An Effect of Urban Forest on Urban Thermal Environment in Seoul, South Korea, Based on Landsat Imagery Analysis

Peter Sang-Hoon Lee,Jincheol Park

doi:10.3390/f11060630

Abstract

The urban heat island effect has posed negative impacts on urban areas with increased cooling energy demand followed by an altered thermal environment. While unusually high temperature in urban areas has been often attributed to complex urban settings, the function of urban forests has been considered as an effective heat mitigation strategy. To investigate the cooling effect of urban forests and their influence range, this study examined the spatiotemporal changes in land surface temperature (LST) of urban forests and surrounding areas by using Landsat imageries. LST, the size of the urban forest, its vegetation cover, and Normalized Difference Vegetation Index (NDVI) were investigated for 34 urban forests and their surrounding areas at a series of buffer areas in Seoul, South Korea. The mean LST of urban forests was lower than that of the overall city, and the threshold distance from urban forests for cooling effect was estimated to be roughly up to 300 m. The group of large-sized urban forests showed significantly lower mean LST than that of small-sized urban forests. The group of urban forests with higher NDVI showed lower mean LST than that of urban forests with lower mean NDVI in a consistent manner. A negative linear relationship was found between the LST and size of urban forest (r = −0.36 to −0.58), size of vegetation cover (r = −0.39 to −0.61), and NDVI (r = −0.42 to −0.93). Temporal changes in NDVI were examined separately on a specific site, Seoul Forest, that has experienced urban forest dynamics. LST of the site decreased as NDVI improved by a land-use change from a barren racetrack to a city park. It was considered that NDVI could be a reliable factor for estimating the cooling effect of urban forest compared to the size of the urban forest and/or vegetation cover.

Highlights

The urban heat island (UHI) effect describes an urbanization-driven phenomenon where the ambient temperature is higher in built-up areas than surrounding rural areas at night.It can result from a variety of different urban settings including anthropogenic influence, microclimate, and geographical and environmental factors, similar to other urban issues [1,2,3,4]
A negative linear relationship was found between the land surface temperature (LST) and size of urban forest (r = −0.36 to −0.58), size of vegetation cover (r = −0.39 to −0.61), and Normalized Difference Vegetation Index (NDVI) (r = −0.42 to −0.93)
Urban forests are an assemblage of a variety of tree species, which can meteorologically affect the thermal environment in urban areas by providing a divergent outflow of cool air toward surrounding areas at surface

Summary

Introduction

The urban heat island (UHI) effect describes an urbanization-driven phenomenon where the ambient temperature is higher in built-up areas than surrounding rural areas at night. It can result from a variety of different urban settings including anthropogenic influence, microclimate, and geographical and environmental factors, similar to other urban issues [1,2,3,4]. Forests 2020, 11, 630 level and replacing the hotter surfaces of urban features such as buildings and roadways with green canopies [1,8] Such beneficial functions expected from the presence of urban forests can become less effective when urban forests are replaced with or disturbed by urban features over time. Mitigation performances of tree species on cooling surface temperature may change during and after disturbance occurred by anthropogenic activities, and urban forests can be more susceptible to such changes due to their location in urban settings

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