Chapter 4. Vegetation
99
specific inundation levels and could be used as general indicators of surface flow
conditions (Stromberg et al. 1996). However, it is not clear that these vegetation
surface hydrology relations can be applied to other rivers characterized by
different geomorphic conditions and hydrologic regimes. Indeed, attempts to
relate vegetation patterns to surface flow conditions have sometimes yielded
contrasting results in different contexts. For example, on the North Fork Kings
River on the west slope of the Sierra Nevada in California, Rhododendron
occidental, Fraxinus latifolia, and Alnus rhombifolia occurred at locations that
were flooded infrequently and for short durations (Harris et al. 1985). However,
in the Transverse Ranges of southern California, Alnus rhombifolia occurred at
locations with the highest stream power (Bendix 1999).
The equilibrium approach may not be appropriate in many arid riparian
settings. Here, flood disturbance may be infrequent, and ecological and geomor-
phic recovery processes may be slow, leading to non-equilibrium relations
between vegetation patterns and surface hydrology (Friedman et al. 1996, Katz
2001). In such situations, riparian vegetation patterns may be most strongly
influenced by time since flood disturbance (and associated successional
processes), depth to groundwater, and/or other sources of floodplain disturbance
(Harris et al. 1985, Friedman et al. 1996, Stromberg et al. 1996, Bendix 1999).
For example, in his study of riparian vegetation of the Transverse Ranges of
southern California, Bendix (1999) found relatively low R2 values for regression
models relating 20-year flood stream power to plant distributions along cross-
floodplain transects. He argued that the statistical relations would have been
stronger if the vegetation data had been collected immediately after the most
recent 20-year flood rather than 7 years later. In his words, "the interpretation of
the results as they regard stream power must include recognition of the role of
history" (Bendix 1999, p. 250). That is, without subsequent flooding, post-flood
vegetation composition and structure will continue to change over time,
reflecting influences other than ongoing fluvial disturbance.
Despite these difficulties, arid-region riparian vegetation patterns can provide
information about historic and present surface flow conditions. The interpretation
of vegetation patterns in this context, however, needs to be based on a sound
understanding of the ecology of the dominant plant species. The following sections
elaborate on this theme and identify vegetation characteristics potentially useful as
indicators of recent high flow. The majority of this discussion is focused on hydro-
mesic riparian vegetation, particularly the ecology of pioneer trees, since these are
the systems and species for which most scientific information is available. Discus-
sions of herbaceous plants and xeroriparian species are also included, although
these groups have not been as extensively studied and many questions remain
regarding the effects of surface flow on these taxa.
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