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Tree Physiology 23, 1113 -1124 2003 Heron Publishing — Victoria, Canada Ecophysiology of riparian cottonwoods: stream flow dependency, water relations and restoration STEWART B. ROOD, 1,2 JEFFREY H. BRAATNE3 and FRANCINE M. R. HUGHES Department of Biological Sciences, University of Lethbridge, AB TIK 3M4, Canada Author to whom correspondence should be addressed (rood @uleth.ca) Department offish and Wildlife Resources, University of Idaho, Moscow, ID 83844 -1136, USA Department of Geography, University of Cambridge, Cambridge CB2 3EM U.K. Received October 17, 2002; accepted March 29, 2003; published online October 1, 2003 Summary Cottonwoods (Populus spp.) are adapted to ripar- ian or floodplain zones throughout the Northern Hemisphere; they are also used as parents for fast - growing hybrid poplars. We review recent ecophysiological studies of the native cotton- woods Populus angustifolia James, P. balsamifera L., P. delt- oides Marsh., P. fremontii S. Watson and P. trichocarpa T. & G. in North America, and P. nigra L. in Europe. Variation exists within and across species and hybrids; however, all riparian cottonwoods are dependent on shallow alluvial groundwater that is linked to stream water, particularly in semi -arid regions. This conclusion is based on studies of their natural occurrence, decline following river damming and dewatering (water re- moval), water relations, isotopic composition of xylem water, and by the establishment of cottonwoods along formerly bar- ren natural channels after flow augmentation in response to the conveyance of irrigation water. When alluvial groundwater is depleted as a result of river dewatering or groundwater pump- ing, riparian cottonwoods exhibit drought -stress responses including stomatal closure and reduced transpiration and pho- tosynthesis, altered 13C composition, reduced predawn and midday water potentials, and xylem cavitation. These physio- logical responses are accompanied by morphological re- sponses including reduced shoot growth, altered root growth, branch sacrifice and crown die -back. In severe cases, mortality occurs. For example, severe dewatering of channels of the braided Big Lost River in Idaho led to mortality of the narrow - leaf cottonwood, P. angustifolia, and adjacent sandbar willows, Salix exigua Nutt., within 5 years, whereas riparian woodlands thrived along flowing channels nearby. The conservation and restoration of cottonwoods will rely on the provision of river flow regimes that satisfy these ecophysiological requirements for survival, growth and reproduction. Keywords: Populus. Introduction Cottonwoods (Populus spp.) occur in riparian zones (river val- ley floodplains) throughout the Northern Hemisphere (John- son 1994, Braame et al. 1996, Naiman and 136camps 1997). Along with willows ( Salix spp.), cottonwoods are ecological pioneers that colonize barren riparian sites, and in semi -arid environments they are the dominant and often exclusive ripar- ian tree species (Rood and Mahoney 1995, Scott et al. 1997, Patten 1998). In wetter regions, such as eastern North America and central Europe, cottonwoods are followed in succession by other hardwoods, and in the Pacific region of North Amer- ica and boreal regions of North America and Europe, they are followed by conifers (Johnson 1994, Polzin and Rood 2000). Riparian cottonwoods provide stream -bank stabilization and wildlife habitat, contribute to aquatic ecosystems, and pro- vide the parental genotypes for natural and artificial inter - specific hybrids ( Eckenwalder 1996, Stettler et al. 1996). Nat- ural hybrids are important for ecosystem biodiversity (Schweitzer et al. 2002, McIntyre and Whitham 2003), and ar- tificial intersectional hybrids often grow vigorously and provide fiber, fuel and wood products. However, cottonwood forests are particularly vulnerable to degradation of riparian zones as a result of livestock production, tree harvesting for fi- ber and fuel, and clearing for agriculture, human settlement and transportation corridors (Rood and Mahoney 1990, Patten 1998, Muller et al. 2002). River damming and water diversion have further contrib- uted to the collapse of cottonwood populations along many rivers (Rood and Mahoney 1990, Braatne et al. 1996). The de- cline of cottonwoods downstream from dams prompted us to review the dependency of riparian cottonwoods on stream flow and the consequences of dewatering (removal of water) on cottonwood water relations. We focus on studies conducted after 1990, thereby complementing previous reviews of cot- tonwood water relations (Blake et al. 1996, Smith et al. 1998), impacts of damming (Rood and Mahoney 1990, Friedman et al. 1998, Nilsson and Bergren 2000), and life history and con- servation ( Braatne et al. 1996, Karrenberg et al. 2002). We have adopted the taxonomic treatment of Populus described by Eckenwalder (1996). t This paper was among those presented at the /7th North American Forest Biology Workshop "Rocky Mountain ecosystems: Diversity, com- plexity and interactions, " sponsored by the Tree Physiology and Forest Genetics working groups of the Society ofAmerican Foresters and held at Washington State University, Pullman, WA.