132 Arboriculture & Urban Forestry 2013. 39(3): 132-135 Costello: Urban Trees and Water Urban Trees and Water: An Overview of Studies on Irrigation Needs in the Western United States and a Discussion Regarding Future Research Laurence Costello Abstract. A review of the literature concerning water needs and water loss from landscape plants is presented. Studies conducted in the field, using lysimeters, and in containers are summarized and discussed. In some studies, crop coefficients or water use coefficients are included. A discussion of the variability found in research methods and the need for a standardized protocol for tree water needs studies is presented. Key Words. Crop Coefficients; Irrigation; Lysimeter; Plant Water Loss; Reference Evapotranspiration; Tree Water Needs; Urban Trees; Water Con- servation. This report provides an overview of studies that have evalu- ated the performance of urban trees under differing levels of irrigation, and summarizes findings that can be used to help determine tree water needs. Papers that measure water loss from tree crowns are included, although they do not necessarily provide an assessment of water needs. The scope of studies is limited to work conducted in the western United States, includ- ing the states of California, Nevada, Arizona, Utah, Colorado, and New Mexico. These states receive little precipitation in the summer months and water management for urban vegeta- tion is of paramount importance. The focus is on urban trees, but other types of vegetation are included, such as shrubs and ground covers. For an extensive listing of literature on land- scape water conservation and management topics prior to 1995, see Santos and Burger (1995). They include nearly 1,300 cita- tions of books, articles, and manuals that address design, con- struction, and maintenance issues related to small and large commercial landscapes, small and large residential landscapes, and public works. For a more recent review of the literature on efficient landscape irrigation, see Hilaire et al. (2008). Thir- teen authors from 11 different academic institutions in six U.S. states contributed to this paper that summarizes how the fol- lowing factors impact the efficient use of water in urban land- scapes: irrigation and water application technologies, design and management strategies, reuse of water resources, social considerations, incentives (economic and noneconomic), and public policy. Similarly, Kjelgren et al. (2000) address key issues associated with water use and conservation in landscapes: plant water needs, irrigation system uniformity, conservable water, and methods of conservation and their implementation. In order to group studies similar in nature, reports have been sorted first according to vegetation type: 1) trees and 2) other plant types. Tree studies are then separated into two categories: 1) studies conducted in the field, and 2) studies ©2013 International Society of Arboriculture conducted in containers or lysimeters. For studies conducted in the field, both broadleaf species and palms are included. From a management perspective, studies evaluating tree per- formance following irrigation treatments are of greatest in- terest because they provide guidance regarding how much water may be needed to maintain a species in good condition. TREES Field Studies Very few field studies have been conducted evaluating water needs of urban trees in the western U.S. Only three studies are reported here, two for broadleaf species and one for palms. Most recently, Schuch et al. (2010) evaluated the perfor- mance of nine tree species commonly planted in the low des- ert of Arizona. Trees were planted in field plots and established for 19 months prior to the initiation of three irrigation treat- ments: available soil moisture in the root zone was depleted by 30%, 50%, and 70%. Twelve months after treatments be- gan, no differences in tree height, caliper, or growth index were found for seven species, while two species receiving the lowest irrigation treatment (70% depletion) showed signs of water stress. Overall quality of all trees was rated as being good based on an assessment of visual appearance. Note that this is an ongoing study and only first-year results have been reported. Costello et al. (2005) evaluated the growth response of three California native oak species to three irrigation levels (0%, 25%, and 50% evapotranspiration, or ETo) in Santa Clara, CA. Container-grown trees were planted into a cultivated loam soil and irrigated uniformly during a one-year establishment period. After a four-year treatment period, no significant differences in trunk caliper were found for any of the irrigation treatments for all three species (Quercus agrifolia, Q. lobata, and Q. douglasii).
May 2013
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