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The effect of shade trees on the air and surface-soil temperature reduction under the canopy was studied in a park in subtropical Taipei City, Taiwan. Ten species of trees and two species of bamboo, which had tightly clustered tall stems and spreading branches resembling trees in shape, were chosen for the study. In the summer of 2007, we measured leaf and canopy characteristics of each species. The microclimate conditions under the tree canopies and an unshaded open space were measured repeatedly at middays without precipitation. In comparison with the nearby unshaded open space, air temperatures under the canopies were 0.64 to 2.52 °C lower, whereas the surface-soil temperatures were 3.28 to 8.07 °C lower. Regression analysis revealed the relative contributions to air cooling effect by the plant's leaf color lightness, foliage density, leaf thickness, and leaf texture (surface roughness) in decreasing order. Foliage density had the greatest contribution to surface-soil cooling followed by leaf thickness, leaf texture, and leaf color lightness in that order. Regression analysis also revealed that solar radiation, wind velocity, and vapor pressure at the site had significant effects on temperature reduction attributable to shade trees or bamboo.
This study investigated the cooling performances of extensive green roofs (EGRs) planted with 12 ornamental plants on rainless days in a subtropical city for 1 year. Imitating the construction of an EGR, 48 modules were constructed and each module was planted as a monoculture with 100 plants each. Plant growth and greening performance were measured every 2 weeks. Temperatures, solar radiation intensities, and substrate water contents were measured continuously and recorded every 5 minutes. The analyzed results showed that both plant species selection and seasonal variation had a significant impact on the noontime cooling benefit. The modules planted with taller plants, more extensive plant cover, higher albedo, and greater canopy volume had a greater noontime cooling benefit. As the seasons changed, the albedo and canopy volume of the modules were primarily responsible for differences in the noontime cooling benefit provided by the different plant species. Over an entire year of observation, the results of this research could inform the selection of plant species by landscape designers for EGRs with the aim of providing greater cooling benefits and aesthetic quality overall four seasons.