Drought Stress and Water Use in Green Ash

Greg Litus and James E. Klett

At the Colorado State University Agricultural Research Development and Education Center, the winter/spring of 2006 was one of the driest on record and ambient soil moisture conditions were the lowest recorded for the 10 years since establishment of the Tree and Turf Research Site. During the summer of 2006, selected plots of green ash (Fraxinus pennsylvanica var. Patmore) trees were exposed to extended drought by restricting irrigation until late August.

Green ash are determinant and leaf elongation is completed by late May or early June. The dry winter and spring resulted in drastically reduced leaf size in plots receiving the least water. Control Plot 1 continued to receive irrigation and shoot growth exceeded the dry plots be a factor of 10-20. After growth cessation leaf temperature, transpiration, photosynthesis and leaf water potentials were measured throughout the summer using a Ciras II gas exchange analyzer and a Shollander pressure chamber. As expected, transpiration and photosynthesis were depressed in comparison to the control trees. During this period predawn leaf water potentials in drought stressed trees progressively dropped to a low of –4.14 MPa while the control plot was maintained with full irrigation such that pre-dawn leaf water potentials did not fall below –0.59 MPa.

On August 29, 2006, irrigation was resumed in Plot 4. The drought stressed trees rebounded from depressed predawn leaf water potentials and mid-day leaf water potentials and stomatal conductance and photosynthesis but did not reach levels similar to control trees. Early season growth limitations and reduced leaf development established an upper limit for transpiration and photosynthesis that was approximately half of the well-irrigated control trees.

The reduction in photosynthesis are expected to reduce carbohydrate stores and may increase the risk of branch dieback and affect following year bud break. As part of this study samples of current year shoots, hydraulically conductive xylem and phloem and roots were collected over a six week period beginning with leaf senescence. These samples will be analyzed for total sugars and starch that comprise the non-structural carbohydrate stores. Carbohydrate stores are required for respiration and early season root and leaf development. Comparison of carbohydrate stores in stressed trees with respect to fully hydrated control trees will provide an estimate of subsequent year risk that results from a single year of drought. This work should be completed in early 2007.

Thanks to: Colorado Nursery Research and Education Foundation (CNREF) and Colorado Agricultural Experiment Station (AES) for partial financial support for this project. Partial in-kind donations of plant material are also appreciated form Fort Collins Wholesale Nursery and Bailey Nurseries Inc..

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