Geoscience Reference
In-Depth Information
and soils, biogeography, and human activities.
These processes and factors often interact with
each other leading to positive or negative feed-
back relationships on local, regional, and global
scales.
Hydroseral succession refers to gradual tran-
sitions involving both water and vegetation in
the life cycle of a wetland under relatively stable
conditions. This is the most common type of
autogenic change brought about by vegetation
succession, soil development, accumulation of
sediment, and other processes operating within
a wetland. Two means for initiating the develop-
ment of peatland are terrestrialization and
paludii cation. Once started, bogs grow upward
and outward through distinct stages of develop-
ment, which lead after several millennia to
mature mire complexes.
Many allogenic factors may inl uence wetland
development. Among the more important are
changes in sea level, crustal movements, climatic
variations, i re, and human activities. During the
past million years, the Earth has experienced
multiple cycles of glaciation and interglaciation
in which sea level varied by
climatic system, namely solar energy, green-
house gases, albedo, and oceanic circulation.
Fresh-water and marine wetlands are key parts
in the global carbon cycle both as carbon sinks
and sources of greenhouse gases. In a general
way, warmer and drier climate would reduce
fresh-water wetlands and release greenhouse
gases, while cooler, wetter climate would favor
wetland expansion and carbon storage. However,
neither climate nor wetlands behave in such
simplistic ways. In fact, the connections between
climate and wetlands may become quite complex.
Fire is an integral part of many ecosystems
and may be used benei cially for management
of some wetland habitats. In other situations, i re
is quite destructive. The potential climatic impact
of i re is mixed. Release of carbon dioxide from
burning forest and peat enhances the green-
house effect, whereas soot and ash in the atmos-
phere increase the albedo. Furthermore, removal
of forested wetland cover by i re leads to an
increase in albedo with a possible cooling effect.
Over the long term, the roles of i re, wetlands,
and climate change are poorly understood and
may have distinct regional variations.
When multiple autogenic and allogenic pro-
cesses interact, there may be unexpected effects
for individual wetlands as well as globally. Given
that even the most basic climatic inl uences,
such as solar energy and volcanic eruptions, are
impossible to predict, forecasting the impact of
climatic change on wetlands is a particularly
daunting task that requires a great deal more
interdisciplinary scientii c investigation. What
is abundantly clear, however, is that human
impacts on wetlands have accelerated the release
of greenhouse gases and conversions of land
cover that increase albedo. The climatic conse-
quences of these contradictory anthropogenic
factors remain to be seen.
100 m. Major
changes in coastal wetlands resulted as the
shoreline migrated. Areas of direct glaciation
were depressed hundreds of meters by ice
sheets, then rebounded when the glaciers disap-
peared. Immediately surrounding ice sheets, the
opposite happened. The combined effects of
changing sea level and crustal movements may
have complicated effects in many regions, with
direct impacts on coastal wetlands. Current
projections are for rising sea level during the
twenty-i rst century with major consequences
for coastal wetlands and human populations.
Water supply and wetland vegetation are con-
trolled to a large extent by climate. Several
factors play primary roles within the Earth's
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