180 Jacobi et al.: Long-Term Fluctuations in Water Status and Crown Dieback of Plains Cottonwood Trees Question #3: How rapidly do moderately to severely stressed trees recover after water is provided? After the addition of water to the canal, water- stressed cottonwood trees responded by in- creasing ψpd by at least 0.10 MPa within two days, on average, over two years of measure- ments (range = 1–3 days) (Table 4). The number of days after water was added until the highest ψpd (least negative number) was reached aver- aged 5.8 days (range = 3–9 days) (Table 4). The time it took for trees to become moderately water stressed (<-0.30 MPa) again after the water was provided and then withheld was 20.6 days on average (range = 8–36 days) (Table 4). Question #4: Do summer precipitation events reduce water stress? Significant rain events of at least 25 mm were needed to provide enough soil water to relieve water stress (defined as an increase of 0.1 MPa in ψpd). The occurrence of 25 mm rain events was rare with only one to six events per growing sea- son noted over eight years (Table 5). On average, 22% of the sites had trees that responded to the 16 significant rain events that occurred in 2001– 2008, but trees at 67% of sites responded during the driest year, 2002 (Table 5). The longest dura- tion of water-stress relief (maintenance of high ψpd), at any of the sites, was seven days (Table 5). Question #5: Can predawn tree water poten- tials predict future crown dieback? Crown dieback in autumn was positively cor- related with the average yearly autumn ψpd (r = 0.78, P = 0.07) and growing season ψpd (r = 0.75, P = 0.08) in the Sand Creek sec- tion (Figure 3C, Sand Creek section). Spring leaf ψpd did not correlate with autumn crown Denver, Colorado, U.S. (2002–2003). SE = standard error. Year 2002 2003 Mean 2 19 2 17 6 5 3 5 3 5 4 4 10 10 4 before water added (SE) -0.67 (-0.13) -1.00 (-0.23) -0.40 (-0.08) -0.31 (-0.07) -0.38 (-0.06 -0.55 (-0.11) z Water source for basin in canal was a water truck or fire hydrant. dieback on the Sand Creek section (data not shown). There were no significant correlations between tree or site averages of yearly spring, growing season, and autumn (ψpd), and au- tumn crown dieback along the Cherry Creek and Toll Gate Creek sections. Trees in the Toll Gate Creek section experienced low ψpd (mod- erate to severe water stress) during the 2002 drought but autumn crown dieback did not continually increase over time and few trees died as a result (Figure 3B). The background amount of crown dieback on the average cot- tonwood along the canal in no-stress sites (most canal-flow days) was 10%–15% (Figure 3A). Comparing tree response at three represen- tative sites that experienced no stress (site 20, 28.5% average soil moisture at all soil depths over all years, SE ± 0.3), moderate stress (site 6, 13.2% average soil moisture, SE ± 0.2), or severe stress (site 2, 9.7% average soil moisture, SE ± 0.1) provided an additional of the response of plains cottonwoods to pro- longed below-normal soil moisture conditions (Figures 4A; Figure 4B; Figure 4C). Trees at the no-stress site had no crown dieback because ψpd was maintained between -0.1 and -0.3 MPa in all years, except for 2002 and 2007 (Figure 4A). Average spring, summer, and autumn ψpd were -0.19 ± 0.02 SE, -0.25 ± 0.01, and -0.29 ± 0.01 MPa, respectively, over all years. Although average autumn ψpd fell to -0.46 MPa during the 2002 drought, crown dieback did not occur that year or in subsequent years (Figure 4A). Percent crown dieback of trees at the moder- ate-stress site increased after the 2002 drought (Figure 4B). Trees averaged 25% dieback over the study period with a maximum dieback of 40% on a few trees the year after the 2002 drought. Pre- Table 4. Mean predawn leaf water potentials (ψpd) and number of days to different levels of ψpd increases of plains cottonwood trees at three study sites over two years after the addition of water to the canalz Site Trees Days of Mean water potential (MPa) Mean water potential (MPa) Days to waterz aſter water added (SE) -0.38 (-0.07) -0.55 (-0.11) -0.25 (-0.03) -0.22 (-0.04) -0.27 (-0.04) -0.33 (-0.06) 1 3 3 2 2 >-0.10 MPa water potential 1 7 9 5 5 3 5.8 Days to highest Days to decrease back to <-0.30 MPa 8 14 36 28 17 20.6 along the High Line Canal, illustration ©2017 International Society of Arboriculture
September 2017
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