Variability of In-Situ Plant Species Effects on Microclimatic Modification in Urban Open Spaces of Nairobi, Kenya

Abstract

Plant species play a key role in microclimate regulation especially in cities where Urban Heat Island (UHI) effects are mostly felt. This study aimed at determining the impact that different tree species have on microclimatic parameters in urban open spaces of Nairobi and the degree to which they reduce UHI implications and improve Thermal Comfort (TC) of inhabitants. Two sites representing two Local Climate Zones (LCZ) were selected for sampling namely; LCZ B: Central Park (Site 1), and LCZ 4: Taifa road (Site 2). Four mature and isolated in-situ plant species with varied tree architecture were chosen for measurements of climatic variables done at 1.1 m above the ground, at the trunk base, 5 m horizontally away from the plant and open in the sun with no trees (control). Leaf area index (LAI) and Physiological Equivalent Temperature (PET) were measured under the trees. In both sites, Ter-minalia mantaly species with a spreading canopy form provided the best cooling effect with a PET reduction of 9.6˚C and 9.3˚C in Site 1 and Site 2, respectively. Tipuana tipu (round canopy form) was the second best with 9.2˚C and 8.2˚C, followed by Cassia spectabilis (vase canopy form) with 8.5˚C and 7.6˚C, and lastly Podocarpus falcatus (pyramidal canopy form) with 7.9˚C and 6.4˚C. Air temperatures in Site 1 (Park) were 2.3˚C, 1.3˚C and 1.0˚C lower compared to those in Site 2 (Street) at 1 pm, 6 pm and 8 am, respectively. A strong negative correlation (S1; r = −0.96, S2; r = −0.8) was obtained between LAI and PET for both sites. This showed that plants with higher canopy densities reduced temperature more, which in turn helps improve human TC.