Black Saturday” fires in 2009 destroyed more than 2000 homes and claimed 173 lives, the most destructive in the coun- try’s history. The cost of fire suppression in the United States reached almost
$2 billion in 2012. (See the national in- teragency Fire Center at www.nifc.gov/ fireInfo/fireInfo_main.html.)
lightning-caused wildfire is a natu- ral disturbance, with a dynamic role in community succession. Many ecosys- tems have properties that influence the intensity and size of wildfires, which in turn create a mosaic of habitats, ranging from totally burned to partially burned to unburned areas. This patchwork of habitats ultimately benefits biodiversity. Fire affects soils, making them more nutrient-rich in some cases, yet more susceptible to erosion in cases of in- tensely hot fires. Fire also affects plants and animals; in fact, some species are adapted to, even dependent on, fre- quent fire occurrence.
Fire-adapted Ecosystems
a number of earth’s grasslands, forests, and scrublands have evolved through interaction with fire and are known as fire-adapted ecosystems. Plant species
in such environments may have dense bark, which protects them from heat, or lack lower branches, which protects them from ground fires. Fire-adapted species typically resprout quickly after fire destroys their branches or trunks (see Chapter 20, Figure 20.16b).
Several north american tree species depend on fire for reproduction. For ex- ample, lodgepole pine and jack pine rely on fire to crack and open the resin that otherwise seals their cones and keeps seeds from being released (Figure 19.1.1). Seedlings of giant sequoia grow best on open, burned sites, without grasses or other vegetation
began with european forestry practices of the 1800s and carried over into forest management in north america. Since then, however, forestry experts have learned that when fire-prevention strate- gies are rigidly followed, they can lead to a buildup of forest undergrowth that fuels major fires. For example, in yel- lowstone national Park, after decades of fire suppression, forest managers began a new policy of letting natural fires burn. in 1988, after one of the driest summers on record, massive fires burned through the park, destroying buildings and about 1.2 million acres of forest and grassland.
competing for resources. Fire- disturbed areas quickly recover with stimulated seed produc- tion, protein-rich woody growth, and young plants that provide abundant food for animals (Figure 19.1.2).
Fire Management
Modern society’s demand for fire prevention to protect property
▲Figure 19.1.1 Recovery after the 1988 Yellowstone National Park wildfires. Decades of fire suppression in yellowstone created a buildup of undergrowth that fuelled the massive, uncontrollable 1988 fires. Ten years later, young lodgepole pine, a fire-adapted species, grow among the burned stands. [Jim Peaco, nPS.]
environment in a manner that favours a different com- munity. During the transitions between communities, species having an adaptive advantage, such as the abil- ity to produce many seeds or disperse them over great distances, will outcompete other species for space, light, water, and nutrients. Successional processes occur in both terrestrial and aquatic ecosystems.
Terrestrial succession An area of bare rock or a dis- turbed site with no vestige of a former community can be a site for primary succession, the beginning stage of an ecosystem. Primary succession can occur on new surfaces created by mass movement of land, glacial retreat, volcanic eruptions, surface mining, clear-cut logging, or the movement of sand dunes. In terrestrial ecosystems, primary succession begins with
the arrival of organisms that are well adapted for colo- nizing new substrates, forming a pioneer community. For example, a pioneer community of lichens, mosses, and ferns may establish on bare rock (Figure 19.18). These early inhabitants prepare the way for further succession: lichens secrete acids that break down rock, which begins the process of soil formation, which en- hances habitat for other organisms. As new organisms colonize soil surfaces, they bring nutrients that further change the habitat, eventually leading to the growth of grasses, shrubs, and trees.
More commonly encountered in nature is secondary succession, which occurs when some aspect of a previ- ously functioning community is still present; for exam- ple, a disturbed area where the underlying soil remains intact. As secondary succession begins, new plants and