Transpiration and cooling potential of tropical urban trees from different native habitats.

Urban trees are widely promoted as a solution to cool the urban environment because of shading and evaporative cooling provided by tree canopies. The extent to which the cooling benefits are realized is dependent not just on the genetically determined traits of trees, but also by their interactions with the atmospheric and edaphic conditions in urban areas, for which there is currently a paucity of information. We conducted a field experiment to compare whole-tree transpiration (Et) of tropical urban species from seasonally dry forest (SDF) (Tabebuia rosea, Lagerstroemia speciosa, Delonix regia, Caesalpinia ferrea, Dalbergia sissoo, Samanea saman) and aseasonal evergreen forest (AEF) (Peltophorum pterocarpum, Sindora wallichii). We examined the dependence of Et on atmospheric conditions (solar radiation (Rn) and vapor pressure deficit (VPD)), as well as on soil moisture level (θv). Daily Et differences between species were large but not statistically significant overall: 2000-3200 g m-2 (leaf area) under sunny conditions and 980-2000 g m-2 under cloudy conditions. The led to a daily latent heat flux (LE) of 770 W m-2 between the species with the highest (2136 W m-2) and lowest (1369 W m-2) daily Et. SDF species had higher daily Et than AEF species, but the difference was only significant under cloudy condition. Rn had a slightly stronger role in influencing transpiration compared to VPD, and species responses to drought stress differed marginally between the two groups. We assessed if two plant functional traits, wood density (ρw) and leaf stomatal conductance (gs), could be used to predict Et. Only gs was shown to be moderately correlated with Et, but more studies are needed to assess this given the limited number of species used in the study.