Geoscience Reference
In-Depth Information
Fig. 5.11. in the mid-1800s,
summer travelers along the
oregon trail would dig for
ice preserved in the peat of
this fen, known locally as ice
Slough. Photo taken along
Highway 287, 40 miles east of
Lander. elevation 6,500 feet.
Photo by Ken Driese.
along the oregon trail were happy to find ice preserved
in the insulating peat there. core samples show that ice
Slough is underlain by 5-10 feet of peat.
Fens sometimes merge with marshes as water depth
increases. in some places the fen community extends
over the deeper water, with the peat forming a floating
mat. eventually, the peat may cover the entire pond or
pothole, with the only evidence of a former pond being
the undulation of the surface when walked on, or the
surprise of one's foot slipping into the water below.
Deciding where the fen ends and a shallow marsh or
wet meadow begins, or even if a wetland contains a fen
at all, can be a challenge. Peat thickness can be used,
but fens at high elevations commonly have thinner peat
than those at low elevations. 32
Wetland scientists customarily identify different
kinds of fens based on acidity and concentrations of
calcium, magnesium, and other minerals in the water.
At the acidic, infertile end of the gradient are poor fens,
similar to bogs. With decreasing acidity and increasing
mineral concentrations, fens are labeled inter mediate,
rich, and extremely rich. in general, the number of plant
species increases with nutrient availability. Most fens
in the Rocky Mountains lie on granite or other rela-
tively nutrient-poor rocks and are intermediate. 33 the
most nutrient-rich fen identified in Wyoming is at
Swamp Lake, in the clark's Fork valley east of Yellow-
stone national Park, adjacent to Wyoming Highway 296
(see fig. 5.10). 34 this large wetland complex lies on gra-
nitic bedrock and glacial deposits, but it benefits from
waterborne nutrients coming from the calcium- and
magnesium-rich limestone and dolomite of the nearby
cathedral cliffs to the south. 35
Fens share many plant species with shallow marshes
and wet meadows (see table 5.1). in the wetter parts
of fens, northwest territory sedge and water sedge are
ubiquitous across a broad elevation range, along with
other sedges. Some unusual graminoids grow in many
fens, such as cottongrass and grass-of-Parnassus. com-
mon forbs are elephanthead lousewort, redpod stone-
crop, and white marsh marigold. Floating peat mats
usually are composed of analogue sedge, mud sedge,
and slender sedge. Along the edge can be found shrubs,
such as diamondleaf willow, mountain willow, Wolf's
willow, small-leaved laurel, and resin birch.
Sometimes mosses are as conspicuous as vascular
plants. 36 Growing on the well-aerated surface of the
peat, they respond readily to differences in surface
water chemistry across small distances. in contrast,
vascular plants rooted in the peat are influenced more
strongly by differences in climate, topography, and
elevation, along with differences in water depth, water
flow, and peat thickness. 37 Different groups of plants are
associated with floating mats, areas of standing water,
 
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