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ERDC TR-04-1
4.3
SPATIAL PATTERNS
Riparian vegetation in semi-arid and arid regions is typically spatially
heterogeneous. Often, distinct vegetation patch types can be readily distinguished
on the basis of species composition, species dominance, and/or vegetation struc-
ture. For example, common vegetation patch types on the San Pedro River flood-
plain in Arizona include Prosopis/Zizyphus, Populus/Sporobolus, Baccharis/
young Populus-Salix, and Scirpus/Juncus associations (Stromberg et al. 1996).
On the Bill Williams River in Arizona, distinct vegetation patches have been
identified on the basis of dominance by Populus-Salix, Tamarix, Prosopis, or
xeric shrubs (Shafroth et al. 2002).
If the spatial distributions of such patches could be shown to reliably reflect the
frequency or probability of flooding, they could potentially be used as indicators of
specific flow levels. This approach has proven successful in some analyses of
humid-region riparian vegetation. In the eastern U.S., riparian vegetation patterns
have sometimes been interpreted as equilibrium associations in which certain
species assemblages are maintained on specific fluvial landforms by the unique
fluvial disturbance regime that characterizes the landform (Hupp and Osterkamp
1985). In this view, species that are strongly associated with particular landforms
may be used as indicators of "hydrogeomorphic" conditions, most notably
conditions of flood disturbance (Hupp and Osterkamp 1985). In certain arid-region
situations, equilibrium interpretations of vegetationsurface hydrology relations
may be valid and useful. For example, daily inundation frequency strongly
influences the distribution of marsh plant associations on the perennial Colorado
River in the Grand Canyon (Stevens et al. 1995). On the perennial Gunnison River
in the Black Canyon of the Gunnison National Monument in Colorado, distinct
herbaceous plant communities have been shown to occur at discrete fluvial
geomorphic locations (e.g., low, middle, and high cobble bars, off-channel pools)
relative to the stream channel (Auble et al. 1994). Geomorphic surfaces at lower
elevations and closer to the channel have higher probabilities of inundation and
support more wetland species than higher and more distant locations. However, on
the San Pedro River in Arizona, similar patterns have been interpreted as resulting
most strongly from variations in depth to groundwater (Stromberg et al. 1996).
Equilibrium interpretations may be valid for particular well-studied river
systems or riparian sites where consistent relations between plant occurrences
and surface flow conditions do occur. For example, on the San Pedro River in
Arizona, channel incision has led to the creation of a distinct set of geomorphic
surfaces and associated vegetation patch types that do occur consistently along
much of the upper river. These surfaces/vegetation patch types typically occur at
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