VI.6 Grasslands
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VI.6 Grasslands Martha Downs and Osvaldo E. Sala OUTLINE 1. Scope of grasslands 2. Provisioning ecosystem services 3. Regulating services 4. Cultural services 5. Supporting services 6. The significance of grasslands This section focuses on the ecosystem services provided by natural grasslands. These regions of the world are mostly limited by water availability, and they exclude anthropogenic grasslands, which derived from forests that were logged and converted into pastures, often to support cattle grazing. Grasslands account for 41% of Earth’s land surface , and 38% of Earth’s 6.8 billion people live in natural grasslands. Grasslands support a diversity of uses, but until recently they have been primarily used for grazing and wood gathering for fuel, with conversion to agriculture at the wet end of their climatic envelope. Alternative uses of these regions—e.g., recreation, conservation, and carbon sequestration—are gaining in societal value, particularly in developed countries. This chapter used the definition of ecosystem services presented above and the categorization developed by the Millennium Ecosystem Assessment with four types of ecosystem services: provisioning services, regulating, cultural, and supporting services. GLOSSARY albedo. Energy reflected from the land or water surface . Generally, white or light-colored surfaces have high albedo, and dark-colored or rough surfaces have low albedo. carbon sequestration. The process of removing carbon dioxide from the atmospheric pool and making it less accessible or inaccessible to carbon-cycling processes. grasslands. Short-stature vegetation dominated by grasses, characteristic of locations with a strong water limitation for at least part of the year. petagram. One trillion million (1018 ) grams. soil texture. Soil texture is described by the proportions of sand (large particles), silt (intermediatesized particles), and clay (smallest particles). Sandy, loose-textured soils allow rapid water infiltration and fast leaching of nutrients. Denser, clayey soils have poor drainage and poor soil aeration. transpiration. The evaporation of water from the leaves, stems, and flowers of plants. Transpiration occurs through small pores, or stomata, on leaf and stem surfaces, which must remain open to take up carbon dioxide. 1. SCOPE OF GRASSLANDS Grasslands occur where there is not enough water to support forests, although temperature also plays a role: cool locations can support forests at precipitation levels that can only support grasslands in warmer climates . For instance, rainfall in the United States generally increases from west to east, and temperature increases from north to south. The grassland–forest boundary thus runs in a diagonal fashion from southeast to northwest, reflecting the issue that lower temperatures characteristic of the north allow forest to grow at lower precipitation than in the south. Grasslands are dominated in general by herbaceous vegetation, mainly grasses and forbs, although shrubs account for an important fraction of grassland biomass in some regions, and grasslands can also support occasional trees. The proportion of shrubs and grasses depends on the texture of the soil and the seasonality of precipitation. Grasslands encompass different vegetation types with different shrub abundance from prairies to steppes. 2. PROVISIONING ECOSYSTEM SERVICES Grasslands, through their support of grazing, produce meat, milk, and blood for many people who depend on animals for their daily protein intake. Grasslands in developed countries are primarily managed by cowcalf producers where calves are sold to fattening operations that feed grain to calves until they become mature and ready for slaughter. In contrast, in developing nations most of the meat production occurs in grasslands themselves. The type of animal used varies enormously depending on cultural and climatic conditions and ranges from goats to camels. Especially in developing countries, residents often harvest shrubs and trees from grasslands for fuel, and the resource can provide a large percentage of household energy use in dryland regions. Overuse of this service can increase soil erosion, decrease recruitment of new plants, and degrade the ecosystem. Grasslands support a wide variety of grazing animals , including sheep, goats, llamas, alpacas, vicuñas, and even muskoxen, which can sustainably produce fiber for household use and sale. The trade-offs between grazing density and grassland sustainability vary with the productivity of the system, which depends largely on soil type and precipitation. Most grassland plant species evolved under some grazing pressure, and a moderate population of grazers can help replicate past conditions and sustain or even increase plant species richness. Increasing grazing intensity to higher levels reduces plant cover, exposing bare ground and increasing soil erosion. The majority of the human diet derives originally from grassland species. Annual grasses and legumes are most abundant in...