Cooling and humidifying effect of plant communities in subtropical urban parks

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Abstract

Urban vegetation has been proved to play an important role in mitigating the heat island effect. However, it is not clear how independent small-scale plant communities affected the microclimate. In this paper, the effects of fifteen plant communities on temperature and relative humidity were investigated from November 2010 to October 2011 in urban parks in subtropical Shenzhen City, China. The canopy density, canopy area, tree height and the background climate conditions under plant communities were measured. The effects of small-scale plant communities on temperature and relative humidity were the most significant at 1400–1500 h during the day. The temperature reduction and relative humidity increase due to small-scale plant communities were higher in summer, followed by autumn, spring and winter. As compared to the control open sites, the temperature reduction due to plant communities ranged from 2.14 °C to 5.15 °C, and the relative humidity increase ranged from 6.21% to 8.30%. We found that multilayer plant communities were the most effective in terms of their cooling and humidifying effect, while bamboo groves were the least effective. Regression results revealed that four factors, namely canopy density, canopy area, tree height and solar radiation, had significant influence on temperature reduction and relative humidity increase.

Introduction

With the rapid progress of urbanization, the urban heat island (UHI) is aggravated mainly because of the reduced density of the green vegetation and the loss of vegetative cover in the urban environment (Akbari et al., 1992, Antrop, 2000, Martinuzzi et al., 2007). Many studies have shown that trees can affect the air temperature and relative humidity through shading, transpiration and evaporative cooling (Hamada and Mikami, 1994, Rosenfeld et al., 1998, Dimoudi and Nikolopoulou, 2003, Georgi and Dimitriou, 2010). The importance of urban vegetation as a measure to mitigate heat island is well known (Oke, 1989, Chang et al., 2007, Shashua-Bar et al., 2010). It was reported, for example, that a 10% increase in vegetated surface reduced the temperature by 0.14–0.32 °C (Kuo, 2000).

Honjo and Takakura (1990) suggested that smaller green areas with sufficient intervals were preferable for effective cooling of microclimate to lumped larger green areas. Shashua-Bar and Hoffman (2000) reported that the cooling effect of small green areas (such as gardens and streets) was significant and developed the Green CTTC model to predict air temperature. Saito et al. (1990/1991) found that the air temperature distribution in an urban area was closely related to the distribution of green cover, and even a small green area of about 60 m × 40 m had the cooling effect. These studies clearly showed that it was important to investigate how the cooling and humidifying effect was affected by small plant communities.

Temperature reductions in urban green areas have been measured, particularly during summer (Shashua-Bar and Hoffman, 2002, Georgi and Zafiriadis, 2006, Potchter et al., 2006). Jauregui (1990/1991) measured the daily maximum and minimum temperature in Chapultepec Park (500 ha) in Mexico City, and found that daily minimum temperature was 3–4 °C cooler in the park. Measures made on clear nights showed that the effect of the park on air temperature was noticeable and its influence reached a distance about the same as its width (2 km). Chang et al. (2007) conducted a survey in 61 parks in Taipei city during the summer and winter. The results showed that air temperature in parks varied in different times and seasons, and the cooling effect of parks may be related to park characteristics, such as park size, trees and paved coverage. Shashua-Bar et al. (2009) found that the combination of shade trees over grass in a hot-arid region was the most effective landscape strategy, which can reduce air temperature by 2k. Bau-Show and Yann-Jou (2010) measured the microclimate conditions under ten tree species and two bamboo species, and found that tree foliage density, individual leaf thickness, leaf texture, and leaf color lightness had significant effect on cooling the shaded air.

The results from previous research on the cooling effect of urban green areas can support urban planning, urban design and the restoration of urban environments. However, to understand the cooling effect of urban green areas requires more data and long-term measurements, not limited to summer. Despite the fact that studies on the cooling effect of entire parks exist for some cities (Barradas, 1991, Upmanis et al., 1998, Yu and Wong, 2006, Hamada and Ohta, 2010), little work has been done on the microclimate of separate plant communities in subtropical areas. The present study seeks to evaluate the diurnal and seasonal variations in the cooling and humidifying effect of fifteen different small-scale plant communities (about 600 m2) in subtropical city of Shenzhen, China. We analyzed the differences between fifteen plant community in terms of their cooling and humidifying effect, and studied the relation between plant community characteristics and cooling and humidifying effect throughout a year to provide guidelines and theoretical reference for urban greening in urban areas to maximize its ecological function in terms of impacts on the microclimate.

Section snippets

Research site

Studies were conducted in Shenzhen, China (22°27′–22°52′ N and 113°46′–114°37′ E), a city with a population of 1.04 million. Shenzhen has typical subtropical oceanic features of a hot summer and mild winter. The annual average air temperature and annual average precipitation are 22.5 °C and 1966.5 mm, respectively. The prevailing wind direction is NE, and mean wind speed was 2.0 m/s. According to Meteorological Bureau of Shenzhen Municipality, the minimum monthly mean air temperature is 12.0 °C in

Diurnal and seasonal variations of the microclimate

The average temperature under the plant communities and the control sites averaged 23.8 °C and 27.5 °C, respectively, while the average relative humidity was 56.4% respectively 49.1%. It was clear that plant communities were cooler and more humid than the open sites. The diurnal variations of temperature (Fig. 3) and relative humidity (Fig. 4) were described as V-shaped curves. More specifically, in summer, temperature and relative humidity under the plant communities at 0800–0900 h were 27.8 °C

Discussion

In this study, we present empirical findings on the cooling and humidifying effect of 15 small-scale plant communities in urban parks. The plant communities had obvious cooling and humidifying effects during the entire year, although the effect varied in different times and different seasons. The effects of plant communities on temperature and relative humidity were the most significant at 1400–1500 h during the day. The temperature reduction and relative humidity increase were large in summer

Acknowledgements

This research was supported by National Key Technology R & D Program in the Eleventh Five-Year Plan of China (Issue No. 2006BAD07B09). The authors would like to thank the students of Beijing Forestry University who helped with field investigation and data collection.

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