Climate in an urban setting differs from that of the surrounding peri-urban and rural zones which brings about UHI (Voogt, 2002; Anjos & Lopes, 2017; Soltani & Sharifi, 2017). Urbanization has led to modifications in ground cover types. The vegetative covers and natural soils are substituted by impermeable materials like asphalt and other building materials (Rushton, 2001; Frazer, 2005) that do not allow water infiltration or absorption and change the natural route of storm water (Brattebo & Booth, 2003). These materials also have low albedo (Voogt, 2002) that store vast amounts of radiation energy and release it later when the sun sets. Architects, Brattebo & Booth (2003) reported that most urban planners select paving and building supplies based on several specialized necessities such as durability and security together with expenses and normally pay little attention to environmental reasons.

Elevations in ambient temperatures may possibly cause a number of problems with regard to an increased urban thermic discomfort in addition to deterioration of city dwellers’ wellbeing (Voogt, 2002; English et al., 2007; Soltani & Sharifi, 2017). Previous studies have revealed that green spaces in urban sections such as urban gardens and community parks can significantly moderate the ultimate UHI implications (Chiesura, 2004; Feyisa, 2014). Vegetation has a vital function in the prevention of heat build-up through processes like shading of buildings and ground, ventilation and evapotranspiration (English et al., 2007). More contribution of vegetation is towards effective storm water control (Akbari et al., 2001; Brattebo & Booth, 2003) and enhances air quality in urban domains. Furthermore, vegetation improves out-of-door Thermal Comfort (TC) (Van Hoof et al., 2010; Elnabawi & Hamza, 2019), offering numerous services of the environment, like carbon storage (Ren, 2011), reducing air pollution and acting as urban biodiversity habitat (Saxe et al., 2001; Georgi & Zafiriadis, 2006; Bellard, 2012).

In Kenya, the mean annual temperature was estimated to have risen by one degree Celsius from 1960 signifying a mean value of 0.22˚C increment every ten years (CCN, 2007; Tibaijuka, 2007). A rise of 1.1˚C to 3.0˚C temperature through the year 2060 to 2100 is projected; the temperatures may rise with almost four degrees Celsius, producing unevenness in precipitation by about twenty percent (Tibaijuka, 2007). As there are increased shortcomings related to escalated urban temperatures and heat surge, a proposal to develop both adaptation and attenuation techniques was made by Chandler (1962). It entailed integration of green zones in urban domains that showed a potential temperature rollback effects to the inhabitants (Chiesura, 2004; Park et al., 2012; Feyisa, 2014). Nairobi formerly had a status as a salubrious residence, it being dubbed as “Green City in the Sun” (CCN, 2007). Conversely, infrastructure development and urban sprawl have vastly contributed to tail-off of forests and most natural sections, like bush lands and mixed rangeland (Tibaijuka, 2007). Thus, vegetation in urban places is increasingly becoming critical to maintaining the quality of a livable environment.

However, there is a dire shortage of information regarding the effectiveness of plants in microclimate control within the diverse urban environments especially in tropical climate settings. Therefore, this study aimed to assess the impact of in-situ plant species on microclimate regulation in the Central Business District (CBD) of Nairobi city. The specific research questions were: how does plant architecture influence the degree of air temperature roll back in the immediate urban settings; when and where is the magnitude of change highest between differing urban surfaces. Information of the study would be useful to urban green managers, landscape designers, arborists and nursery plant suppliers when making decisions with regard to planting design, installation and overall maintenance. Further, the findings contributed to decision-making on urban open space management in relation to plant selection and microclimate regulation in Nairobi.

AT is much higher in urban regions resulting from human activities which include construction, burning of fossil fuels and use of motor vehicles (Akbari et al., 2001; Atkinson et al., 2007; Morris, 2010) that highly contribute to production of anthropogenic heat causing the buildup of UHI (Voogt, 2002). UHI alters the human thermal comfort and poses adverse environmental and health implications to the urban residents (Voogt, 2002; Santamouris et al., 2017). In this study, the plant species in both the park and the street were able to regulate microclimate regardless of their external environments. Notably, the temperatures in the park were lower compared to the temperatures at the street and this cut across all the species. The park was able to reduce the AT by 3.3% (2.3˚C) than the street at 1 pm when it is mostly hot and by 2.6% (1.3˚C) in the evening at 6 pm when the effect of UHI is felt.

The park was also able to increase the RH by 8.2% and 9.3% than the street both at 1 pm and at 6 pm, respectively. This shows that improving tree cover within urban open spaces will help reduce AT and increase RH making the out-of-door spaces comfortable for the urban dwellers (Narita et al., 2008; Loughner et al., 2012; Feyisa, 2014). When urban greenery is lost, the lesser its surface area will be, the lesser the rate of evapotranspiration (Leuzinger et al., 2010; Lin et al., 2017) and the lesser the cooling effect of the vegetation will be felt (Saxe et al., 2001). This poses a lot of environmental and health risks to the city dwellers. Urban planners, policy makers and ecologists should work towards protection and conservation of the already existing green spaces and even aim to increase tree cover by certain percentages. They should also create numerous patches of green spaces (Mélissa Giguère, 2009; Anjos & Lopes, 2017) and not single-big greenery patches (Robinson, 2004). Likewise, habitual street tree maintenance will provide an economical approach for urban landscape improvement (Jim, 2013).

From the study, there were significant differences among the species’ performances as a result of their canopy densities shown by LAI (Table 3). Plants species with higher canopy densities have larger surface areas for evapotranspiration (Loughner et al., 2012; Shahidan, 2015; Sodoudi et al., 2018) and providing shade which in turn reduces temperatures to a greater extent compared to trees with less canopy densities. The results also showed a surface temperature difference of 9.1% (6.4˚C) in the street warmer than the park at 1 pm (Table 4). This shows that materials used to make the surfaces in streets have low albedo and tend to absorb more heat (Voogt, 2002; Anjos & Lopes, 2017), compared to the natural surfaces in parks (Govindarajulu, 2014) which are also permeable (Chiesura, 2004; Feyisa, 2014; Stewart & Oke, 2012).

Increasing albedo for construction materials used by architects and construction managers will help reflect a huge amount of radiation instead of absorbing it, including the roofing materials used (Mélissa Giguère, 2009). Narita et al. (2008) propose parking lots should at least be 35% paved and 65% vegetated. There was a notable 11% (0.3 m/s) difference in street WV, being higher than that of the park at 1 pm (Table 4). This is because streets tend to have high wind velocities from the moving vehicles and also as a result of the street canyon effect caused by the adjacent tall buildings on each side of the street (Pauleit, 2003; Narita et al., 2008). On the other hand, parks are mostly open and wind velocity is equally distributed and neutral as a result of having trees which act as wind breakers (Chiesura, 2004; Feyisa, 2014).

Minimizing damages caused by trees to construction and pavements is achievable through development of a proper planting design. Some factors to be considered during installation include the crown height and width, the utility clearance as well as the size of the planting pit. Besides the aesthetics and functionality of the plants, considering the urban trees’ architectural aspect is essential in regulating microclimate in Nairobi. Avoiding tree species having drooping branches, weak trunks or having thorns is imperative for safety purposes. To avoid damaging the roads or surrounding pavements, the use of root barriers and root trainers is essential. Moreover, the trees should be planted approximately six meters away from any building. On open spaces with poor soil or compacted surfaces, using various tree planters can be an ideal alternative.

Urban landscape planners and designers of Nairobi city should consider tree species with strong trunks, spreading canopies as well as rounded canopy forms, such as Tipuana tipu tree species. Similarly, evergreen trees with more foliar densities similar to Terminalia mantaly, but with raised crowns are suitable for microclimate regulation. For Nairobi city’s streetscapes, some of the trees that were not informed by the study include the Ficus benjamina species. This is because of its invasive nature in terms of canopy and root overgrowth, which is not favorable near construction. However, they can be used in the urban parks and green spaces away from any infrastructure. Generally, a habitual maintenance of the existing urban trees is vital in reducing structural failures and their potential hazardous risks.

From this study, we found out that plants with dense canopies and low SVF reduce temperatures more than plants with less dense canopies. Temperatures in open built and paved areas within Nairobi city are higher than the temperatures in green spaces and they highly contribute to the UHI effects felt by the city dwellers. Nairobi city residents are more likely to suffer no thermal stress in parks, but will experience warm moderate heat stress in built areas during hot seasons. There is therefore need to increase greenery especially evergreen tree species with dense foliage crowns, as well as spreading, rounded and vase canopy forms within the urban open spaces. This will help regulate climatic factors and enhance the thermal comfort of the city’s inhabitants. Additionally, during plant selection, Nairobi’s landscape planners should consider native tree species that are resistant to the unfavorable urban environmental conditions. Such measures should be used to influence relevant policies within the local government and to sensitize other stakeholders with interests in the urban environment and greening of Nairobi city.

This study was supported by ER-Africa (LOCLIM-3 Project) through NRF-Kenya. We thank Nairobi City County through the Department of Environment for issuing us with permits to conduct the surveys. We are also grateful to the staff of JKUAT and Freie University Berlin (FUB) who assisted in survey campaigns.

The authors declare no conflicts of interest regarding the publication of this paper.

Onyango, S. A., Mukundi, J. B., Adimo, A. O., Wesonga, J. M., & Sodoudi, S. (2021). Variability of In-Situ Plant Species Effects on Microclimatic Modification in Urban Open Spaces of Nairobi, Kenya. Current Urban Studies, 9, 126-143. https://doi.org/10.4236/cus.2021.91008