5.5. Rangelands (Grasslands, Savannas, and Deserts)
Rangelands here are taken to include deserts (cold, hot, and tundra), grasslands
(unimproved), scrub, chaparral, and savannas (after, e.g., Allen-Diaz, 1996).
This section does not consider improved grasslands or croplands in detail because
they are covered in Section 5.3. It does partly cover
tundra because it is an important grazing system; that ecosystem is discussed
in more detail in Section 5.9.
Ecosystems within rangelands are characterized by low-stature vegetation because
of temperature and moisture restrictions; they are found on every continent
(Allen-Diaz, 1996). They are adapted to great variations in temperature and
rainfall on an annual and interannual basis, but they generally are confined
to areas that have about one unit of precipitation to every 16 units of evapotranspiration
(the ratio that is characteristic of drylands; Noble and Gitay, 1996). They
often are referred to as "pulse systems" (Noble and Gitay, 1996).
In many countries, human activities in rangelands have evolved in response
to variable and often unpredictable climate. Human practices include pastoralism;
subsistence farming (Allen-Diaz, 1996); and, more recently, commercial ranching
(Canziani and Diaz, 1998). Rangelands also are important for many national economies
in terms of foreign cash (e.g., through tourism). They are important stores
of biodiversity, including ancestors of many of the cereals (World Bank, 1995),
and have high levels of endemism (Barnard et al., 1998). Rangelands are used
primarily for grazing and hence livestock production (Squires and Sidahmed,
1997). Thus, they are important for food (mostly livestock, but also wild fruits),
fuelwood, wood poles for construction, and feed (Campbell et al., 1997). Other
key services are biodiversity, water cycle, and carbon stores. Some of these
products and services can be given economic valuation; however, only a small
component of the total economic value is represented by products that can be
given a market value (Campbell et al., 1997), which suggests that nonmarket
values are quite important for some rangelands.
Rangelands are adapted to grazing and other disturbances, such as fire, flood,
and insect herbivory (Allen-Diaz, 1996). Vegetation tends to be sparse and thus
is not considered worth mechanical harvesting; however, the sparse grass/herbaceous
cover is efficiently harvested by grazers (Williams, 1986). In many cases, episodic
fires (Bock et al., 1995) are important for providing new and lush growth for
grazers, and fire sometimes is used to manage grass-woody shrub balance (Noble
et al., 1996), which is important for livestock and meat and wool production
(see, e.g., Chapter 12). Human management can be critical
to the status of these systems with or without climate change. Previous IPCC
reports concluded that fluctuating rainfall and temperatures along with increased
human activity (especially in more tropical systems) has led to land degradation
and eventually desertification in many areas (Bullock and Le Houérou,
1996; Gitay and Noble, 1996; Canziani and Diaz, 1998).
In this section, food/fiber, biodiversity, and carbon stores are examined in
detail (see Table 5-1). Water as a resource is considered
elsewhere in the report, at the global level (Chapter 4) and in many of the
regional chapters (Chapters 10-12).