Riparian Cottonwoods: Hydrology, Hydraulics and Health
Stewart Rood1*, David Pearce2, Karen Zanewich2 and Larry Flanagan2
1Department of Biological Sciences, University of Lethbridge, Alberta, Canada; rood@uleth.ca
2Department of Biological Sciences, University of Lethbridge, Alberta, Canada
Four cottonwoods, or riparian Populus species, occur in western North America and these interbreed to produce interspecific hybrids that further increase regional biodiversity. In dry regions, these cottonwoods may provide the only native trees, with distributions restricted to floodplain zones along perennial channels. There, water from the adjacent stream infiltrates to recharge the alluvial groundwater that provides a primary water source for the phreatophytic cottonwoods. With this association, cottonwoods are reliant on instream flow patterns that are sufficient to recharge the alluvial groundwater, especially during warm and dry intervals when precipitation is limited. During these periods, foliar stomatal closure reduces transpirational water use but stem water potentials may still decline, and cottonwoods are exceptionally vulnerable to xylem cavitation, embolism that breaks the capillary strands of water. This can induce branch sacrifice that is displayed by precocious senescence of specific branches, or in severe cases, the whole tree crown. This presentation will coordinate aspects of stream and groundwater hydrology, and the hydraulic architecture and water relations of cottonwood species and native hybrids. It will then consider diagnostic measures to detect drought stress, as part of a strategy to inform dam operations for the conservation of healthy woodlands.