Table of Contents Roger Kjelgren, Daryl Joyce, and David Doley Subtropical–Tropical Urban Tree Water Relations and Drought Stress Response Strategies ....... 125 Abstract. Understanding native habitats of species successful as subtropical and tropical urban trees yield insights into how to minimize urban tree water deficit stress experienced during monsoonal dry periods. Equatorial and montane wet forest species rarely subject to drought are generally absent in subtropical and tropical cities with pronounced monsoonal dry seasons. Species native to monsoonal dry forests appear to have wide environmental tolerances, and are successful as urban trees in many tropical cities. Monsoonal dry forest species have a tendency to be deep rooted to avoid drought, with leaf habits falling along an avoidance to tolerance spectrum. Dry deciduous species, typi- cally found on more fertile soils, maximize growth during the monsoonal wet season with high photosynthesis and transpiration rates, then defoliate to avoid stress during the dry season. Evergreen tree species, typically found on less fertile soils, have a higher carbon investment in leaves that photosynthesize and transpire less year-round than do dry deciduous species. Dry deciduous tree species are more common urban trees than dry evergreen species explicitly due to more ornamental floral displays, but also implicitly due to their ability to adjust timing and duration of defoliation in response to drought. An empirical study of three tropical species exhibiting a range of leaf habits showed isohydric behavior that moderates transpiration and conserves soil water during drying. However, dry evergreen species may be less adaptable to tropi- cal urban conditions of pronounced drought, intense heat, and limited rooting volumes than dry deciduous species with malleable leaf habit. Key Words. Climate Change; Drought Deciduous; Drought Physiology; Dry Evergreen; Lagerstroemia loudonii; Pterocarpus indicus, Swietenia macrophylla; Urban Forestry; Water Stress; Wet Evergreen. Laurence Costello Urban Trees and Water: An Overview of Studies on Irrigation Needs in the Western United States and a Discussion Regarding Future Research ........................................................ 132 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 Conservation. Peter B. May, Stephen J. Livesley, and Ian Shears Managing and Monitoring Tree Health and Soil Water Status During Extreme Drought in Melbourne, Victoria ...................................................................................................................... 136 Abstract. Drought can lead to mortality in urban tree populations. The City of Melbourne, Victoria, Australia, manages a large popula- tion of trees that provide important ecosystem services and cultural heritage values. Between 1997 and 2009 Melbourne was affected by a serious drought resulting in significant tree health decline. Elms and planes in particular, were badly affected. This paper presents data from a survey of tree health status, and of studies of retrofitted buried drip line irrigation. A study of soil wetting in autumn of 2009 found that the use of drip irrigation had, in most cases, little or no effect on soil moisture levels and a modeled study of tree water use showed that water delivered by drip irrigation provided only a fraction of the water required by a mature tree. By con- trast, drip irrigation in late winter was able to recharge soil moisture levels. Mechanisms responsible for the decline in tree health seen during the drought are discussed. While the drought has temporarily been alleviated, climate change scenarios for southern Australia suggest that increased rainfall variability and drought events will be more common. The experi- ences gained during the recent drought event provide useful information for urban tree managers planning for the future. Key Words. Australia; Climate Change Strategy; Drip Irrigation; Drought; Melbourne; Platanus × acerifolia; Retrofitted Irrigation; Soil Mois- ture; Tree Health, Tree Water Use; Ulmus procera. ©2013 | International Society of Arboriculture | ISSN:1935-5297
May 2013
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