Long-term fluctuations in water status and crown dieback of plains cottonwood trees

Abstract

Plains cottonwood trees (Populus deltoides subsp. monilifera (Aiton) Eckenw.) occur in riparian and urban areas in semi-arid regions of western United States. The amount of water necessary to maintain healthy trees while reducing water use are management objectives along the High Line Canal in Denver, Colorado, U.S. Canal-flow days, soil moisture, precipitation, cottonwood predawn leaf water potentials (pd) and crown dieback were measured annually from 1997-2008. Five key findings were: 1) percent soil moisture did not correlate with pd, 2) trees became moderately water-stressed (pd < -0.3 MPa) and severely water-stressed (< -0.5 MPa) 45 and 51 days after canal flow ended, respectively, 3) pd of water-stressed trees increased 24-48 hours after water was added, 4) only 22% of rain events of 25 mm relieved stressed conditions for seven days, and 5) yearly average autumn and growing season pd correlated with autumn crown dieback at low soil moisture sites (few canal-flow days), but not at moderate or high sites (more canalflow days). Trees with pd > -0.30 MPa generally had low crown dieback, whereas trees with pd < -0.50 MPa exhibited moderate or severe crown dieback. Plains cottonwoods are relatively resilient to single-year drought, and moderately stressed trees may recover and can withstand 25%-40% crown dieback if water is available the following year for >14 days. Irrigation timing and the frequency needed to minimize cottonwood future crown dieback can be obtained from autumn and growing season predawn leaf water potentials.