128 kok that were much older than the street tree population. These specimen trees were dominated by evergreen species from dry evergreen forests, particularly in the genus Ficus, rather than by deciduous species. However, only four of the most com- mon specimen trees were also common as street streets, and two of these four were deciduous. Limited crossover between old specimen and street tree species suggests that evergreen spe- cies even from dry habitats did not perform as well as decidu- ous species in urban areas, resulting in proportionally greater adoption of deciduous species as street trees. The apparent heat tolerance limitation of evergreen species is consistent with Woodruff’s (2010) observation that the current distributions of evergreen tropical forests, certainly from aseasonal, but pos- sibly also seasonal habitats (Trisurat et al. 2009), reflect their temperature limits. Thus, the circumstantial evidence suggests that evergreen tropical tree species are less suited to higher urban heat island temperatures in subtropical and tropical cities. Empirical Studies Recent studies illustrate the differences in adaption among sub/ tropical evergreen versus deciduous species used as urban street trees. Kjelgren et al. (2008) investigated water use of three tropi- cal tree species varying in leaf habit and commonly used in Bangkok’s streetside population. In plotting the frequency dis- tribution of water use rates expressed as the ratio of daily water use (mm day-1 ) to local reference evapotranspiration (the plant factor, Kp), the dry deciduous species Lagerstroemia loudonii (closely related to Star of India, Lagerstroemia speciosa) had the highest use (Figure 1, modified from Kjelgren et al. 2008). Consistent with reports that dry deciduous species have hydrau- lic architecture (Choat et al. 2005) supporting higher stomatal conductance (Ishida et al. 2006), L. loudonii Kp values reached a maximum of between 40%–50% of reference evapotranspi- ration (ETo ). Higher stomatal conductance and transpiration Kjelgren et al.: Tree Water Relations and Drought Stress Response rates for sub/tropical dry deciduous species would be consistent with high carbon gain over the monsoonal wet period, in con- trast to evergreen species that can photosynthesize year round. By contrast, the Kp of both Pterocarpus indicus (angsana) and Swietenia macrophylla (mahogany) was lower at 20%–30% of ETo . Swietenia macrophylla is a dry evergreen species from ply greater sensitivity of transpiration to environmental factors that can affect stomatal opening, such as vapor deficits and wind. When subjected to drying conditions, all three species exhib- ited a similar response to water stress (unpublished data). All of the trees were growing in containers when subjected to substrate drying. The stomatal conductance quickly declined in all three species, moderating internal water potential. Since isolated trees are well ventilated (Jarvis and Morison 1981), stomatal con- ductance to water vapor is closely coupled to the atmosphere, exerting close to a direct 1:1 relationship between incremental stomatal closure and transpiration rate (Jarvis 1985). Stomatal closure at incipient substrate drying reduces evaporative strain on internal water potential, an isohydric response to water stress that slows the depletion of root zone water (Schultz 2003). All three species maintained an isohydric response as stomatal con- ductance declined to approximately 30% of well-watered levels. However, at this point, P. indicus isohydric control failed and internal water potential declined quickly as conductance fell to about 10% of well-watered levels. Total leaf area of P. indicus was substantially greater than the other two species, such that the obvious explanation is that it depleted substrate water content to what was, in effect, the permanent wilting point. The studies in Bangkok’s monsoonal climate indicate that three commonly used sub/tropical tree species exhibit an iso- hydric water stress response strategy, similar to many temper- ate woody species (Shultz 2003). This isohydric response favors lower stomatal aperture, transpiration, and possibly photosyn- thesis in exchange for slower depletion of root zone water— in essence, a “save it for a rainy day” strategy (Kjelgren et al. 2009). Combined with the deep rooting common in monsoonal forest species (Schenk and Jackson 2005), and traits that ei- ther favor drought avoidance or drought tolerance through leaf morphological characteristics (e.g., smaller, denser leaves; Wright et al. 2002), monsoonal dry forest species that are ei- ther deciduous or evergreen appear to be well-equipped to tol- erate drought as street trees in subtropical and tropical cities. monsoonal regions in South America. Its lower transpiration rate would slowly deplete soil water during the monsoonal dry period and allow extended carbon gain. P. indicus can be facul- tative deciduous when conditions become dry, but is otherwise evergreen. P. indicus is found along sandy seashores, suggesting tolerance to drought and salt under stressful conditions. However, it can aggressively grow and expand habitat under more favor- able conditions. Notable in comparing these three species is that the distribution of Kp values is much broader for L. loudonii, approaching ETo on occasion. High stomatal conductances im- Figure 1. Monsoonal dry season (January–April) water use nor- malized to depth units (mm/day) for container grown Pterocarpus indicus, Swietenia macrophylla, and Lagerstroemia loudonii, three sub/tropical trees species common in Bangkok, Thailand’s street- side population, where n is the number of daily observations over six replicates per species (modified from Kjelgren et al. 2009). ©2013 International Society of Arboriculture Acknowledgments. Thanks to the U.S. State Depart- ment and Council for International Exchange of Scholars Fulbright Fellowship program and the University of Queensland for their support that led to this publication.
May 2013
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