182 Jacobi et al.: Long-Term Fluctuations in Water Status and Crown Dieback of Plains Cottonwood Trees dawn leaf ψ in 2002 dropped below -0.5 MPa for both growing season and autumn ψpd, but these low values were not sustained through subse- quent years, so crown dieback did not increase. Average spring, summer, and autumn ψpd were -0.21 ± 0.015SE, -0.35 ± 0.01, and -0.49 ± 0.03 MPa, respectively. Trees at the severe-stress site increased in crown dieback starting in 2003, and most trees were dead by 2008 (Figure 4C). After 2003, canal flow ended (Figure 4C), and percent crown dieback increased every year because water stress was never relieved (Fig- ure 4C). Average spring, summer, and autumn predawn ψpd were -0.26 ± 0.03SE, -0.41 ± 0.03, and -0.56 ± 0.03 MPa, respectively, over all years of available data. During the last two years of data, when most trees had died or were dying, the average summer ψpd were -0.53 ± 0.04 MPa in 2007 and -1.04 ± 0.05 MPa in 2008. DISCUSSION Percent soil moisture did not relate well with cot- tonwood water status (ψpd) in this study. Predawn leaf ψpd is oſten used to represent the equilibrium between the non-transpiring plant and available soil moisture, but oſten these two variables have wide discrepancies (disequilibrium), especially in well-watered soils (Sellin 1999; Donovan et al. 2001). Native plains cottonwoods growing where the groundwater depth was less than 3 m had weak to moderate relationships between soil moisture content and ψpd in a study along Colorado rivers during experimental flooding (Williams and Cooper 2005). Groundwater depth in the current study averaged >7 m, and soil moisture monitor- ing only measured down to 3.7 m, which may have contributed to weak relationships between soil moisture and tree ψpd along the HLC. Re- searchers do not recommend the expense and complexity of monitoring soil moisture to assess the water status of canal-side or urban trees. It was more direct and easier to obtain ψpd than to drill holes, install soil access tubes, and use a soil- moisture gauge. The neutron probe gauge used in this study required radioactive exposure monitor- ing and reporting, tasks most land managers and professionals may not want to be burdened with. The time it took for plains cottonwood trees to become water-stressed, when water was with- ©2017 International Society of Arboriculture held from the canal, depended on the presence and frequency of canal flow in spring and most likely the quantity of the soil moisture reservoir. When water was provided in the spring for sev- eral weeks and then the canal flow ended, it took approximately 45 and 51 days on average for trees to reach moderate stress (ψpd < -0.30 MPa) or severe stress (ψpd < -0.50 MPa), respectively. These data indicate that plains cottonwoods can survive severe droughts (as in 2002) if there is a minimum of 14 days of soil saturation during the growing season in following years (Table 2). The plains cottonwoods along the HLC appar- ently have different amounts of soil moisture res- ervoirs to draw on after canal flow ends, because some trees exhibited immediate water stress, and some trees took three months to exhibit severe water stress (range = 23 to 92 days) (Table 2). Other species of cottonwoods are also sensitive to water stress and show similar levels of dieback and mortality within two weeks to a few years (Scott et al. 1999; Cooper et al. 2003; Coble and Kolb 2012), as witnessed along the Sand Creek section of the canal where most trees died (Fig- ure 3C; Figure 4C). Severe crown dieback tends to occur the year water is halted (i.e., by pump- ing or mining activities) and mortality of trees occurs within two to three years (Scott et al. 1999; Cooper et al. 2003; Coble and Kolb 2012). Water-stressed cottonwood trees increased their ψpd after a short-term addition of water was provided within 24 to 72 hours, and remained less stressed for 8 to 36 days. The relatively rapid response of trees to the addition of water to the soil profile was because the trees roots were still alive and able to uptake the water placed in the canal. This conforms with data that illustrated plains cottonwoods maintained high ψpd dur- ing water table dry-downs in hot, mid-summer temperatures because of short-duration peaks in river flow (Cooper et al. 2003), in contrast, others have noted no physiological responses after the addition of water to soil surfaces (Cox et al. 2005; Williams and Cooper 2005). At sites where exper- imental flooding of soil surfaces occurred, a pre- vious loss of shallow roots due to the long-term (several years) regulated flow regime of the river was noted (Williams and Cooper 2005). Thus, the water experimentally added to the soil surface
September 2017
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