Geoscience Reference
In-Depth Information
The opposite kind of feedback is demon-
strated by glaciers and snow cover. During a
period of global climatic cooling, glaciers, pack
ice, perennial snow, and ice sheets expand in
coverage, leading to an increase in the Earth's
albedo, greater rel ection of incoming solar
energy, and a cooler climate. In this case, further
climatic cooling favors still more snow and
ice coverage and still more global cooling. This
positive feedback relationship may lead to a
prolonged ice age, which has happened several
times in the Earth's past (Hambrey and Harland
1981; Caputo and Crowell 1985; McCay et al.
2006). Of course, the Earth's climatic system is
much more complicated than just clouds and ice
sheets
-
there are many other processes in oper-
ation that contribute to the net result. Wetlands
are important components of this environmental
system, as they both store carbon and are
sources for greenhouse gases (carbon dioxide
and methane).
in regions of former glaciation, such as Scan-
dinavia, northern Russia, the northern United
States, and much of Canada. Lakes and estu-
aries may be formed in many other settings
by ground-water solution, volcanism, mean-
dering rivers, sea-level change and crustal
movements.
• Paludii cation - happens when ground water
rises to the surface and creates anaerobic soil
conditions. A change in climate, for example,
could lead to increased ground-water
recharge and reduced evapotranspiration
with the net result that the water table rises
toward the surface. Changes in surface drain-
age, caused by crustal movements, stream
captures, beavers, soil development or human
activities, may increase ground-water levels.
Once the soil becomes saturated, wetland
conditions are established and may lead
eventually to peat accumulation.
Both pathways initiated bogs in Scandinavia.
Many bogs developed from lakes left by the
retreating ice sheet at the close of the last glacia-
tion, some 14,000 to 10,000 years ago (Fig. 8-1).
Other bogs appeared when crustal rebound
took place as the ice load was removed; regions
below sea level rose into the terrestrial and
fresh-water environment. Ini lling by lacustrine
sediment led to fen conditions, which set the
stage for further hydroseral succession into bogs
The general sequence of individual bog devel-
opment took place in i ve stages (Fig. 8-2;
Masing 1997):
8.2 Hydroseral succession
Hydroseral succession refers to the gradual tran-
sition involving both water and vegetation
beginning with open-water habitat and ending
with a raised bog or forest cover. The concept
was developed by Clements (1916) and pro-
moted by Tansley (1939). The general sequence
starts with open water of a shallow lake, pond
or estuary and progresses to aquatic vegetation,
emergent vegetation and terrestrial fen, and
culminates with a raised bog or dry woodland
habitat. This theme has many variations and
diversity of development. According to Charman
(2002, p. 145), “hydroseral succession is by far
the most important and commonly encountered
example of autogenic change in peatlands.” Two
main pathways may initiate the development of
peatlands:
• Stage 1 - Centrally thinned wooded bog or
open fen, surrounded by a bog pine forest
(Fig. 8-3). Lake sediment and sedge peat
have ini lled the lower part of the basin to
the level where the upper surface is cut off
from mineral soil or surface runoff from the
surrounding terrain. At this point,
Sphag-
num
moss may colonize the surface.
• Stage 2 - Centrally open raised bog with
beginnings of bog hollows (Fig. 8-4).
Sphag-
num
peat accumulation has raised the bog
surface above the surrounding moat and
created a nutrient-poor, acidic environment
in which few trees could survive. Moss
• Terrestrialization - begins with pre-existing
lakes or other shallow water bodies that
were created by various geomorphic pro-
cesses. The fate of all lakes is to ini ll with
sediment and eventually be transformed into
wetlands. This scenario is especially common
