Republican Rivers in eastern Colorado, seedlings and juveniles of Populus
deltoides and Salix amygdaloides were almost entirely confined to the active
fluvial zone, although adults occurred over a much larger area (Katz 2001).
Similarly, on the Hassayampa River in Arizona, young (10-year-old) Populus
and Salix cohorts occurred close to the river channel, while older cohorts
occurred farther away and at higher floodplain elevations (Stromberg et al.
Although the presence of woody riparian pioneer seedlings and saplings may
be a good indicator of recent fluvial disturbance, their absence at a given location
in the riparian zone does not necessarily indicate a lack of high flow. Seed
germination and seedling establishment may be prevented by a variety of factors,
even within a regularly flooded zone. The relative timing of high flows and seed
dispersal may result in an absence of seedlings. That is, if peak flows occur
earlier or later than the period of seed release for a given riparian species,
dispersed seeds may not encounter suitable seed beds and germination will not
occur. Even if high flows do coincide with seed dispersal, the shape of the flood
hydrograph may preclude seedling establishment. Specifically, for optimal
seedling establishment of riparian Populus species, flow discharge will decline
gradually after seed germination in order for seedling roots to remain in contact
with the receding moisture zone (Mahoney and Rood 1998). Seed germination
and seedling establishment may also be prevented by factors such as temperature,
substrate pH, and salinity (Siegel and Brock 1990, Shafroth et al. 1995b).
Seedlings that do become established in association with ordinary high flows
generally experience high mortality rates. In arid-region riparian environments, a
primary cause of seedling mortality is a lack of available moisture to plant roots.
Although drought-intolerant seedlings of riparian pioneer species may become
event, survival is unlikely if soil moisture levels are low and alluvial groundwater
levels are out of reach of plant roots. For example, first-year seedlings of Salix
lasiolepis on Schultz Creek in northern Arizona experienced close to 100%
mortality, primarily as a result of desiccation during the summer months (Sacchi
and Price 1992). Similarly, 75% of 37,000 first-year P. wrightii seedlings
censused in Lyle Canyon in southeastern Arizona in 1983 died from desiccation
in their first summer (Bock and Bock 1989). After one to two years, Populus and
Salix saplings are still vulnerable to desiccation, and mortality rates of 100%
have been observed in response to groundwater decline (Shafroth et al. 2000).
At the other extreme, floods are an equally important mortality agent for
riparian seedlings. In experimental studies, P. fremontii, S. gooddingii, P.
sericea, and B. salicifolia have been shown to possess significant inundation
tolerance, surviving saturated soil conditions for 58 days with no reductions in