Chapter 4. Vegetation
(Stromberg 2001a, Galuska and Kolb 2002); and Tamarix chinensis, Baccharis
salicifolia, and Tessaria sericea (Stromberg 1997).
In addition to regeneration from seed, many riparian species are also capable
of asexual reproduction. The ability of damaged individuals to resprout in place
(by roots and/or shoots) and the ability of water-dispersed fragments to become
established may be important traits in flood-prone habitats (Karrenberg et al.
2002). Cottonwoods commonly resprout from fallen branches or toppled trees,
and some species also propagate by root suckering or by the shedding of small
branchlets (Braatne et al. 1996). Established P. fremontii and S. gooddingii
seedlings can sprout vegetatively following flood-induced prostration and burial
(Stromberg 1997), a pattern of "flood training" common to many riparian
species. Shrub species such as B. salicifolia, Hymenoclea monogyra, and T.
by stem-sprouting following flooding (Stromberg
1993c). H. monogyra also reproduces clonally by propagation from dispersed
root and stem fragments (Stromberg et al. 1997a).
Fluvial Processes and Tree Recruitment
Seedlings that become established within the active channel or at low heights
along the channel edge are very vulnerable to removal by subsequent flows
(Stromberg 1997, Auble and Scott 1998, Rood et al. 1998). Longer-term riparian
pioneer seedling survival and recruitment into older age classes occur on bare
patches characterized by both adequate moisture and protection from lethal levels
of physical disturbance (Auble and Scott 1998, Mahoney and Rood 1998, Rood
et al. 1998). Such protected sites may occur in localized geomorphic situations
such as the downstream ends of islands (Scott et al. 1997) but are more
commonly found outside of the active channel area. Mahoney and Rood (1998)
reviewed the literature on cottonwood seedling establishment and determined
that successful recruitment occurred between approximately 60 and 150 cm
above the base flow elevation. Presumably, the lower elevation limit is deter-
mined by erosional processes, while the upper limit results from the combination
of seedling root elongation potential and the depth of the capillary fringe above
the riparian water table.
Successful cottonwood recruitment depends on dynamic and episodic fluvial
processes that create opportunities for seedling establishment on moist disturbed
patches in safe geomorphic positions. The exact nature of the important fluvial
effects depends on the geomorphic context, with different processes operating
along meandering, braided, and bedrock streams (Scott et al. 1996, Cooper et al.
2003). Along meandering rivers the migration of river bends is accomplished by
moderate flows that progressively erode banks on the outsides of bends and