Chapter 4. Vegetation
107
growth, while Tamarix was intolerant of this (Vandersande et al. 2001).
However, flood-induced seedling mortality rates can be very high in natural
settings. For example, P. fremontii, S. gooddingii, and T. chinensis seedlings that
established following a 25-year flood on the Hassayampa River in Arizona had
96%, 96%, and 100% mortality following a 15- to 20-year flood that occurred
two years later (Stromberg 1997). Flood-induced mortality is not restricted to
seedlings; in 1981, a succession of two 35-year flash floods removed an entire
cohort of 11-year-old P. wrightii saplings in Lyle Canyon in Arizona (Bock and
Bock 1989).
Mechanisms associated with flood-induced mortality of riparian plants
include the physical force of floodwaters (which results in stem breakage or
mobilization of underlying sediments) and the physiological stress of prolonged
inundation (Friedman and Auble 1999). Although often difficult to tease apart,
these factors may operate alone or in tandem to reduce seedling numbers. For
example, Acer negundo seedlings on the Gunnison River in Black Canyon of the
Gunnison National Monument suffered significant mortality from extended
inundation in 1995, though shear stress clearly influenced the spatial distribution
of older individuals, with no established trees having survived shear stresses that
exceeded the critical shear stress by more than 24 Pa (Friedman and Auble 1999).
Burial effects may also be important, though many riparian species appear to be
fairly tolerant of sediment deposition (e.g., P. fremontii, S. gooddingii, and T.
ramosissima seedlings) (Levine and Stromberg 2001).
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