Arboriculture & Urban Forestry 39(3): May 2013 Arboriculture & Urban Forestry 2013. 39(3): 125–131 125 Subtropical–Tropical Urban Tree Water Relations and Drought Stress Response Strategies Roger Kjelgren, Daryl Joyce, and David Doley 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 toler- ances, 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, typically found on more fertile soils, maximize growth during the mon- soonal 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 adapt- able to tropical 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. Urban trees are an increasingly important quality of life issue in tropical cities as economic growth swells their increasingly affluent urban populations (Nilsson 2005). The understanding and management of urban trees in tropical cities (including sub- tropical cities where cold does not seasonally limit growth), par- ticularly the street tree population, is based on a modest body of scientific knowledge. Temperate (seasonal cold limiting growth) urban tree understanding and management does not necessarily translate well to tropical trees. Tropical trees are adapted to a wide range of rainfall conditions ranging from year-round rainfall to monsoonal climates, where heavy wet seasons vary in duration and periodicity in alternation with often-pronounced dry seasons. Freestanding urban trees growing along streets, in street me- dians, or on private properties are a critical foundation for both a healthy human population and a healthy economy (Tzoulas et al. 2007). The UN World Health Organization recommends at least 9 m2 of urban greenspace per capita to mitigate unde- sirable environmental effects and to provide aesthetic ben- efits (Deloya 1993). Urban forests are particularly important to healthy cities in developing countries, which constitute some of the world’s largest metropolitan areas. Greenspace and urban tree plantings become imperative where the rate of urbaniza- tion is greatest in developing countries, particularly in smaller cities of ~500,000 people in Asia and Africa (UN-ESA 2003). Tropical cities in developing countries have a diverse pool of potential tree species available from tropical forests (Jim and Liu 2001). The selection of those tree species best suited for tropical urban conditions depends upon matching above and belowground space (Jim 2001) and on matching urban climate to species from an appropriate tropical forest type. Selecting urban trees from an appropriate tropical forest type also depends upon where a tropical city falls along the seasonality gradient of rainfall distribution. This gradient ranges from aseasonal wet, with significant rainfall every month and dry periods rare- ly longer than a week to a monsoonal climate with alternating dry and wet seasons of varying length and periodicity. Gener- ally, forest tree species in aseasonal wet climates can be de- scribed as wet evergreen, while those in monsoonal climates are either deciduous or dry evergreen. While not addressing the many nuances among tropical forest types, this generalization provides a functional conceptual framework for understand- ing tree adaptations to drought in tropical and subtropical cities. Cities in equatorial wet climates, such as Singapore in South- east Asia, logically use many trees from equatorial wet evergreen aseasonal forests. However, cities in wet equatorial climates also use drought-adapted trees from monsoonal climates (Tee and Wee 2001). This is similar in strategy to temperate cities using tree species from colder climates. Tropical monsoonal dry forest species are adapted to forest environments subject to several months of low rainfall (Miles et al. 2006). Tropical dry forest species either avoid drought with a deciduous leaf habit or tolerate drought with evergreen foliage (Santiago et al. 2004). When grown in a wet, aseasonal climate predominated by evergreen species, drought-deciduous species typically re- tain their leaves most of the year, sometimes shedding foliage briefly during short dry periods (Brodribb and Holbrook 2005). ©2013 International Society of Arboriculture
May 2013
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