108
ERDC TR-04-1
4.6
HERBACEOUS PLANT PATTERNS
Compared to woody species, herbaceous riparian species have received
considerably less research attention, especially with regard to the effects of
surface streamflow on plant patterns and dynamics. However, because herba-
ceous plants generally live for much shorter lengths of time than woody plants,
herbaceous vegetation patterns may be more likely to reflect present (or recent)
ecological conditions. Therefore, to the extent that herbaceous vegetation
patterns in riparian systems reflect characteristics of surface flow, they have the
potential to be useful indicators of recent inundation levels. Several studies have
noted that individual herbaceous plant species tend to occur in specific locations
or within particular "patch types" within the riparian zone. For example, species
such as Nasturtium officinale (water cress) and Veronica anagallis-aquatica
(water speedwell) are common in wet, stream-edge habitats in central and south-
ern Arizona, while within-channel sandbars are more commonly dominated by
Polypogon monspeliensis (rabbit's foot grass) and stream banks are often domi-
nated by Cynodon dactylon (bermuda grass) and Melilotus albus (sweet clover)
(Stromberg and Chew 1997, Stromberg et al. 1997b). Vegetation patch types that
occur on higher alluvial surfaces, such as P. velutina woodlands and P. fremontii-
S. gooddingii forests, usually support a different suite of herbaceous species
(Stromberg et al. 1991, 1997b, Stromberg and Chew 1997). If such patterns could
be related to specific streamflow characteristics, herbaceous plant distributions
might be a reliable indicator of the ordinary high water level.
Although it is probably not realistic to document the inundation tolerances or
responses of all herbaceous species present at a particular riparian site, the classi-
fication of plant species into functional groups that share ecological traits and
tolerances is a useful approach in this context (Stromberg et al. 1996, Stromberg
and Bagstad, in prep). Ecological characteristics of herbaceous plants that have
been identified as important in riparian systems include longevity (annual,
biennial, perennial), growth form (rhizomatous, caespitose, taprooted), reproduc-
tive mode (sexual vs. asexual), and competitive ability (Friedman et al. 1996,
Stromberg et al. 1996, 1997b, Stromberg and Bagstad, in prep). Wetland indica-
tor classes can also be used to designate plant functional groups (Reed 1988,
Stromberg and Bagstad, in prep). The five recognized wetland indicator classes
are based on the probability of species occurrence in wetlands and include
Obligate Wetland (>99% frequency of occurrence in wetlands), Facultative
Wetland (6799%), Facultative (3466%), Facultative Upland (133%), and
Obligate Upland (<1%). Plant rankings are adjusted for different geographic
regions and can be determined by accessing the U.S. Fish and Wildlife Service
Previous Page
