Diversity of plant species could have a different influence on urban microclimate and thermal comfort. However, the magnitude of different plant species to ameliorate urban microclimate by cooling the urban microclimate and improving thermal comfort based on their allometric properties at any particular time of the day is unknown for urban environments. This paper presents the result of a study conducted in Kenya to quantify the attenuation effects of plant species on microclimatic parameters and thermal comfort as influenced by plant allometric properties.The microclimatic and instantaneous scales were adopted to analyse in-loco degree of influence of individual tree on microclimate. The choice of species was based on the search of independently isolated mature plant species with different allometric properties. Using this criterion, four species were selected in Uhuru Park, and five species were selected in Central Park for evaluation. Relative Percentages of variation of microclimatic parameters and discomfort index as influenced by plant species were calculated. The results showed differences in performance. Ficus benjamina (12.00%) presented the highest ability to reduce thermal discomfort followed by Cassia spectabilis (10.19%),Warburgia ugandensis (8.37%), Ficus religiosa (7.86%), Callistemon citrinus (5.72%), followed by Dypsis decaryi (4.48%), Bambusa vulgaris (3.87%), Terminalia mantaly (3.91% ) and Schinus molle (2.82%).The diurnal discomfort index of all the analysed tree species ranged 20°C to 25°C from 11.00 am to 18.00 pm, which meant that discomfort was expressed by < 50% of the population who sat under the shade.The differences in microclimate control are due to specific tree allometric properties of the analysed and the individual sample species, like structure and density of the treetop, size, shape and colour of leaves, tree age and growth.
Key words: Discomfort index, environmental parameters, scale, plant species.