Chapter 4. Vegetation
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4.8
INDIVIDUAL RESPONSE
Floods may produce both positive and negative effects on established plants.
Positive effects of floods on arid-region riparian trees include higher growth rates
and greater foliage area, stem basal area, and riparian forest patch width on rivers
with higher flow volume (Stromberg 1993b, 2001b). These effects result
primarily from augmented floodplain moisture levels produced by flood-induced
replenishment of shallow alluvial aquifers, and possibly from the fertilization
effects of sediment-laden flows (Stromberg 2001b). Floodwaters may also flush
salts from floodplain soils, resulting in more favorable growing conditions for
some species (Vandersande et al. 2001). Although growth rate may be a reliable
indicator of flow volume at a site and may integrate hydrology over several years
(which may be beneficial for the present purpose), it is unlikely to be a useful
field indicator of ordinary high water, since within-site variations may not be
detectable, and it is not easily measured during a single field visit.
The damaging effects of flood flows on established individuals include death and
wholesale removal, or lesser impacts such as flood-training, burial, breakage, and
abrasion. Large floods may scour riparian forests. For example, a large flood
removed most of the riparian forest on Plum Creek in Colorado in 1965 (Friedman et
al. 1996). Flooding on the Hassayampa River in Arizona removed 30% of the
Populus-Salix forest, 35% of the Prosopis woodland, and 90% of the B. salicifolia
shrub vegetation in 1993 (Stromberg et al. 1997a). Following non-lethal flood dam-
age, many riparian plants sprout vigorously from stem bases or prostrated stems left
in situ. Shrub species such as B. salicifolia are especially adept at this mode of recov-
ery (Stromberg et al. 1997a). Often, the effects of past flooding are not visible at the
ground surface, but excavation of tree and shrub stems will reveal repeated episodes
of flood-induced prostration, burial by sediment, and resprouting (Everitt 1968).
There are several sources of visible evidence of recent flooding that could be
used to help identify recent high flow levels. First, when floodwaters carry debris
that can abrade standing stems, flood scars are often produced. Such scars will
eventually "heal" and become covered by bark, making them visible only in stem
cross-sections. However, if flooding has been recent, flood scars will still be
visible and can provide an indication of the height of floodwaters and the spatial
extent of inundation on the floodplain. Flood debris deposits can also provide
evidence of the spatial occurrence of past flood flows. In general, larger floods
will carry and deposit larger kinds of debris, such as whole trees, and such debris
will persist for relatively long periods on the floodplain. Smaller flows will carry
and deposit finer kinds of debris, including leaves and twigs, which persist for
shorter periods of time following deposition.
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