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1116 ROOD, BRAATNE AND HUGHES <br />cipitation. Riparian trees were thus reliant on groundwater that <br />originated from stream water during dry periods but were op- <br />portunistic and would take up water from local precipita- <br />tion when available, a pattern consistent with conclusions by <br />Snyder and Williams (2000). <br />Why cottonwoods are dependent on stream flow <br />These observations collectively demonstrate that riparian cot- <br />tonwoods are dependent on stream flows. The trees are espe- <br />cially dependent along streams in semi -and regions where the <br />riparian groundwater is recharged by stream water. However, <br />rivers influence several other conditions that contribute to the <br />suitability of riparian zones for cottonwoods and that are im- <br />pacted by damming and flow regulation (Table 2). <br />Flowing water has the capacity to erode alluvial sediments, <br />which are transported downstream and redeposited. Both the <br />Figure 2. Native balsam pop- <br />lars (Populus balsamifera) that <br />have become established along <br />a previously intermittent natu- <br />ral drainage channel east of <br />Calgary, Alberta that was fur- <br />ther excavated and is used to <br />convey irrigation water (Uni- <br />versal Transverse Mercator: <br />5645 km N, 310 km E). <br />scour and deposition zones provide barren areas suitable for <br />seedling recruitment. With damming and diversion, the sedi- <br />ment patterns are altered (Table 2), and when dams are oper- <br />ated to attenuate flood peaks there can be a reduction in <br />sediment movement downstream. Furthermore, suspended <br />sediments settle out in reservoirs and this can produce a down- <br />stream zone of sediment degradation and down - cutting. Both <br />reduced sediment movement and sediment depletion can de- <br />grade the suitability of the riparian area for cottonwood <br />establishment. <br />Reservoirs also trap floating materials including woody <br />debris and seeds or vegetative fragments suitable for clonal <br />reproduction. Hydrochory, i.e., water -based propagule dis- <br />persal, can be diminished by alterations to stream flow and the <br />imposition of reservoirs (Nilsson et al. 1991, Nilsson and <br />Bergren 2000). <br />Table 2. Ecological processes associated with stream flow that influence riparian cottonwoods. <br />Process <br />Description <br />References <br />Water availability <br />Particularly along losing streams of semi -arid <br />Smith et al. 1991, Stromberg and Patten 1991, 1992, <br />regions, water from the stream infiltrates the <br />Busch et al. 1992, Busch and Smith 1995, <br />alluvial water table and is thus available for <br />Rood et al. 1995, 1998, Van Splunder et al. 1995, <br />cottonwoods and other phreatophytes. <br />Stromberg 1997, Shafroth et al. 1998, Cooper et al. 1999, <br />Scott et al. 1999, 2000, Horton et al. 2001 b, 2001 c <br />Geomorphic disturbance <br />High flows produce sediment erosion, transport <br />Rood and Mahoney 1995, Scott et al. 1996, 1997, <br />and deposition that create nursery sites suitable <br />Cordes et al. 1997, Stromberg 1997, Friedman et al. 1998, <br />for cottonwood establishment. <br />Rood et al. 1998, 1999, Polzin and Rood 2000, <br />Kalischuk et al. 2001, Shafroth et al. 2002 <br />Vegetation exclusion <br />High flows retard encroachment of inundation- <br />Rood and Mahoney 1995, Rood et al. 1999, <br />intolerant upland vegetation into riparian zones. <br />Polzin and Rood 2000 <br />Hydrochory <br />Streams transport seeds and vegetative propagules <br />Nilsson et al. 1991, Andersson et al. 2000, <br />and deposit these at appropriate positions. <br />Nilsson and Bergren 2000, Polzin and Rood 2000 <br />TREE PHYSIOLOGY VOLUME 23, 2003 <br />