Arboriculture & Urban Forestry 41(6): November 2015 urban sites may be 8°C to 10°C greater when com- pared to temperatures in surrounding rural loca- tions (Oke 1987; Harp et al. 2002). Due to increased radiation found in urban sites, atmospheric drought stress (when the rate of transpiration from tree leaves exceeds capacity of tree roots to absorb water (Cregg and Dix 2001) is common for trees growing in urban forests. Atmospheric drought stress is en- hanced in urban trees due to increased temperatures, greater VPD (Cregg and Dix 2001), and limited soil moisture. Vapor pressure deficit is calculated as the difference between the amount of water vapor in the air compared to what the air could hold at satura- tion vapor pressure (Campbell and Norman 1998), and is one of the forces that enhance water vapor movement from tree leaves (Cregg and Dix 2001). Due to the interaction of low humidity and heat loading, in urban sites, daytime VPD oſten exceeds 5 kPa, while in mesic environments daytime VPD generally remains near 2 kPa (Bush et al. 2008). Be- cause saturation vapor pressure increases exponen- tially with temperature, at a given relative humidity, VPD also increases as temperature increases. There- fore, greater air temperature (as found in urban heat islands) can dramatically increase evapora- tive demand for urban trees (Montague et al. 2000; Cregg and Dix 2001; Montague and Kjelgren 2004). Urban forests provide numerous benefits (Jack- Scott et al. 2013). Urban trees increase property values (Anderson and Cordell 1988), improve air quality, reduce heat island effects through shading and transpiration (reduce cooling costs) (McPher- son et al. 1997), and reduce stormwater runoff (Sanders 1984). In addition, urban trees may provide socioeconomic benefits (increase neighborhood unity, reduce violence and crime rates, alleviate stress, and provide spiritual fulfillment) to urban residents (Dwyer et al. 1992; Zhang et al. 2007; Troy et al. 2012). Research also indicates urban popula- tions have positive attitudes toward urban trees, and value urban tree shade, aesthetics, improvement of air quality, and the ability of urban trees to reduce noise (Lohr et al. 2004; Jack-Scott et al. 2013). The ability of urban trees and forests to mitigate effects of urban heat islands depends on maintain- ing a healthy canopy cover (Fahey et al. 2013). However, the average life span of urban trees is very short. An early report by Foster and Blaine (1978) indicated average life span of urban street trees in 335 Boston, Massachusetts, U.S. to be 10 years. That is, on average, trees needed to be replaced every 10 years. Sklar and Ames (1985) indicated estab- lished, urban street trees in Oakland, California, U.S., had an annual mortality rate of 6% to 8%. In a another study conducted in Oakland, California, annual mortality rate of newly planted, urban street trees averaged 19% over a two-year period (Nowak et al. 1990). A more recent study suggests urban street trees in Baltimore, Maryland, U.S., have an annual mortality rate of 6.6% (Nowak et al. 2004). Others (Berrang et al. 1985) indicate life spans of urban trees can be substantially less when compared to trees growing in rural or native sites. Gas exchange (stomatal conductance, transpira- tion rate, photosynthetic rate) of woody plants is controlled by the response of the plant to its environ- ment (Montague et al. 2000), and urban heat loads can have a strong influence on urban tree physiol- ogy, growth, and survival. Leaves of woody plants placed over non-vegetative surfaces intercept more sensible heat and surface longwave radiation when compared to plants over a vegetative surface (Mon- tague et al. 2000; Montague and Kjelgren 2004). Consequently, trees over non-vegetative surfaces oſten have greater leaf temperature, increased expo- sure to VPD, and lower stomatal conductance (gs) when compared to plants over vegetative surfaces (Montague et al. 1998; Montague et al. 2000; Mon- tague and Kjelgren 2004). How transpiration is influ- enced by increased evaporative demand depends on the extent to which a plant regulates stomatal opening (Choudury and Monteith 1986). To regu- late transpiration, stomata of many woody plants close as VPD increases (Choudury and Monteith 1986; Montague et al. 2000). Plants that maintain open stomata dissipate more energy through tran- spirational, evaporative cooling, but transpire more water. Plants that close stomata transpire less water, but limit photosynthetic rate (A) and increase respi- ration due to greater leaf temperatures (Montague et al. 1998; Bauerle et al. 2003; Montague et al. 2004). Depending upon species and climate, woody landscape plants grown over non-vegetative surfaces in urban-like environments may have increased gs and transpiration (Potts and Herrington 1982; Zajicek and Heilman 1991), or decreased gs and transpiration (Kjelgren and Clark 1993; Montague et al. 1998; Montague et al. 2000; Montague et al. ©2015 International Society of Arboriculture
November 2015
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