Vegetation Classification and Mapping Project Report - USGS

Appendix C: Global and local plant community descriptions
C.3 Shrubland
C.3.4. Atriplex canescens / Pleuraphis jamesii Shrubland
Base map class
NatureServe common name
NatureServe code
Four-wing.Saltbush./.Galleta.Shrubland
Fourwing.Saltbush./.James’.Galleta.Shrubland
CEGL001288
Summary:
This broadly defined shrubland association has been reported from the Colorado Plateau and Uinta
Basin and may occur in the Chihuahuan Desert and eastern California. As defined, this association
occurs on two distinct types of landforms: alluvial flats and stream terraces with fine-textured, alkaline,
or saline silty clay loam soils, or on upland flats covered by eolian sand deposits. The vegetation
is characterized by a sparse to moderately dense shrub layer (10-40% cover) dominated by Atriplex
canescens with Pleuraphis jamesii dominating the herbaceous layer. Associated shrubs include Ericameria
nauseosa, Ephedra torreyana, Chrysothamnus viscidiflorus, Krascheninnikovia lanata, Gutierrezia
sarothrae, Artemisia bigelovii, or Opuntia polyacantha, depending on topographic position and
substrate. Other graminoids include Achnatherum hymenoides and Sporobolus cryptandrus on sandy
sites, and Bouteloua gracilis and Sporobolus airoides on fine-textured soil. Forbs generally have low
cover and may include Sphaeralcea grossulariifolia and Chenopodium spp. Introduced or weedy species,
such as Bromus tectorum, Descurainia pinnata, and Salsola kali, are common on some sites.
Classification confidence: 2 - Moderate.
Classification comments:
Globally. As this association is currently defined, it is characterized only by the codominance
of Atriplex canescens and Pleuraphis jamesii. Its relation to several similar associations is also
unclear. Most of the information available suggests that the sparse shrub canopy is more typical
and that this association should not be classified as a shrubland (Miller et al. 1977, Francis
1986, Von Loh 2000). Because it has a wide distribution (from southern Great Plains to the
Mojave Desert), stands occur in diverse habitats (clayey bottomland to sand dunes) and vary
from a sparse to moderate shrub canopy. It is likely that, when more information becomes
available and the needed classification work is completed, this association will be subdivided.
The diagnostic species require warm sites with somewhat alkaline soils, so this association
should be relatively widespread throughout the Colorado Plateau and western Great Plains.
That it is not may reflect spotty sampling of the region, or it may indicate that this association
has been mostly altered by grazing. This association may represent a form of Atriplex canescens
/ Sporobolus airoides Shrubland that has been degraded by domestic livestock grazing. The
concept of Atriplex canescens / Sporobolus airoides Shrubland includes Pleuraphis jamesii in
the herbaceous layer. In addition, the species lists for related and equivalent communities cited
in the References section include both Pleuraphis jamesii and Sporobolus airoides in the herbaceous
layer. It is possible that these associations should be combined, as it appears they are
distinguished primarily by which of the two grasses is dominant. It is possible that summerlong
grazing has reduced or eliminated Sporobolus airoides from many stands in the Colorado
Plateau, leaving Pleuraphis jamesii and Bouteloua gracilis as more grazing-resistant species.
Distinguishing these two associations because of different intensities of historic grazing may
not be ecologically valid.
Petrified Forest National Park. Since Atriplex canescens / Pleuraphis jamesii Shrubland can be
a degraded vegetation community with many dead annuals and a low cover of shrubs, this map
class served as a catch-all in some cases to map many of the flat areas with a mixed herbaceous
community and low shrub cover that includes a low cover of Atriplex canescens.
C23