Arboriculture & Urban Forestry 43(5): September 2017 OBJECTIVES This study was initiated to quantify the amount of water (i.e., canal-flow days or amount of pre- cipitation) necessary to maintain plains cotton- wood tree health (minimal crown dieback) when canal water flow days were reduced in the HLC in Denver, Colorado. The study attempted to pro- vide answers to the following five questions: 1. Can percent soil moisture predict tree pre- dawn leaf water potential? 2. How long does it take for trees to become water-stressed when water is withheld? 3. How rapidly do moderately to severely water-stressed trees recover after water is provided? 4. Do summer precipitation events reduce tree water stress? 5. Can predawn tree water potentials predict future crown dieback? MATERIALS AND METHODS Fifteen study sites were established along the HLC in the Denver, Colorado, metro area in 1997, with eight more sites added in 1999 (Table 1; Figure 1). The study area without irrigation was a typical semi-desert grassland and shru- bland, with an average elevation of 1,610 m and where cottonwood trees only occurred along waterways. The soils in the Sand Creek, Toll Gate Creek, and Cherry Creek canal sections were classified as loamy sand, silt loam, and loamy sand, respectively (Figure 1). The soil moisture depends on both winter and summer precipi- tation, where winter precipitation (October– April) averaged 229 mm and average summer precipitation (May–September) averaged 173 mm for a yearly average of 402 mm (based on 30-year averages from the National Oceanic and Atmospheric Administration, Denver Stapleton, Meteorological Station, #0522220). Thirty-year average winter temperature is 4.2°C, and av- erage summer temperature is 19.5°C, with an average summer maximum of 27.0°C. Sites were selected in 1997, and were located 71 to 103 km downstream from the beginning of the canal (Table 1; Figure 1). At each site, three to seven plains cottonwood trees, growing within 0.5 to 175 3.0 m of the canal edge and within 30.0 m of each other, were tagged for repeated observa- tions and data collection. Each tree had branches within 3 to 4 m of the ground, so leaves could easily be collected for water potential readings. Tree diameter at breast height (DBH, 1.4 m from the ground level) was measured in centi- meters with a diameter tape; height was mea- sured in meters with an inclinometer or laser. Mean tree DBH was 63.6 cm (SE = 2.08), and the mean height was 15.5 m (SE = 0.83) (Table 1). Soil moisture, ψpd, and percent crown dieback were collected yearly at some sites, while spo- radically at others, from 1997 to 2008 (Table 1). Percent crown dieback was defined as the per- cent of the tree crown with recent branch death indicated by failure to leaf out or premature defoliation. Percent crown dieback was evalu- ated by the same researcher in the spring (May– June) and in autumn (September) of each year. Figure 1. Map of plains cottonwood study sites, site num- bers, and canal sections along High Line Canal, Denver, Colorado, U.S. (1997–2008). ©2017 International Society of Arboriculture
September 2017
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