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ERDC TR-04-1
patterns of riparian vegetation: (1) mid- to high- elevation riparian plant
communities, predominantly mixed-deciduous riparian forests, and (2) low-
elevation riparian plant communities, predominantly Populus-Salix forests and
Tamarix or Prosopis woodlands. Examples of higher-elevation riparian
community types occur in Zion National Park, Utah (Harper et al. 1991), in the
mountains of southern California (Bendix 1994), and in the mountains of Arizona
and New Mexico (Bock and Bock 1989, Szaro 1990, Stromberg 1993b, 2001a).
Examples of lower-elevation riparian plant community types occur along the
lower Rio Grande in New Mexico (Howe and Knopf 1991, Durkin et al. 1995)
and on the San Pedro, Hassayampa, Bill Williams, and lower Colorado Rivers in
Arizona (Stromberg 1993c, 1998, Busch and Smith 1995, Shafroth et al. 2002).
After elevation, the second most important factor influencing riparian plant
community distributions is often local moisture availability. Johnson et al. (1984)
classified riparian vegetation into three types: Hydroriparian, Mesoriparian, and
Xeroriparian. Hydroriparian systems occur on sites with hydric soils or substrates
that are almost never dry (i.e. perennial or near-perennial river reaches), Meso-
riparian systems occur on sites with nonhydric soils and substrates that are
seasonally dry (i.e. intermittent reaches), and Xeroriparian systems occur on sites
that only infrequently experience moisture in excess of direct precipitation (i.e.
ephemeral streams). Because details of riparian site moisture status are often not
reported in the literature, broader moisture categories will sometimes be used in
this review. In particular, riparian ecosystems at the relatively wet end of the
riparian moisture gradient may be termed "Hydro-mesic riparian" systems when
there is insufficient information to categorize them as clearly Hydroriparian or in
cases where moisture conditions have likely changed over time. Although these
distinctions are based on hydrologic conditions, geomorphic factors (e.g.,
channel form, degree of channel stability) are also likely to differ somewhat
predictably between categories.
Additional factors influence riparian plant community patterns, usually at
local scales. For example, reach-scale stream power or stream gradient, valley
width or cross-sectional area, geomorphic setting (e.g., canyon, arroyo, shallow
wash), geologic substrate, and fire history have all been shown to influence
riparian vegetation patterns (Szaro 1990, Bendix 1994, Minckley and Brown
1994, Evans 2001). To some extent these influences are superimposed on the
elevation and moisture-driven biotic patterns. For example, riparian-zone fires
can change the species composition of plant communities because of differences
in fire tolerance and regeneration ability among species (Busch 1995). In certain
cases local factors may override the first- and second-order effects of elevation
and moisture on community patterns. For example, high-elevation plant
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