220.127.116.11. Changes in Temperature
Temperature is an important factor controlling many ecological and physical
functions of wetlands. Primary productivity and microbial activity are both
controlled to a certain extent by temperature conditions. Temperature also affects
evapotranspiration rates and has an impact on the water regime. Because higher
temperatures and drying of the surface soil usually occur together and interactively
affect the ecosystem processes, it is not always possible to separate their
Although Gorham (1991) suggests that the effects of temperature will be overshadowed
by the impacts of water-level drawdown on northern peatlands, the impacts of
temperature increases on wetlands on permafrost may be drastic (Gorham, 1994a).
A fairly small increase in temperature might initiate large-scale melting of
permafrost, with thermokarst erosion and changed hydrological regimes as a consequence
(Billings, 1987). Work by Vitt et al. (1994) and Halsey et al. (1997) has demonstrated
clearly the dynamic association between the distribution of peatlands, peatland
types, and the presence or absence of permafrost in North America. This association
is strong enough that it has been used as a proxy method for inferring climatic
variability during the Holocene (Halsey et al., 1995). This might imply shifts
from black spruce/Sphagnum/lichen communities on permafrost to wetter fen communities,
with subsequent changes in carbon cycling.
18.104.22.168. Land-Use Change
Land-use change may create multiple pressures on wetland habitats. Area estimates
of the scale of direct development of tropical peatlands vary and provide only
an imprecise picture of the current situation (Immirzi et al., 1992; Maltby
and Immirzi, 1996). In southeast Asia, agriculture and forestry are the major
peatland land uses. Toward the end of the 1980s, it was estimated that in Indonesia
alone 3.7 Mha (18% of the total peat swamp forest) had undergone some form of
development (Silvius et al., 1987).
Cultivation of tropical peatlands involves measures that radically change the
hydrological regime and consequently influence vegetation and soil processes.
Forests are cleared and effective drainage installed. In many places in southeast
Asia, cultivation of horticultural and estate crops has met with mixed success,
and some previously converted peatlands have been abandoned, although peat-forming
vegetation has failed to reestablish (Immirzi et al., 1992). Reasons for failure
include poor water management and persistent infertility of the soil (Rijksen
et al., 1997).
The total area of tropical peatland drained or otherwise altered during forestry
management is not known. Silvius et al. (1987) suggest that as much as 0.11
million km2 of peatlands in Indonesia (i.e., as much as 50% of the
total resource) are possibly being exploited for forestry purposes. In Malaysia,
most of the remaining peat swamp forest outside limited conservation areas has
been logged (Immirzi et al., 1992). Sustainable-yield forestry is likely to
be the most appropriate form of land use for peat swamp forest, but such methods
applicable to peat swamps have yet to be developed, let alone implemented (Immirzi
et al., 1992).
Use of peatlands for forestry usually brings about smaller changes in the ecosystem.
In floodplain swamps and peatlands of the more continental areas of North America,
often only tree stands are managed (Dahl and Zoltai, 1997), but in northwestern
Europe and the southeastern United States, forestry use includes artificial
drainage (Richardson and McCarthy, 1994; Päivänen, 1997). Some 0.15
million km2 have been drained for forestry, mostly in Scandinavia
and Russia (Päivänen, 1997). In these cases, much of the original
vegetation (Laine et al., 1995) and functions (Aust and Lea, 1991; Minkkinen
et al., 1999) are preserved during forestry management. About 70% of the expansive
peatlands in North Carolina (6,000 km2) have been entirely or partially degraded
through draining, ditching, or clearing (Richardson and Gibbons, 1993).
Peat harvesting for energy or horticultural use has the most drastic impact
on the ecosystem; vegetation is removed with the topsoil prior to harvesting,
and most of the accumulated peat gradually is extracted (Nyrönen, 1996).