Marshes, Playa Wetlands, Wet Meadows, and Fens - Mountains and Plains: The Ecology of Wyoming Landscapes - page 77

Fig. 5.10. Fens are common south of
Mammoth Hot Springs in Yellowstone
national Park. northwest territory
sedge and water sedge are abundant on
the usually wet organic soils; diamond-
leaf willow, Wolf's willow, resin birch,
and shrubby cinquefoil are found along
the margin; wet meadows and lodgepole
pine occur on mineral soils that are
somewhat drier.

of their widely recognized ecological and hydrological
values. thus, considerable controversy can arise about
whether a tract of land is in fact a wet meadow or a
grassland or shrubland. Such disputes are settled based
on the abundance of obligate hydrophytes, the pres-
ence of hydric soils, and an understanding of the site's
hyd rolog y.

by precipitation that has not passed through the soil or
rock and contains only the nutrients that settle on the
wetland in dust, rain, or snow. 28 Both kinds of peatland
store far more water and nutrients than an equal volume
of mineral soil, though many of the nutrients are not
available to plants because they are chemically bound
in organic material. Mosses do well in this environment,
probably because the reliable supply of surface water
minimizes water stress late in the summer and they are
able to obtain the small amount of nutrients required
by such small plants. Peat in fens is composed mostly
of slowly decomposing sedges and mosses. the true peat
moss—sphagnum—is found in some of them. 29
Fens are most common in glaciated mountain land-
scapes, such as where potholes have formed and in val-
leys. they also occur on slopes where a reliable source of
groundwater comes to the surface in springs and seeps,
sometimes developing under the canopy of trees. 30
Most mountain fens are small, between 1 and 10 acres.
Fens in the alpine tundra can be associated with palsas,
which are vegetated domes caused by frost heaving. 31
Fens also occur in sagebrush steppe in the Green
River Basin and on the Sweetwater Plateau in west-
central Wyoming. the latter site, known as ice Slough
(see fig. 5.11), is the subject of a historical marker west
of Jeffrey city. it became famous because immigrants

Fens

Fens are rich in graminoids and forbs, but shrubs are
more common—especially willows and a shrub known
as resin birch (figs. 5.10 and 5.11). the peat substrate is
saturated all year. Many people think of fens as bogs,
but wetland ecologists have determined that all peat-
lands in the Rocky Mountains are fens. the distinction
is based on nutrient availability and acidity. Bogs have
low nutrient availability and are highly acidic (pH <
5). in contrast, fens are fed by groundwater containing
relatively high nutrient content and are either basic or
mildly acidic (pH 5-8). Some of the plants are different
as well.
the differences between bogs and fens are caused
by their sources of water: fens are supported by waters
that have moved through the soil or underlying rock,
from which nutrients are dissolved; bogs are supported