176 Jacobi et al.: Long-Term Fluctuations in Water Status and Crown Dieback of Plains Cottonwood Trees Table 1. Distance from beginning of canal, mean cottonwood tree size attributes, sample size (n = number of trees), and data collection years for each study site (within canal section) along the High Line Canal, Denver, Colorado, U.S. (1997– 2008). Hyphen (-) = data not collected. Canal section Cherry Creek Site # 8 9 10 11 12 13 14 15 Toll Gate Creek 6 7 18 19 20 21 Sand Creek 1 2E 2W 3 4 5 16 17 23 25 Distance from canal beginning (km) 84.3 81.3 78.4 75.5 75.2 74.4 74.0 72.0 95.2 89.0 96.0 94.4 92.8 89.8 102.4 102.0 102.0 102.1 98.6 98.2 102.1 99.2 102.1 102.1 Tree DBH Tree height Trees Years of data collection (cm) (m) 68.8 34.5 77.1 48.0 89.5 29.2 77.3 40.4 83.9 24.7 116.7 86.1 28.6 23.2 113.9 103.1 103.1 35.0 85.4 135.1 22.0 39.0 23.6 38.5 Soil Moisture and Root Distribution Soil moisture was monitored to compare to ψpd as a measure of tree water status. Soil moisture was monitored at four sites (2W, 6, 17, 20) in August 1999, and July through September 2000–2003, and at five more sites (2E, 16, 19, 23, 25) from 2001– 2003 with a neutron probe (Troxler 4300, Troxler Electronic Laboratories, Inc., Research Triangle Park, North Carolina, U.S.). At each of the soil moisture monitoring sites, three aluminum soil access tubes were placed in the canal bank in two sets, 5 m apart, at 1.0, 7.6, and 15.2 m perpendicu- lar and away from the edge of the canal. Ground- water observation wells were also placed 1.0, 7.6, and 15.2 m perpendicular to the edge of the canal at sites 2, 6, 17, and 20. Percent soil moisture was monitored every two weeks at depths of 0.3, 0.9, 1.5, 2.1, and 2.7 m. Depth to groundwater was measured the same date when soil moisture data were collected at groundwater wells at sites 2, 6, 17, and 20. The canal bottom and normal high- water depth was surveyed at each site with a transit to determine and record their relationship to the depths percent soil moisture. These depth data were then used to determine what soil moisture ©2017 International Society of Arboriculture 14.7 13.2 22.2 15.4 18.6 12.5 16.8 12.9 20.6 9.8 17.0 19.0 11.9 10.0 16.9 20.7 20.7 14.3 21.8 17.4 8.8 14.7 9.2 14.4 (n) 5 5 5 5 5 5 5 5 5 5 3 3 3 3 5 7 7 5 5 5 3 3 3 3 Predawn leaf water potential (ψpd) 1997–1999, 2004–2008 1997–1999, 2004–2008 1997–1999, 2004–2008 1997–1999, 2004–2008 1997–1999, 2004–2008 1997–1999, 2004–2008 1997–1999, 2004–2008 1997–1999 1997–2008 1997–2001, 2004–2008 1999–2006 1999–2008 1999–2008 1999–2001 1997–1999 1997–2003, 2006–2008 1997–2003 1997–2002 1997–2001 1999–2003 1999–2003 2001–2003, 2007–2008 2001–2003, 2007–2008 Soil - - - - - - - Depth to moisture water table - - - - - - - - - 1999–2003 2001–2003 - - - - 2002–2003 1999–2003 2001–2003 - - 2001–2003 1999–2003 - - - - - - - - 2001–2003 - 1999–2003 2001–2003 2001–2003 - 2001–2003 - readings would be expected to be influenced by canal flow. Canal flow is presented as the num- ber of days of flow based on in-canal staff gages. Root distribution was measured in 2001 on one large cottonwood (78 cm DBH) growing on the canal bank one meter from the canal edge along the Sand Creek canal section where there was no surrounding urban development. A 2 m wide × 2 m deep trench, located two meters from the tree and perpendicular to the canal, was excavated from the center of the canal to five meters away from the edge of the canal. All roots were counted on the trench surface in 0.6 m wide × 1.8 m deep sections and placed into diameter classes of small (0.001–0.3 cm), medium (0.31–2.5cm), and large (>2.5 cm). Leaf Water Potential Measurements Predawn (10 pm–2 am) leaf water potentials (ψpd) were obtained from two leaves, each from a different branch using a pressure chamber (PMS Instrument Company, Corvallis, Oregon, U.S.). Water potentials were obtained every two weeks, until mid- to late-summer when ψpd dropped below -0.3 MPa, and then ψpd were obtained
September 2017
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