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ECOPHYSIOLOGY OF RIPARIAN COTTONWOODS and consequently may not support phreatophytic cottonwoods and willows. Cottonwoods become established when the chan- nel flow is augmented, further demonstrating the reliance of cottonwoods on stream water (Figure 2). Isotopic analyses Stable isotopes of oxygen and hydrogen occur naturally and, because of fractionation accompanying evaporation and other physical processes, specific water sources have particular iso- topic compositions. Generally, there is minimal fractionation in stem tissues, and thus xylem water reflects the isotopic com- position of the source water taken up by roots (Dawson and Ehleringer 1991). Busch et al. (1992) used isotopic analyses to confirm the re- liance of riparian cottonwoods on alluvial groundwater origi- nating from river water. For both the Bill Williams and the lower Colorado rivers in Arizona, 2H and 180 ratios consis- tently showed that P. fremontii S. Watson xylem water was 1115 Figure 1. Riparian zones along two channels of the Big Lost River in Idaho (July 1993). The upper photograph shows a channel with continued flow and healthy streamside sandbar wil- lows (Salix exigua) and narrowleaf cot- tonwoods (Populus angustifolia) further back. The lower photograph shows a channel in which flow is fully diverted during the irrigation season, which has led to woodland mortality over a 5 -year period. similar to alluvial groundwater, which was in turn similar to river water (although evaporative enrichment of 2H occurred over the summer). Other researchers have conducted isotopic analyses that further demonstrate the reliance of riparian cot- tonwoods on water originating from stream flow (Table 1). However, there are several complications associated with isotopic analyses (Dawson and Ehleringer 1991). For exam- ple, along the Red Butte Creek near Salt Lake, Utah, ground- water from hill -slope aquifers flows into the streamside zone and supplements the water originating from the stream. A study by Kolb et al. (1997) with box elder (Acer negundo L.), another riparian phreatophyte, revealed seasonal differences. The 's0 composition of xylem water indicated that large and small trees along a stream in Arizona were solely reliant on groundwater with an 180 composition similar to that of the ad- jacent stream throughout the dry summer period, whereas, at other times, these trees obtained up to one -half of their water from shallow surface water that originated as recent, local pre- TREE PHYSIOLOGY ONLINE at http: //heronpublishing.com