Adaptive physiology of riparian cottonwoods and hybrid poplars

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Cottonwoods are Populus species that are adapted to riparian (floodplain) zones throughout the Northern Hemisphere. Interspecific hybrids occur naturally in zones where species overlap and are also artificially produced to create hybrid poplars, the fastest-growing trees in Canada. Due to the impacts of river damming and diversion and other human influences, cottonwood populations have collapsed across western North America. I propose to further investigate the ecophysiology of riparian cottonwoods and their conservation. Investigations will involve three overlapping areas, hydraulics, hormones and hybridization. Studies of hydraulics will investigate water fluxes linking streams, riparian substrates, roots, shoots and the atmosphere. Aspects of tree water relations will investigate hydraulic conductance and vulnerabilities to xylem cavitation, air embolism in the xylem vessels that destroys the capacity to conduct water. The studies will compare and contrast the four Canadian cottonwood species, study differences between males and females, and will also investigate naturally occurring Populus hybrids from the complex hybrid zone in southwestern Alberta, and artificial intersectional hybrids. The studies will investigate the physiological processes responsible for adaptation to differing environments and we have identified some 'survivors', trees that have persisted after chronic stress and may be uniquely drought-adapted. Plant hormones studies will investigate the roles of the hormone gibberellin (GA) in the coordination of cottonwood growth responses to changing environmental conditions and studies will also investigate consequences of transgenic modification of GA level and action. These investigations will (1) reveal ecophysiological adaptations of cottonwoods and poplars, (2) contribute to the restoration of native cottonwoods through river regulation and water management, and (3) identify heritable physiological traits that may enable the development of drought-adapted hybrid poplars.