Arboriculture & Urban Forestry 39(3): May 2013 xylem function (Brodribb et al. 2003). A large decrease in sto- matal conductance somewhat decoupled from water potential can be explained by a feed-forward process mediated by LAVPD and enhanced by large leaf size, where a small reduction in conductance from soil drying reduces transpirational cooling. Reduced transpirational cooling would increase leaf tempera- ture and LAVPD, which, in turn, would push conductance even lower and LAVPD higher in a feed-forward loop. Buildup of leaf-level abscisic acid (ABA) has been linked to stomatal clo- sure in trees, not only during soil drying (Bauerle et al. 2003), but also in very dry air conditions that causes high LAVPD (Bauerle et al. 2004). Thus, a large-leafed species could reach rapid stomatal closure from small changes in soil drying and decreased internal water potential due to leaf heating that, in turn, triggers ABA buildup to the point of inducing leaf senescence. Wet Evergreen Equatorial wet evergreen forests largely, but not exclusively, fall within +/- 10 degrees latitude of the equator in three global regions. These regions are the Amazon basin in western South America, the Congo in central Africa, and in Southeast Asia from Malaysia to eastern New Guinea (Kottek et al. 2006). A simple characterization of these forests is aseasonal annual precipitation of over 2,000 mm, with each month receiving at least 60 mm of rainfall (Kottek et al. 2006) and resulting in no distinct dry season. Since water is not limiting, wet evergreen forest species com- pete for more limited nitrogen and light (Graha et al. 2003). Wet evergreen species typically have shallower root systems than do monsoonal dry forest species (Schenk and Jackson 2005). Shallower roots are more effective at scavenging scarce nitro- gen from forest-floor dry matter decomposition (Santiago et al. 2004). With less biomass needed for deep root production, wet evergreen species invest more biomass in leaves (Santiago et al. 2004) that are long lived and more efficient at nitrogen use (Wright et al. 2002). Light is particularly limiting for seedling recruitment. Whole plant shade tolerance is a key adaptation that allows seedlings to persist in the understory until a gap allows enough light for vigorous growth (Baltzer and Thomas 2007). As a trade-off for adaptation to light and nitrogen scaveng- ing, aseasonal climate wet evergreen species are less tolerant of soil and atmospheric water deficits (Brenes-Arguedas et al. 2008). That is, wet evergreen species are less drought toler- ant at the physiological level than species found in monsoonal dry habitats (Baltzer et al. 2007), with less desiccation-tolerant leaves (Baltzer et al. 2008) and xylem hydraulic properties (Baltzer et al. 2009), particularly at the seedling stage (Kursar et al. 2009). The wet evergreen members of co-generic spe- cies pairs have higher aboveground growth rates than do relat- ed species from monsoonal habitats with distinct dry seasons (Baltzer et al. 2007). Wet evergreen species are also not toler- ant to atmospheric water deficits. Adapted to year-round wet and humid conditions, upper canopy tree species in aseasonal wet forests maintain high photosynthesis rates at low levels of vapor deficits, but exhibit steep stomatal closure and reduced photosynthesis as vapor deficits increase (Cunningham 2006). Tropical wet evergreen forest species are essentially spe- cialists attuned to a relatively specific set of environmental conditions (Woodruff 2010), where drought is absent (Baltzer et al. 2007). Wet evergreen forests also function within a nar- 127 row temperature range (Woodruff 2010), thus physiologi- cal and morphological mechanisms to survive greater water and temperature ranges are not evident (Baltzer et al. 2007; Baltzer et al. 2008; Baltzer et al. 2009; Kursar et al. 2009). A subset of wet evergreen tropical forest habitats includes montane forests. These forests exist in cooler, higher elevation climates than do lowland wet evergreen forests in a broader latitudinal range within the tropics (Bubb et al. 2004). Higher elevation translates to cooler temperatures and lower vapor deficits and evapotranspiration, resulting in a more favorable water balance (Tanaka et al. 2003). Montane forests may be subject to monsoonal dry periods. Evapotranspiration peaks during the dry season, and the general presence of deep soils suggests that water stress is not a decisive factor in character- izing this forest type (Tanaka at al. 2008). Likely due to their temperature limits and limited adaptability to water stress (Foster 2001), montane species are not typically found in urban tree populations in either monsoonal or wet equatorial cities. In general, wet evergreen forest species are not typically used in monsoonal sub/tropical cities because they are, in essence, specialists (Woodruff 2010) adapted to nutrient and light lim- ited habitats, and unlikely to tolerate soil and atmospheric water deficits during monsoonal dry periods (Kjelgren et al. 2011). WATER STRESS – TROPICAL URBAN TREES Urban heat island effects are characteristic of all cities and arise from increased sensible and re-radiated heat from impervious surfaces (Rizwan et al. 2008). In tropical and subtropical regions, cities with pronounced monsoonal dry seasons may experience particularly intense heat islands (Roth 2007). While vegetation can mitigate against heat is- lands (Roth 2007), urban vegetation in tropical cities will be affected nonetheless by elevated temperatures from climate change, in general, and through attendant increased heat load- ing from asphalt (Kjelgren and Montague 1998) and other non-transpirating surfaces (Montague and Kjelgren 2004). In particular, the crowns of freestanding isolated street trees will be subject to higher heat loading (Kjelgren and Clark 1993). The degree of heating depends on sensible heat dissipation as a function of leaf size (Leuzinger et al. 2010) and stoma- tal conductance, which determines the degree of coupling of the leaf with the atmosphere (Jarvis and McNaughton 1986). Temperature interacts with limited soil water due to confined root zones, due to either limited volume or depth (Bondarenko 2009), to impose water stress on urban trees (Close et al. 1996). Studies of street tree populations in Bangkok, Thailand (Thai- utsa et al. 2008), and Bangalore, India (Nagendra and Gopal 2010), suggest that dry-deciduous species are more tolerant of monsoonal dry urban climates. Both of these cities have pronounced monsoon- al climates with a four- to six-month dry season and a majority of deciduous species in their street tree populations. In each city, ev- ergreen species comprised only one-third of the top 15 most com- monly used street tree species, and those evergreen species used as street trees were largely from drier and harsher habitats than most species from wet equatorial forests (Kjelgren et al. 2011). The use of deciduous tree species in tropical cities in Asia appears to be the result of informal selection processes. Thai- utsa et al. (2008) also surveyed older, large specimen trees (growing on private property not along a city street) in Bang- ©2013 International Society of Arboriculture
May 2013
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