Predawn Leaf Water Potentials Indicate Crown Dieback and Water Status

Summary: Assessment of predawn leaf water potentials affords direct, easy ways of ascertaining water status of canal-side and urban trees in Denver, Colorado.

A study conducted from 1997 to 2008 scrutinized the relationship between soil moisture and crown health of plains cottonwoods (Populus deltoides var. monilifera) along High Line Canal, an agricultural irrigation canal that passes through Denver, north central Colorado; a stand of plains cottonwoods along Arkansas River, south central Colorado: Dave Powell, USDA Forest Service (retired), Bugwood.org, CC BY 3.0 United States, via Forestry Images

An article in Arboriculture & Urban Forestry for September 2017 approaches water needs in healthy trees by analyzing crown dieback, High Line Canal-flow days, predawn leaf water potentials, precipitation and soil moisture.

The article Long-Term Fluctuations in Water Status and Crown Dieback of Plains Cottonwood Trees broaches shallow-rooted, water-stressed roots above and in water tables underneath saturated zones. Betsy Goodrich of USDA Forest Service in Wenatchee, Washington, and William Jacobi and Ronda Koski at Colorado State University in Fort Collins consider water-stressed physiological responses. Decreased stomatal conductance and reduced leaf water potentials draw severely water-stressed cottonwoods toward branch, leaf or tree mortality through cavitated (bubble-filled) or embolic (blocked) xylem vessels.

Plains cottonwoods that endure extended dry, hot weather often exhibit low midday xylem water potentials without experiencing, contrary to cottonwood-related laboratory studies, roots-to-shoots water-transporting tissue cavitation.

Measurements of predawn and midday leaf water potentials furnish high correlations even though midday increases sometimes follow daily predawn decreases through branch dieback and stomatal closure.

Generating cottonwood-related crown dieback field research and guarding canopy cover and canal water use guide the 10-year study of 23 semi-desert grassland sites with waterside cottonwoods. The study has percent crown dieback, as percentage with recent branch death-indicated leaf-out failure or premature defoliation, evaluated for spring and autumn, May-June and September 1997-2008. It indicates non-predictability of predawn leaf water potentials, identified in bi-monthly and weekly information-collecting, from percent soil moisture, identified by information-gathering August 1999 and July-September 2000-2003.

No water additions after Cherry, Sand and Toll Gate Creek runs in Denver, Colorado, jump-start on average moderate water stress in 45 days and severe in 51.

Water-stressed cottonwoods know minimum two-day and maximum 5.8-day recovery times for predawn leaf water potentials after water additions and 20.6-day sustainability without additions before moderately re-stressing.

Summer rainfall below 25 millimeters (0.98 inches) leaves Cherry and Sand Creek loamy sand and Toll Gate Creek silt loam soil moisture below water stress-relieving levels. Average growing-season and yearly autumn predawn leaf water potentials maintain positive correlations with autumnal Sand Creek crown dieback even though no such predictability manifests itself elsewhere. Percent soil moisture nurtures nothing predictable about water status because of unpredictable interactions of ground water, non-transpiring trees and predawn leaf water potentials in well-watered soils.

Measurements of predawn leaf water potentials offer easier, more direct analyses of canal-side and urban tree water status than drilling holes, installing tubes and using gauges.

Frequency and presence of springtime canal flow and quality of soil moisture reservoir prompt the passage of time between canal water withholding and tree water stress.

Fourteen growing-season days of soil saturation subsequent to severe drought quicken tree survival, versus crown dieback in drought years and mortality within two to three years. Water-stressed trees in the study respond relatively rapidly to water additions to soil profiles because the sections' cottonwoods retain roots sufficiently alive to uptake canal water. The three researchers suggest not relying upon summer rainfall because of infrequency of suitable precipitation levels, rare impacts on urban trees and short-lived soil moisture relief.

The study touts predawn leaf water potentials for, unlike midday leaf water potential measurement of greatest daytime stress, showing overall water status after non-transpiring, recovery periods.

Management objectives for the High Line Canal in Denver, north central Colorado, focus on balancing the amount of water required for healthy tree maintenance with reduced water use; south-facing view along one of narrow portions of Denver's High Line Canal near East Iliff Avenue and South Yosemite Street; June 6, 2008, 12:03: J. Pock, CC BY SA 3.0 Unported, via Wikimedia Commons

Acknowledgment

My special thanks to:
talented artists and photographers/concerned organizations who make their fine images available on the internet;
University of Illinois at Urbana-Champaign for superior on-campus and on-line resources.

Image credits:

A study conducted from 1997 to 2008 scrutinized the relationship between soil moisture and crown health of plains cottonwoods (Populus deltoides var. monilifera) along High Line Canal, an agricultural irrigation canal that passes through Denver, north central Colorado; a stand of plains cottonwoods along Arkansas River, south central Colorado: Dave Powell, USDA Forest Service (retired), Bugwood.org, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=1210051

Management objectives for the High Line Canal in Denver, north central Colorado, focus on balancing the amount of water required for healthy tree maintenance with reduced water use; south-facing view along one of narrow portions of Denver's High Line Canal near East Iliff Avenue and South Yosemite Street; Friday, June 6, 2008, 12:03: J. Pock, CC BY SA 3.0 Unported, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:High_Line_Canal-Denver_Iliff_and_Yosemite.JPG

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