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Nonforest terrestrial ecosystems are the single largest type of land surface cover (>51%) in North America. They are extremely diverse and include nontidal wetlands (bogs, fens, swamps, and marshes), ecosystems of the polar domain (tundra and taiga), traditional rangeland ecosystems (grasslands, deserts, and savannas), and improved pastures. These ecosystems are major components of every region of North America; they constitute about 80% of the land cover of western North America and nearly 100% of the land cover above the 75th parallel. They provide forage for 80 million cattle, sheep, and goats and 25 million deer, elk, antelope, caribou, and buffalo, as well as most of the breeding and feeding grounds for waterfowl in North America (Child and Frasier, 1992; WRI, 1996). Nonforest ecosystems are the source of most surface flow and aquifer recharge in the western Great Plains and the extreme northern regions of North America. Municipal, agricultural, and industrial sectors in these regions depend on nonforest ecosystems for the quantity and quality of water required for economic sustainability. The quality and quantity of water derived from nonforest ecosystems depend on the management these lands receive and the average annual and extreme climatic events they encounter.
Nontidal wetlands in North America include a variety of ecosystems such as bogs, fens, swamps, marshes, and floodplains. Classification systems are many and varied. These wetlands are distributed throughout North America, principally in a band extending from the New England states to Alaska. There are additional significant areas in the Mississippi Valley, the "Prairie Pothole" region, the many coastal wetlands (e.g., the Mississippi River delta, the Everglades, and the Okefenokee), the Atlantic coastal marshes and Fraser River estuary, the former Great Kankakee and Great Blackwater swamps, the Hudson Bay Lowlands, the Peace-Athabasca-Slave delta, the Mackenzie delta, and the Queen Maud Gulf on the Arctic Ocean (Mitsch and Gosselink, 1986; Ecological Stratification Working Group, 1995; IPCC 1996, WG II, Chapter 6). See Section 8.3.3 for a more detailed discussion of wetlands impacts.
Rangelands are characterized by native and introduced vegetation-predominantly grasses, grasslike plants, forbs, shrubs, and scattered trees. These lands are extremely varied: They include the tallgrass, mixed, and shortgrass prairie regions of central North America; tundra and taiga areas in the polar domain; annual grasslands of California; chaparral regions of Arizona and California; sagebrush shrub steppe and pinyon-juniper woodlands in the intermountain region of western North America; and the Chihuahuan, Sonoran, and Mojave deserts in the southWestern portion of North America. The associated ecosystems are complex and are affected by many interacting biotic and abiotic components, and their health depends on the interaction of climate, soils, species competition, fire, grazing, and management. These ecosystems provide a wide array of goods and services, including forage, water, and habitat for wildlife and domesticated livestock and open space for recreational activities, and they are the source of many of the raw materials needed to sustain our industrial society (i.e., pharmaceuticals, precious metals, minerals, construction materials, natural gas, oil, and coal) (Heady and Child, 1994).
Although some rangelands are fragile and easily disturbed by anthropogenic activity (Belnap, 1995), others are resistant to change. Semi-arid and arid ecosystems are considered among the most sensitive because these ecosystems often are water-limited and have marginal nutrient reserves (OIES, 1991; IPCC 1996, WG II, Chapter 2).
Current levels of uncertainties associated with the functioning and adaptive capacity of nonforest ecosystems under variable and changing climate and the possibility of critical thresholds limit our ability to identify the relative sensitivities of these ecosystems (and the potential impacts of changing climates). It is understood, however, that these ecosystems are sensitive to climate variability and that the impacts can vary depending on the resilience and resistance of the ecosystem to the stresses applied (e.g., changes in precipitation, CO2, temperature, fire, land use, and land cover and management). Researchers also believe that the impacts of CO2 enrichment and shifts in temperature and precipitation regimes are likely to be greatest when they are reinforced by other destabilizing forces. Lack of information about how these other factors interact with climate change also limits our understanding of ecosystem response. Also of concern are the relative sensitivities of species at the ecotones between vegetation types, such as between grasslands and woodlands and between woodlands and forests (Polley, 1997).
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