Arboriculture & Urban Forestry 43(5): September 2017 weekly. Yearly ψpd were first obtained within a few weeks of leaf expansion in the spring (20 May – 10 June), and ended in late-summer before leaf abscission zone formation (03–15 Septem- ber). During simulated canal runs, additional ψpd data were collected. Researchers developed three ψpd ranges to represent none to mild wa- ter stress (-0.10 to -0.29 MPa), moderate water stress (-0.30 to -0.50 MPa), and severely stressed (<-0.5 MPa). Ranges were based on published ψ of cavitation events (Tyree et al. 1994; Cooper et al. 2003), and personal knowledge from collect- ing ψpd and crown dieback measurements for ten consecutive years. Midday leaf water poten- tials (ψmd) were collected three times during the summer of 1999 within 12 hours of the ψpd for comparison, but due to high variability between measurements within a tree and across days, all analyses were conducted using only ψpd. Water Stress Recovery Simulated canal runs were conducted 29 July to 02 August 2002 at site 2, 13–14 August at site 19, and 04–15 August 2003 at sites 2, 6, and 17, to determine how fast water-stressed trees would recover (measured by ψpd). Canal-run tests also determined how long reduced-water stress in cottonwoods would be maintained fol- lowing the canal run. Basins were created at the sites by putting temporary dams in the canal. Water was trucked in or obtained from nearby fire hydrants on six different days to provide 0.3 m of water in the canal each time water was added. At site 2, the basin was 87.2 m × 3 m × 0.3 m deep; at site 6, the basin was 90 m × 3 m × 0.3 m; at site 17, the basin was 50.5 m × 3 m × 0.3 m; and at site 19, the basin was 62 m × 3 m × 0.3 m. Trees were monitored daily during basin tests to determine how rapidly they responded in their ψpd (an increase of >0.10 MPa) to the addition of water, and how rapidly they became water stressed again (<-0.30 MPa.). Data Analyses All statistical analyses were performed using SAS© 9.4 software (SAS Institute, Cary, North Carolina, U.S.). Relationships between per- cent soil moisture and ψpd were assessed using Pearson’s correlation coefficients, with P < 0.05 177 considered significant. Researchers compared percent soil moisture at all depths, at the tubes closest to the canal, and ψpd taken within 24 hours of the soil moisture reading at that site. The length of time to initial stress was the number of days between the last run of the canal in the spring and the monitoring date when average ψpd for a site fell to -0.30 MPa or less. If there was no spring canal run, then the starting date was 15 May. The effect of rainfall events on ψpd were visually analyzed by plotting the occur- rence of every rain event and the following ψpd for 2001 through 2008 for sites 8–14. If there was an increase (>-0.1 MPa) in ψpd after a rain event, it was assumed the rain event affected the water potential. A rain event amount was de- fined as the cumulative rainfall during one to three consecutive days of rain. The duration of the rain effect on the tree ψpd was based on the number of weekly readings that were significant- ly higher (i.e., less stressed) after a rain event. Researchers tested the relationship between ψpd and percent crown dieback in autumn of each year, using initial spring (20 May – 10 June), growing season (average of all readings), and autumn (last reading between 15 August – 15 September) ψpd with Pearson’s correlation coef- ficients. Water potential data and percent crown dieback values were both skewed and there- fore square root transformed prior to analyses. Regression modeling did not produce a model that was strong enough to predict crown dieback. RESULTS Canal Water Flow Water flow in the canal varied yearly and was regu- lated by Denver Water (Table 2). Factors affecting water flow in the canal included amounts of moun- tain snowpack and precipitation during the cur- rent year (Don Kennedy, Denver Water, personal communication). Water flowed for over 100 days in some years, but during years of drought, such as in 2002, no water was allocated (Table 2). Water- flow rate influenced the distance that the water traveled in the canal. Water releases with low flow rates did not move water far down the canal, leav- ing some downstream study sites without water. ©2017 International Society of Arboriculture
September 2017
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