126 While monsoonal drought-adapted tree species can be used in an aseasonal wet evergreen climate, the reverse does not appear to be true. Street trees in Bangkok, Thailand, were dominated by deciduous species, mostly native to Southeast Asia but also from Africa and South America (Thaiutsa et al. 2008). This was due largely to deciduous species typically having more ornamental floral displays. Dry evergreen species were more common as large specimen trees, but most were older, often declining specimens found in protected urban locations (Thaiutsa et al. 2008). Similarly, Nagendra and Gopal (2010) reported that deciduous trees dominat- ed the street-side tree population in Bangalore, India. Of the ever- green species, only one was ostensibly from a wet evergreen forest. Understanding differences in ecological physiology between dry and wet tropical forest species can explain the relative dis- tribution and abundance of deciduous and evergreen species in tropical cities, and inform how they can be managed for drought. Combined with insights into forest change from modeled projec- tions under increased temperature scenarios and paleo-climate reconstructions, urban landscapers in tropical cities can poten- tially select appropriate deciduous and evergreen tropical spe- cies that are best adapted to future hotter and drier conditions. The purpose of this paper is to consider how an appreciation of monsoonal tree species adaptations to variable drought can inform sub/tropical urban tree understanding and management. TROPICAL FOREST TYPES AND ADAPTATION Monsoonal Dry Forests The climate of most subtropical and tropical cities has one or several dry periods long enough that water stress and negative growth effects emerge if not understood and managed prop- erly. Monsoonal dry forests are found within tropical latitudes where the dry season is long enough to cause routine soil water deficits despite episodic rainfall, while the wet season duration is sufficient to support a forest canopy. Because of the extend- ed dry season, monsoonal species appear to be characteristi- cally more deeply rooted than other forest types (Schenck and Jackson 2005), although coarse textured soil can sometimes create dry-deciduous islands in otherwise evergreen forests (Bohlman 2010). Dry forests are a significant ecosystem in the monsoonal tropics whose rich animal and plant biodiversity, particularly Kjelgren et al.: Tree Water Relations and Drought Stress Response than do deciduous species (Doley 1982; Wright et al. 2002), but they must tolerate low leaf water potentials and survive desic- cation (Pittman 1996; Brodribb et al. 2003) during the extended dry season. Dry evergreen forest species typically have lower sto- matal conductance and photosynthetic rates than do deciduous species (Choat et al. 2005; Ishida et al. 2006) and a lower hy- draulic conductance that limits stomatal conductance and lowers internal water potentials (Brodribb et al. 2003; Ishida et al. 2006). Deciduous species in monsoonal dry forests have a tendency to occupy more nutrient rich soils, where fire can determine a particular forest subtype (Miles et al. 2006). Dry-deciduous spe- cies typically have high transpiration rates and hydraulic conduc- tivities that maximize productivity during wet season foliation. This forest type then enters deciduous leaf senescence to de- foliate at some point during the dry season (Choat et al. 2005; Ishida et al. 2006). Deciduous dry forest tree species, in fertile soil where the cost of nutrient loss is not limiting, exhibit a remarkable array of adaptations in response to extreme season- al wetness and drought. Drought-deciduous species vary with respect to leaf longevity and length of dormancy in response to drought (Elliot et al. 2006) and in hydraulic properties. Some species may undergo xylem cavitation and defoliate in response to modest water stress at relatively less negative internal water potentials and then initiate refoliation with new xylem as rainfall increases the soil water content with onset of the wet monsoon season (Brodribb et al. 2002). Other species under mild water stress avoid cavitation entirely and maintain functional xylem while defoliated. This form of drought sensitivity minimizes soil water depletion, maximizes soil water content, and allows refo- liation and sunlight capture before the onset of the wet season and subsequent increased competition for sunlight (Brodribb et al. 2003; Elliot et al. 2006). Some drought-deciduous species have high exchangeable trunk water storage capacity due to their wood anatomy, which facilitates the avoidance of low soil water potential effects by such species (Borchert and Pockman 2005). Hydraulic signaling appears, however, to be an incomplete in understory environments with greater illumi- nation, is under threat from a range of anthropogenic activi- ties (Miles et al. 2006). The biodiversity of tropical dry forests yields many economically important timber and medicinal spe- cies (Johnson and Grivetti 2008) and is also a source of genetic diversity for breeding programs (Purushothaman et al. 2000). Where soil nutrient levels are low, evergreen species are typi- cally favored, sometimes mixed with deciduous species (Choat et al. 2005; Ishida et al. 2006). Dry evergreen forest species must tolerate intense solar radiation, heat, and drought during the dry season, and low light and high rainfall during the wet season (Graha et al. 2003). The relatively small leaves of dry evergreen forest species relative to deciduous species minimize intercep- tion of solar radiation and maximize convective and radiative cooling during the dry season (Pittman 1996). Evergreen species with lower leaf nitrogen concentrations, greater leaf longevity, and greater internal recirculation of nutrients have a lower invest- ment in nitrogen per unit of photosynthetic machinery per year ©2013 International Society of Arboriculture explanation for drought-induced dormancy. Changes in hydrau- lic properties do not correlate to gas exchange reduction prior to senescence in many tropical deciduous species (Brodribb et al. 2002; Brodribb et al. 2003). The speed of signal transfer and the distances over which the signals must be transported in trees would not result in timely stomatal responses to diurnal variations in environmental conditions. Leaf size and heating may provide a possible alternative explanation for drought-induced dormancy in these species. A number of drought-deciduous species have rela- tively large leaves, such as are rarely found in full sun habitats [e.g., Tectona grandis L. f. (teak) and a number of dipterocarp species (Sales-Come and Holscher 2010)]. Large leaves can be maintained only if high transpiration rates facilitate evaporative cooling and maintain photosynthetically optimum leaf temperatures, a char- acteristic consistent with high stomatal conductance and wide xy- lem vessels capable of high hydraulic conductance as are found in drought-deciduous species (Choat et al. 2005; Ishida et al. 2006). High transpiration rates, particularly in species with large leaves, suggest a possible defoliation signal in drought-decidu- ous species. These species appear to have stomata sensitive to leaf-air vapor pressure difference (LAVPD) (Ishida et al. 2006). Mild, dry season water stress may trigger a rapid and large drop in stomatal conductance in response to a small change in soil water potential, with minimal cavitation and negligible loss of
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