© 2003 Heron Publishing—Victoria, Canada
Ecophysiology of riparian cottonwoods: stream flow dependency, water relations and restoration
Stewart B. Rood (1, 2), Jeffrey H. Braatne (3) and Francine M. R. Hughes (4)
1. Department of Biological Sciences, University of Lethbridge, AB T1K 3M4, Canada / 2. Author to whom correspondence should be addressed (rood@uleth.ca) / 3. Department of Fish and Wildlife Resources, University of Idaho, Moscow, ID 83844-1136, USA / 4. Department of Geography, University of Cambridge, Cambridge CB2 3EN, U.K. / Received October 17, 2002; accepted March 29, 2003; published online October 1, 2003
Summary
Cottonwoods (Populus spp.) are adapted to riparian 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 cottonwoods Populus angustifolia James, P. balsamifera L., P. deltoides 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 removal), water relations, isotopic
composition of xylem water, and by the establishment of cottonwoods along formerly barren 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 pumping, riparian cottonwoods exhibit drought-stress responses including stomatal closure and reduced transpiration
and photosynthesis, altered 13C composition, reduced predawn and midday water potentials, and xylem cavitation. These physiological responses are accompanied
by morphological responses 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 narrowleaf 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
.
