Long-Term Fluctuations in Water Status and Crown Dieback of Plains Cottonwood Trees

• Published in: Arboriculture & urban forestry 2017-09, Vol.43 (5)
• Main Authors: Jacobi, William, Koski, Ronda, Goodrich, Betsy
• Format: Article
• Language: English
• ISSN: 1935-5297; 2155-0778
• DOI: 10.48044/jauf.2017.015

Plains cottonwood trees (Populus deltoides subsp. monilifera (Aiton) Eckenw.) occur in riparian and urban areas in semiarid 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 (ψpd) and crown dieback were measured annually from 1997–2008. Five key findings were: 1) percent soil moisture did not correlate with ψpd, 2) trees became moderately water-stressed (ψpd < -0.3 MPa) and severely water-stressed (< -0.5 MPa) 45 and 51 days after canal flow ended, respectively, 3) ψpd 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 ψpd 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 ψpd > -0.30 MPa generally had low crown dieback, whereas trees with ψpd < -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.