Thailand's catastrophic flood: Bangkok tree mortality as a function of taxa, habitat, and tree size

Urban areas have unique environments such as high air temperature, soil compaction and disturbance, and air and water pollution often impose plant stresses. Natural disasters can impose even greater stresses on plants in such areas. In 2011 Bangkok, Thailand experienced its worst flooding in over two decades, inundated up to a meter for a month. Here we report on tree tolerance to submerged, anoxic soils based on mortality. We identified over 6500 trees by taxa: 395 species in 219 genera within 60 families, which were categorized into three groups: susceptible (>50% mortality), tolerant (<50% mortality), and highly tolerant (no mortality). Among all the species, 18% were categorized as flood susceptible, 75% as tolerant, and 7% as highly tolerant. The floods resulted in decreased overall species richness by 18%, particularly in the Magnoliaceae and Lauraceae families, for which the mortality was 100% and 66%, respectively. Flood susceptible species were mostly from high rainfall habitats such as the hill evergreen forest. As expected, highly flood tolerant species were from the mangrove forests, beach/strand forests, and swamp plant communities, where the root zones are persistently saturated with lower quality water. Unexpectedly, many species native to higher temperature, drier, often deciduous lowland habitats were found to be flood tolerant, and were from the cultivated fruit and ornamental species. The results also indicated that smaller and younger trees suffered more mortality than larger and more mature trees. Therefore, during the tree selection process for planting in urban environments that may be at risk of frequent flooding, species from a wide spectrum of ecological habitats should be considered; particularly those characterized by one or more environmental stresses, such as drought, salt, heat, or saturated soils.