Arboriculture & Urban Forestry 43(5): September 2017 Arboriculture & Urban Forestry 2017. 43(5):173–185 173 Long-Term Fluctuations in Water Status and Crown Dieback of Plains Cottonwood Trees William R. Jacobi, Ronda D. Koski, and Betsy A. Goodrich 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 (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 aſter canal flow ended, respectively, 3) ψpd of water-stressed trees increased 24–48 hours aſter 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 cor- related with autumn crown dieback at low soil moisture sites (few canal-flow days), but not at moderate or high sites (more canal- flow 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. Key Words. Colorado; Cottonwood; Crown Dieback; Mortality; Populus deltoides; Semi-Arid; Water Potential; Water Stress. In semi-arid regions of the western United States, plains cottonwood trees (Populus deltoides subsp. monilifera (Aiton) Eckenw.) occur naturally in ri- parian areas, floodplains, along irrigation water- courses in rural areas, and are planted in urban landscapes. Although cottonwood trees are com- mon shade trees in moderately dry environments and can tolerate short-term periods of drought, they need periodic sources of water to avoid mor- tality (Rood and Mahoney 1995; Scott et al. 1999; Amlin and Rood 2003; Rood et al. 2003; Fichot et al. 2015). Landowners residing in urban areas with limited rainfall have a desire to preserve the health of these trees for their aesthetic and eco- logical values, but the amount of water necessary to maintain tree health is not always known. De- fining this relationship can help sustain limited resources. A unique opportunity to study the re- lationships between cottonwood crown health and soil moisture in an urban environment occurred as part of a long-term study of cottonwoods along an agricultural irrigation canal passing through the metropolitan area of Denver, Colorado, U.S. The canal was managed by Denver Water, the provider of water to residents, businesses, and communi- ties of the Denver metropolitan area. Completed in 1883, the 109 km High Line Canal (HLC) was designed to deliver water to the dry plains north- east of Denver (Skari 2003). Changing land-use practices from agriculture to urban development reduced the need for many irrigation canals along the urban Front Range of Colorado, including the HLC. Many large diameter (80–100 cm) cotton- wood trees were common along the banks of the canal, and urban park agencies and municipalities responsible for adjacent land management wanted to maintain these large shade trees as compo- nents of the urban forest. Following decreases in canal use for water transport, this study was ini- tiated in 1997 to determine the minimum water needs of the cottonwood trees growing along the HLC to minimize branch dieback and mortality. Previous research on the water requirements of various species of cottonwood (Populus spp.) ©2017 International Society of Arboriculture
September 2017
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