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ECOPHYSIOLOGY OF RIPARIAN COTTONWOODS Stream flow alterations can indirectly influence cotton- woods by altering inundation patterns that impact other plant species (Table 2). Upland plants are naturally excluded from floodplain zones because of their intolerance to flooding. With changes in flood patterns, upland plants can encroach on ripar- ian zones and compete with cottonwoods and willows. Water relations Water relations relevant to the ecophysiology of riparian cot- tonwoods include the traditional soil (substrate)— plant —atmo- sphere continuum (Braatne et al. 1992, Kozlowski and Pallardy 1997). Analyses of water relations must also consider the origins of shallow alluvial groundwater from precipitation, stream flow and regional groundwater. Gaining versus losing streams Streams commonly obtain water from the adjacent groundwa- ter and thus groundwater flow is from the riparian zone into the stream. This hydraulic pattern is typical for forested re- gions that comprise most of North America and Europe. Such rivers are referred to as "gaining" streams because they gain water from adjacent alluvial and hill -slope aquifers. In and and semi -arid regions, the hydraulic linkage is often reversed. Local precipitation is lower than potential evapo- transpiration and makes a limited contribution to groundwater. Consequently, during dry periods, water often flows from the stream into the riparian groundwater. Such streams are re- ferred to as "losing" streams because they lose water to the alluvial aquifer. There are several approaches to discriminating between gaining and losing streams. Upland forests have a positive bal- ance between precipitation and evapotranspiration and are generally associated with gaining streams. The hydraulic linkage can also be verified through direct measurement of groundwater elevations. For gaining streams, the adjacent ri- parian groundwater occurs above the stream surface. In con- trast, along losing streams, the alluvial groundwater table tends to slope downward away from the stream. For example, in a sediment mixture of sand, gravel and cobble, the water ta- ble was determined to recede with a slope of about 0.5 % (i.e., 0.5 in per 100 in distance) through cottonwood zones away from the St. Mary River and Willow Creek in southwestern Al- berta, indicating that these are losing streams (Rood and Mahoney 1995, Amlin and Rood 2003). Isotopic analyses of water can also be used to determine groundwater hydrology (Busch et al. 1992, Kolb et al. 1997). For a losing stream, the riparian groundwater is similar in iso- topic composition to the stream water, whereas along a gaining stream, the isotopic composition of riparian groundwater re- sembles groundwater from adjacent upland regions and, in turn, local precipitation. However, seasonal mixing of water sources and seasonal reversal of water fluxes, in addition to physical fractionation, can complicate interpretation of the isotopic composition of water. In semi -arid regions, riparian cottonwoods are the principal natural trees (Braatne et al. 1996, Patten 1998). In such areas, 1117 the streams are typically losing streams and the cottonwoods are thus reliant on the water that infiltrates from the stream into the riparian groundwater (Rood and Mahoney 1995). Conse- quently, these systems are particularly vulnerable to de- watering as a result of damming and diversion (Rood and Mahoney 1990, Friedman et al. 1998). Physiological water relations Many independent studies have demonstrated that cottonwood mortality downstream from dams is largely caused by drought stress (Rood and Mahoney 1990, Braatne et al. 1996, Patten 1998). Symptoms of drought stress include reduced growth and altered water relations within a sequence of physiological and morphological changes (Table 3). Studies of the physiological consequences of groundwater depletion can be grouped into four categories. The first study type involves the use of artificial systems for deliberate water table manipulation in partially controlled environments, usu- ally greenhouses or garden plots (Mahoney and Rood 1991, 1992, Segelquist et al. 1993, Hughes et al. 1997, Horton and Clark 2001). For example, rhizopods are experimental devices that contain vertical growth tubes (cylinders with substrate) linked to a water reservoir (Mahoney and Rood 1991). The wa- ter level in the reservoir is manipulated, leading to correspond- ing changes in the water table level in the growth tubes. Multiple growth tubes and multiple rhizopods permit compar- ative study of the effects of different water table regimes, as well as other factors such as substrate texture (sediment parti- cle size), on different riparian species and genotypes. The second type of study involves field comparisons, typi- cally with groundwater depletion caused by damming, diver- sion, groundwater pumping or sediment excavation (Smith et al. 1991, Busch et al. 1992, Scott et al. 1999, 2000, Horton et al. 2001a, 2001 b, 2001c, Amlin and Rood 2003, Cooper et al. 2003). Compared with the first approach, these field compari- sons are generally less controlled in terms of the extent of groundwater depletion and typically suffer from pseudo - replication. Multiple plants are monitored, but all are typically within a particular grove associated with the groundwater modification. However, the collective results of different field studies and controlled environment studies provide confidence in these analyses (Table 3). The third line of study involves comparisons of eco- physiological characteristics of cottonwoods collected from different environments and brought to a particular study site, typically a common garden or greenhouse. The different geno- types are often members of a particular species that originate from natural riparian zones with different climates (Sparks and Black 1999, Dunlap and Stettler 2001, Rowland 2001), al- though some studies consider different cottonwood species (Kranjcec et al. 1998). The fourth approach involves studies of the physiology of water relations of hybrid poplars in silvicultural plantings (Bassman and Zwier 1991, Braatne et al. 1992, Tschaplinski and Tuskan 1994, Tschaplinski et al. 1994, 1998). These stud- ies often consider different genotypes and particularly hybrid families consisting of clones of two (or more) parental species TREE PHYSIOLOGY ONLINE at http: //heronpublishing.com