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ERDC TR-04-1
adjacent upland habitats (Bloss and Brotherson 1979, Warren and Anderson
1985, Leitner 1987, McArthur and Sanderson 1991), by the subtle effects of
small changes in hydrologic conditions on plant distribution patterns or
vegetation structure in and along dry stream channels, and by the scarcity of
hydrologic data (e.g., gauge records) for small ephemeral streams.
Despite these difficulties a few published studies have identified distinct
plant occurrence patterns in xeroriparian settings. Warren and Anderson (1985)
found that plant distributions varied with drainage area along desert washes
dissecting bajadas in Organ Pipe National Monument in Arizona. Certain species
(e.g., Cercidium microphyllum) tended to occur both in smaller washes and in
uplands, while others occurred only in medium-sized (e.g., Ephedra nevadensis,
Celtis pallida, Zizyphus obtusifolia) or large (e.g., Baccharis sarothroides,
Hymenoclea salsola) washes and not in uplands. Hupp and Osterkamp (1996)
present similar data for the same geographic area. They report that on the
smallest washes, upland plants (Ambrosia deltoidea, Larrea tridentata) occur on
lower, middle, and upper parts of banks. Larger and wetter washes may support
less-xeric species (e.g., Chilopsis linearis, Acacia greggii) on the lower parts of
banks but upland species on the upper banks. However, larger and drier washes
do not support these species, instead supporting plants with apparently lower
moisture requirements (e.g., Acacia constricta) (Hupp and Osterkamp 1996). In a
study of bajada vegetation in southwestern Utah, McArthur and Sanderson
(1991) identified 14 species that tended to occur preferentially in small channels
dissecting the slope surface (e.g., Chyrsothamnus paniculatus, Encelia
frutescens, Gutierrezia microcephala), 3 that tended to occur in upland areas
(Acamptopappus sphaerocephalus, Krameria parviflora, Larrea tridentata), and
13 that exhibited no distributional tendency (e.g., Yucca brevifolia, Ambrosia
dumosa, Ephedra nevadensis, Lycium andersonii, Opuntia spp.).
While these studies provide important information about xeroriparian
systems, they do not explicitly address the relationships between species
distributions and surface flow characteristics. That is, there appear to be no
published studies addressing the role of specific surface flows in influencing the
reproduction, growth, survival, or spatial distributions of xeroriparian species.
Although xeroriparian vegetation patterns likely reflect surface flow patterns
quite strongly because of the inaccessibility of the water table at most
xeroriparian sites, we do not know which kinds of surface flows (e.g., in terms of
most pronounced impact. Without further research it may be difficult to use
xeroriparian vegetation patterns as evidence for the ordinary high water mark on
ephemeral streams.
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