Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 104

2.4 Riparian Vegetation
Riparian vegetation, by definition, is controlled in both form and species distribution by fluvial-geomorphic
forms and processes, which, in turn, are products of prevailing hydrologic conditions. Riparian vegetation,
channel form, and streamflow are mutually adjusted features of the bottomland landscape; alteration of
one will result in compensating adjustment of the others. Severe degradation of the channel typically
removes most of the riparian zone from the influence of fluvial processes for all but the highest flows.
The influence of the riparian zone on fluvial processes and aquatic biota is, likewise, substantially
reduced, which usually leads to deterioration of aquatic ecosystems and water quality. Although some
research has been conducted on the use of vegetation to mitigate the effects of channel incision (Shields
et al., 1993 and 1995), substantially less research has been devoted to the description and interpretation
of the role that riparian vegetation plays during incision and subsequent recovery. The basic organization
and content of this section is patterned after Hupp (1988), but the authors have integrated much of the
recent literature to the basic discussion of concepts and applications.
2.4.1 Zoning of Riparian Vegettion
In equilibrated fluvial systems characteristic vegetation species and patterns have adapted to the prevailing
environmental processes associated with particular fluvial landforms (Ma et al., 2006; Hack and Goodlett,
1960; Zimmermann and Thom, 1982). Interdisciplinary (fluvial-geomorphic and plant ecological)
approaches clearly show that riparian vegetation is an integral part of an equilibrated fluvial system.
Moreover, during natural geomorphic recovery from degradation, invasive or ruderal plants may play an
important and sometimes critical role in the re-establishment of equilibrium conditions (Osterkamp and
Costa, 1987; Hupp, 1992; Friedman et al., 1996a). Hickin (1984) listed five ways in which vegetation affects
fluvial geomorphology:
(1) by creating flow resistance on most fluvial surfaces;
(2) by increasing bank strength through root mass development;
(3) by increasing sedimentation on channel bars;
(4) by providing large woody debris (LWD) that may affect numerous hydraulic processes, including
debris jams, flow deflection, and bank armoring; and
(5) by increasing sediment deposition and stability on banks and other low fluvial surfaces.
All of these effects can be seen along streams, particularly during channel recovery following incision (Hupp,
1992; Fetherston et al., 1995; Diehl, 1997).
Specific riparian plant species grow on specific fluvial landforms. The typical landforms that may be
found along alluvial streams are shown in Fig. 2.51. Some species may be restricted to only one landform,
others may occur on two adjacent landforms, and still others may occur on most fluvial landforms (Fig. 2.51).
The likelihood of a given species vigorously growing on a particular landform is a function of the
suitability of the site for germination and establishment, and the ambient environmental conditions at the
site that permit persistence at least until reproductive age. The distribution pattern may be limited by the
tolerance of a species to a specific disturbance or stress regime, as well as by tolerance for other more
diffuse interactions including competition, for which one set of factors drives the limits at one extreme,
while another set drives the limits at the other extreme. In fluvial systems, the distribution of vegetation
across landforms may be driven largely by the tolerance of species to specific geomorphic processes at
the severe end of a stress-equilibrium relation and by competition with other riparian species at the other
end. In temperate regions, where water is abundant, vegetation distribution can be related to the distribution
of fluvial landforms (Hupp and Osterkamp, 1985). In arid and semi-arid environments, bare sites for
colonization are relatively abundant, but water availability is limiting. Thus, in dry climates, vegetation