Riparian Landscapes - Mountains and Plains: The Ecology of Wyoming Landscapes - page 54

Fig. 4.9. this row of aging plains
cottonwood trees is not likely to be
replaced as the trees die. the riparian
meadow in this area, along Walker
creek in converse county, has Baltic
rush and various sedges. cattails grow
in standing water, and introduced
plants (such as timothy and smooth
brome) are found nearby. Photo by Ken
Driese.

creek, near cheyenne, P. G. cooke wrote in 1845, “for
forty miles we have seen but one tree—five miles off—
and not a bush or shrub.” 10 the near absence of trees in
some areas could have been caused by various factors,
including frequent prairie fires, large mammal brows-
ing, insufficient flooding for tree seedlings to become
established on point bars, or stable channels dominated
by grasses and sedges that reduced the chance of tree
seedling establishment (as discussed below). it seems
unlikely that the lack of seed dispersal is the explana-
tion, as many riparian species, such as cottonwoods,
have seeds that are both buoyant and easily spread by
water and wind.
Riparian meadows on the eastern plains, with no or
only a few trees and shrubs, are commonly dominated
by nebraska sedge, Baltic rush, prairie cordgrass, redtop
bentgrass, and plains silver sagebrush, plus introduced
plants, such as Kentucky bluegrass, smooth brome,
sweetclover, canada thistle, hound's tongue, and tim-
othy. in areas where salts accumulate in the soil, such as
on terraces that are flooded less frequently, greasewood,
inland saltgrass, and alkali sacaton are common. Flood-
plain marshes with standing water commonly have
dense growths of cattail (see chapter 5).
trees have invaded some meadows and shrublands,
possibly because dams and reservoirs reduced the fre-
quency of tree-damaging floods.11 11 Also, return flows
from irrigated land may have favored the trees by pro-

viding water late in the summer. However, if ground-
water drops to a depth where the large trees cannot
obtain the water they need, woodlands give way to
meadows and shrublands. extended droughts can lead
to the death of cottonwoods, just as shifting channels
and altered irrigation practices can.
the riparian mosaic at the confluence of the Big-
horn and Shoshone rivers in northern Wyoming illus-
trates landscape patterns that can be found elsewhere
(fig. 4.11; see also fig. 4.8). Fertile soils and water avail-
ability for irrigation, combined with a relatively long
growing season, sustain one of the most important agri-
cultural regions in the state. Much of the native ripar-
ian vegetation has been lost, but remnants persist in
the Bighorn canyon national Recreation Area. Plains
cottonwood woodland is the most conspicuous vegeta-
tion type along the rivers. Associated species include
peachleaf willow, silver buffaloberry, and the intro-
duced Russian olive. Various shrubs, forbs, and grasses
are found as well (see table 4.1). Shrub and herbaceous
plant cover is low where tree density is high because
of inadequate light beneath the woodland canopy and
competition from trees for the available water.
interspersed in the woodlands at low elevations are
shrublands dominated by basin big sagebrush, plains
silver sagebrush, greasewood, rubber rabbitbrush,
sandbar willow, skunkbush sumac, western snowberry,
Woods' rose, chokecherry, silver buffaloberry, saltcedar,