Global estimates for the area of wetlands vary according to the definition of wetland used. Spiers (1999) reports global estimates for natural freshwater wetlands of 5.7 million km2. The estimate by Matthews and Fung (1987) is 5.3 million km2, which represents approximately 4% of the Earth's land surface, although a more recent estimate by Lappalainen (1996) is somewhat larger, at 6.4 million km2. This compares well with Finlayson and Davidson's (1999) estimate of about 7 million km2, including 1.3 million km2 of rice paddy.
Maltby and Proctor (1996) estimate that peatlands cover about 4 million km2 (±4%), constituting about 75% of wetlands. More than 90% of peatlands are in temperate, boreal, and subarctic regions. The total area of tropical peatlands is estimated to be 0.37-0.46 million km2 (i.e., approximately 10 % of the global resource), but the full extent is uncertain (Immirzi et al., 1992).
Many wetlands have irregular wetting and drying cycles, driven by climate. To date, little attention has been given to the impact of climate change on these less regular cycles of wetlands in semi-arid and arid regions (Sahagian and Melack, 1998). Changes in the area of these wetlands can be immense but could be monitored by using area-based parametersfor example, functional parameters and wetland extent expressed in terms of ha-days (Sahagian and Melack, 1998).
Species that form wetland plant communities are adapted to varying degrees to life in a flooded environment. These phenomena show large spatial variability, and different species show varying degrees of susceptibility to them, so it is not surprising that wetland vegetation exhibits such a high degree of variation in species composition (Crawford, 1983). Peatland plant communities have been observed to change over long periods of time, reflecting the peat accumulation process and leading to gradually drier conditions. This inherent changeability of wetland communities results largely from their occurrence in environments where a single extremely variable habitat factorwater supplyis predominant (Tallis, 1983). Consequently, land use and climate change impacts on these ecosystems can be expected to be mediated through changes in the hydrological regime.
Primary production in wetland communities is highly variable (Bradbury and Grace, 1983; Lugo et al., 1988). Generally, wetland communitieswhich are dominated by trees, sedges, and grasseshave higher production rates than those characterized by shrubs and mosses. Organic matter produced in many wetlands is accumulated partially (2-16%Päivänen and Vasander, 1994) as peat. A necessary antecedent condition for peat formation and accumulation is an excess of water stored on the mineral soil or sediment surface. This arises in humid climatic regions where precipitation exceeds evaporation (Ivanova, 1981; Clymo, 1984) or in more arid climatic regions where lateral inputs of water via surface runoff and/or groundwater seepage are sufficient to exceed evaporative demand (Glaser et al., 1996).
Tropical peatlands play an important role in maintenance of biological diversity by providing a habitat for many tree, mammal, bird, fish, and reptile species (Prentice and Parish, 1992; Rieley and Ahmad-Shah, 1996; Page et al., 1997); some of these species may be endemic or endangered. In common with other peatlands, tropical systems have a significant scientific value that goes beyond their plant and animal communities.
Within the peat also lies a repository of paleoenvironmental and paleogeochemical information that is extremely important in understanding past climatic conditions. These paleorecords are used to estimate rates of peat formation or degradation, former vegetation, climatic conditions, and depositional environments (Morley, 1981; Cecil et al., 1993; Moore and Shearer, 1997).
Other reports in this collection