Evapotranspiration of Woody Landscape Plants

Abstract

Landscape trees and woody shrubs are important components of residential, commercial and municipal sites, and are nearly ubiquitous in modern non-nomadic societies. Attractive landscaping can increase property values from 6 to 20% in the USA (Hardy et al., 2000; Stigarll & Elam, 2009). Proper selection and placement increases energy efficiency for heating and cooling of structures, provides storm water management, air and noise pollution abatement and carbon sequestration in urban and suburban areas (Dwyer et al., 1992; McPherson et al., 1997; Nowak et al, 2006; Sanders, 1984). An advantage of urban trees is that they are located at the source of highest CO2 and pollution concentrations, and thus can have the greatest impact, if healthy. McPherson et al. (1999) calculated dollar benefits of the existing urban forest in Modesto, California compared to its annual tree budget cost. Including storm water and air pollution abatement, energy saving and other tangible cost, benefits outweighed cost by 2 to 1. Non-tangible aspects, such as aesthetics, positive psychological being and wildlife habitat are improved when tree and shrub quality are maintained (Hartig et al., 2010; Ulrich, 1986). Healthy maintained landscapes have a calming effect, especially for those who are under stress or depression (Ulrich, 1986). Though most beneficial in populated areas, urban sites where woody plants are transplanted are often areas not conducive to normal growth. These sites often have constrained soil volumes (Krizek & Dubik, 1987; Lindsey & Bassuk, 1991). This is frequently due to underground utilities or street and building foundations (Ruark et al., 1983). Highly compacted soils are also a major problem (Chi, 1993). If drainage is poor or nonexistent, anaerobic conditions can lead to tree decline and death (Berrang, et al., 1985; Krizek & Dubik, 1987). Conversely, where drainage is acceptable, soil volume and/or plant available water are often inadequate to supply more than a few days of transpirational demands (Lindsey & Bassuk, 1991). In addition to soil volumes and soil water availability, urban environments are often vastly different from natural woodlands or rural settings. Heilman et al. (1989) planted Ligustrum japonicum Thunb. 0.5 m from exterior walls facing the cardinal directions, actual evapotranspiration (ETA) increased with exposure to sun due to emission of long wave radiation from walls, which increased plant temperature during and after sun exposure. Maximum sap flow was about 30% higher at the west wall than other directions. Whitlow et al. (1992) measured temperature, humidity and solar radiation at street level in a study of urban trees in Manhattan, New York City. They were compared to concurrent measurements a few blocks away in Central Park during the summer for a three year period. Duration of hours of direct sunlight was severely truncated due to shading by tall