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ERDC TR-04-1
4.4
ADAPTATIONS OF WOODY PIONEER SPECIES
Recruitment
One of the primary modes of riparian plant adaptation to flooding is via
reproduction. In much of arid and semi-arid North America, riparian forests are
dominated by pioneer species that depend on fluvial processes for the creation of
suitable seedling establishment sites. Most research on the regeneration of
pioneer riparian species has focused on cottonwoods and willows, members of
the Populus and Salix genera of the Salicaceae (willow) family. These species
produce abundant small wind- and water-dispersed seeds in the spring or early
summer (Braatne et al. 1996, Shafroth et al. 1998). For example, in Arizona,
peak seed release of P. fremontii occurs during February through April or May
(Fenner et al. 1985, Shafroth et al. 1998), while in Colorado, peak seed release of
P. deltoides occurs in June, with a mean seed mass of 5.4 104 grams per seed
(Friedman et al. 1995). Individual trees may produce hundreds of thousands to
millions of seeds (Braatne et al. 1996, Mahoney and Rood 1998). Seeds are
germinable when dispersed, and only remain viable for a few weeks after
dispersal (Young and Young 1992). Similar to Populus and Salix species,
Tamarix also produces large numbers of very small wind- and water-dispersed
seeds (Brock 1994). However, Tamarix seeds are dispersed throughout the spring
and summer (Warren and Turner 1975). Thus, the reproductive strategy of many
riparian pioneer species is to saturate the environment with large numbers of
seeds that are capable of germinating immediately upon arrival at a suitable site
(Shafroth et al. 1995a).
Initial seedling establishment of pioneer riparian trees typically occurs on
"fluvial disturbance patches," which are moist and free of competing vegetation
and plant litter (Auble and Scott 1998). Cottonwood seedlings are intolerant of
shade and rarely establish within intact herbaceous vegetation (Friedman et al.
1995, Katz et al. 2001) or beneath forest canopies (Johnson et al. 1976). Seed-
lings of riparian Populus (Segelquist et al. 1993) and Salix (Horton and Clark
2001) species are also intolerant of desiccation, relying on the constant availabil-
ity of moisture for survival. Because of these constraints, in any given year
Populus and Salix seedlings usually become established adjacent to, or within,
the active channel zone where bare moist substrate is available for colonization
(Everitt 1968, Stromberg 1993c, 1997, Friedman et al. 1997, Galuska and Kolb
2002). Similar establishment patterns have been observed for seedlings of other
riparian species, such as Alnus tenuifolia, Salix laevigata, and S. lutea (Russell et
al. 2003); Alnus oblongifolia, Fraxinus velutina, and Platanus wrightii
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