Tropical Street Trees and Climate Uncertainty in Southeast Asia

Urban trees are a critical quality of life element in rapidly growing cities in tropical climates. Tropical trees are found in a wide variety of habitats governed largely by the presence and duration of monsoonal dry periods. Tropical cities can serve as a proxy for climate change impacts of elevated carbon dioxide (CO 2 ), urban heat island, and drought-prone root zones on successful urban trees. Understanding the native habitats of species successful as tropical urban trees can yield insights into the potential climate impact on those habitats. Species from equatorial and montane wet forests where drought stress is not a limiting factor are not used as urban trees in cities with monsoonal dry climates such as Bangkok and Bangalore. Absence of trees from a wet habitat in tropical cities in monsoonal climates is consistent with model and empirical studies suggesting wet evergreen species are vulnerable to projected climates changes such as lower rainfall and increased temperatures. However, monsoonal dry forest species appear to have wider environmental tolerances and are successful urban trees in cities with equatorial wet climates such as Singapore as well as cities with monsoonal climates such as Bangkok and Bangalore. In cities with monsoonal dry climates, deciduous tree species are more common than dry evergreen species. Although dry deciduous species generally have better floral displays, their prevalence may in part be the result of greater tolerance of urban heat islands and drought in cities; this would be consistent with modeled habitat gains at the expense of dry evergreen species in native forest stands under projected higher temperatures from climate change. Ecological models may also point to selection of more heat- and drought-tolerant species for tropical cities under projected climate change.