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Seed Germination and Establishment Seed longevity and conditions required for germination of riparian poplars, including plains cottonwood, are relatively well understood. Plains cottonwood produces an abundance of small seeds that are dispersed by wind and moving water. Seeds ripen at different times across individual trees and dispersal can occur over a six-week period (Engstrom 1948). Seed dispersal typically coincides with peak flows resulting from snowmelt runoff and spring thunderstorms. Under natural conditions seeds lose germinability within 1 to 2 weeks (Moss 1938, Ware and Pen found 1949, Kapustka 1972). Thus, seeds must encounter suitable germination sites soon after release. Under favorable conditions, seeds can germinate within 12 to 24 hrs (Engstrom 1948, Ware and Pen found 1949). Germination rates in excess of 80\ have been reported for poplar species on constantly moist, unvegetated substrates (Ware and Penfound 1949, Fenner et ale 1984). Following germination, young seedlings require a continuously moist substrate (Read 1958) during at least the first week of growth (Moss 1938). There is some evidence that young seedlings are susceptible to physical disturbance from flooding and heavy rains (Moss 1938, Engstrom 1948); thus, moisture provided by alluvial groundwater is desirable. Ideal conditions for seed germination and establishment typically occur on freshly deposited alluvial substrates along river and stream channels (Read 1958) in channel positions low enough to provide adequate moisture but high enough to escape scour from subsequent floods. Along meanderin9 rivers, successful establishment is character1stically associated with medium to large floods (Bradley and Smith 1986). Early Growth Once seedlings are established, root growth must keep pace with declining river stage and the associated alluvial groundwater table. Whereas seedling root growth may proceed slowly over the first few weeks, subsequent growth is much more rapid (Moss 1938). Manipulative experiments indicate that seedlings of some cottonwood species can keep pace with alluvial water table dec11nes of up to 8 em/day, although survival was less than 25\ (Mahoney and Rood 1991). For surviving seedlings, root extension was maximized relative to shoot'extension (Segelquist et ale 1993), which supports observations that a declining water table promotes seedling root growth to greater depths than does a static water table (Fenner et ale 1984). Furthermore, Segelquist et a1. (1993) suggest that cottonwood seedling survival and growth are functions of both rate and depth of groundwater decline. Along the Hassayampa River in Arizona, Fremont cottonwood seedlings established in flood plain positions up to 1.0 m above the alluvial water table (Stromberg et a1. 1991). Those seedlings occupying the higher positions (0.5-1.0 ml were subject to mortality associated with rap1dly declin1ng water tables. Thus, following establishment, root growth of 239