Plant Moisture Stress of Green Ash Trees in Contrasting Urban Sites

Abstract

On three dates in the late summer and early fall of 1994 we measured leaf water potential and gas exchange on green ash trees in 1) a park-like stand on the University of Nebraska-Lincoln campus, 2) in large planters (5.1 m2) in an asphalt parking lot and 3) in small planters (1.8 m2) in the same parking lot. The trees on the campus had consistently higher pre-dawn leaf water potential (\|/) and photosynthetic rates (Ps) than trees growing in the nearby parking lot. Within the parking lot, trees grown in the small planters had lower \|/ and Ps than trees grown in large planters. The results support the supposition that urban environments increase plant moisture stress. Differences in growth and physiological performance between trees in the two planter types in the parking lot suggest that the adverse effects of the 'heat island' may be at least partially offset by adjusting the size of planter opening. Furthermore, models based on projected crown area, such as those developed by Bakker and Lindsey and Bassuk, provide useful guides for planning the amount of soil volume needed to ensure the survival and healthy growth of street trees.