286 Martin and Stutz: Tree Health in Phoenix, Arizona, U.S. Arboriculture & Urban Forestry 2013. 39(6): 286–291 Tree Health in Phoenix, Arizona, U.S. Chris A. Martin and Jean C. Stutz Abstract. Two studies of tree diversity, visual health, and mortality in the Phoenix, Arizona, U.S. metropolitan basin were conducted as part of the Central Arizona Long-Term Ecological Research project. For one study, tree diversity, mortality, and visual health were determined in 2010 at 204 sites, encompassing both the Phoenix metropolitan basin and in the surrounding Sonoran Desert. In another study, records of tree visual health and mortality were taken during the winter months (2003–2007) at 65 non-residential sites across an urban to rural gradient. Average tree mortality rates were 4.2% per annum. Crown condition was rated as very good or good for the majority of trees. Poor pruning practices and abiotic injuries, such as trunk sunscald, were observed on 70% and 23%, respectively, of trees in non-residential areas. Disease and pest problems were detected in 41% of urban trees, including wood decay, Verticillium wilt, sooty canker, and ash decline. Based on these data, researchers suggest that urban forest health in Phoenix is being negatively impacted by extensive wounding of trees, particularly in non-residential settings, possibly caused by excessive crown manipulation through pruning. Key Words. Arizona; Phoenix; Tree Diversity; Tree Mortality; Urban Forest; Urban Heating. Cities first appeared as complex social structures nearly 10,000 years ago. Many of the earliest cities developed in arid cli- mates near reliable fresh river water resources (Redman 1999). “Oasis” is a term that is often used to describe desert cities with abundant perennial fresh water resources because of their many concentrated garden landscapes and increased tree cover. The Phoenix metropolitan area in Maricopa County, Arizona, U.S. is an archetypical modern day oasis urban center, and is situated at the northeast fringe of the lower Sonoran Desert in the broad Salt River basin. During the 20th century, construc- tion of local water storage reservoirs along the regional Aqua Fria, Gila, Salt, and Verde Rivers, as well as the Central Ari- zona Project, which transports water from the Colorado River, have enabled the rapid growth of this city during the latter half of the 20th century into a major metropolitan center with diverse tree plantings. United States (2010) census data in- dicates that the population of Maricopa County increased by 24.2% since 2000 to more than 3.8 million (U.S. Census 2010). Many urban forests, especially in arid systems, have been cre- ated and managed by humans (Whitney and Adams 1980; Martin 2008). The constructed urban forest in the greater Phoenix area is of necessity, irrigated, and has been characterized as having a lower species richness but a much larger species pool and canopy cover compared to the surrounding Sonoran Desert (Martin et al. 2003; Walker et al. 2009). Because of this, the carbon acquisition potential of local irrigated landscape vegetation has been esti- mated to be more than eight times greater than that of vegetation in the surrounding Sonoran Desert (Martin and Stabler 2002). Landscaping, or the practice of creating, installing, and managing outdoor human living environments for the enhance- ment of everyday quality of human life, is shaped by a complex interaction of socioeconomic values, the needs of society, and technologies, such as irrigation and air conditioning (Motloch ©2013 International Society of Arboriculture 1991). Before 1960 and the advent of the common use of air conditioning, one of the principle ways that trees in Phoenix served the needs of society was to make outdoor living spaces more habitable by ameliorating extremely high summer tem- peratures through shading and evapotranspirational cooling. To accomplish this, landscape designs were characterized by the presence of fast-growing broadleaf deciduous shade trees, such as mulberry (Morus alba) or Texas umbrella tree (Melia azeda- rach). Since 1960, the functional role of outdoor landscapes in the Phoenix area has waned as residents generally spend an in- creasingly greater portion of time inside controlled atmospheric environments. Especially since 1990, landscape function, as a measure of the value of outdoor living space, has been super- seded by landscape form driven by increasing public interest in conservation of water resources and native Sonoran Desert flora. Evidence for this change in the role of Phoenix outdoor landscapes may be seen in the widespread planting of desert- adapted trees, such as blue palo verde (Parkinsonia florida) and South American mesquite hybrid (Prosopis alba x Prosopis chilensis). Presently, the role of outdoor amenity landscapes may be to visually enhance the human living experience, while reflecting an attempt to conserve natural resources and offer city-dwellers an opportunity to comfortably experience nature. Urbanization can cause a dramatic shift in plant diversity and community composition, especially in arid regions. This might be explained by the broad introduction of nursery-cultivated exotic species within an urban area. Urban tree diversity and abundance in Phoenix has been shown to be positively correlated with socioeconomic status (Hope et al. 2003; Martin et al. 2004). Recently, researchers reported that since 1970 the abundance of urban forest cover in Phoenix, as measured by NDVI (nor- malized difference vegetation index), has increasingly become positively correlated to average income (Jenerette et al. 2011).
November 2013
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