Abstract—Use of vegetation as a bioindicator of atmospheric heavy metal accumulation has received more attention worldwide due to the fact that this method has been found to be effective, cheap and simple to use. This study aims to find out the levels of heavy metals found in tree bark layers and to test the factors (exposed sides of tree to traffic and size of tree trunk) that affect atmospheric heavy metal accumulation in the bark of Cassia fistula, which is a common tree commonly found along the road sides and planted for shading and decoration. In order to provide the necessary information with regard to using a native tree species as a bioindicator, heavy metals emitted from road traffic including Cr, Cu, Fe, Ni, Pb and Zn were investigated. The results showed that only Cu, Fe and Zn were detected. Their concentrations were highest in the cork layer, decreased in the second (chlorenchyma), third (phloem) layers and increased in the innermost layer (vascular cambium). Heavy metals found on the outer most layer of bark definitely came from atmospheric pollution due to direct exposure to the environment. However, the concentrations in the vascular cambium were higher than in the chlorenchyma and phloem. This is probably due to some diffusion that took place in the xylem, which is connected to the cambium layer. Concerning the factors affecting the heavy metal accumulation in the tree bark, the size of Cassia fistula tree trunks (5 - 30 cm) was not significantly correlated (p > 0.05) with the heavy metal concentrations found in the bark. This was also true of the sides of the exposure of the tree in relation to the traffic source. This means that the bark sampling can be done easily without any limitations with regard to the size of tree trunk and direction of exposure. Index Terms—Tree bark, heavy metal, bioindicator, Cassia fistula, atmospheric pollution. I. INTRODUCTION Heavy metals including Cr, Cu, Fe, Pb and Zn are important pollutants in urban areas and their levels are directly related to the number of motor vehicles present [1]; [2]. The most toxic form of heavy metals is likely found to be the atmospheric contaminants either because of the great quantity involved or the widespread dispersion of these contaminants, which may originate from many different exposure pathways [3]. Recently, the sheer volume of vehicles in Asian countries has been increasing annually. Consequently, the monitoring of heavy metals emitted through road transportation has become necessary, particularly in mega cities. The biomonitoring of tree bark is significant interest because it is cheap, easy and there are many choices of tree species. Examples of the trees species that have been successfully used to monitor the levels of aerial heavy metals were Fraxinus pennsylvanica [4], Pinus massoniana L. [5], Azadirachta indica [6], Casuarina equisetifolia and Delonix regia [7]. Since the most important criteria in the selection of a bioindicator is common occurrence [8], the species of the selected tree is specific to the study area in terms of the abundance of the tree species present. Moreover, most of the studies concerned with the use of trees as a bioindicator have been conducted in Europe and a number of other developed countries. In Southeast Asia and other tropical zones, the criteria used for the selection and the sampling of the trees in terms of environmental quality assessment is still fairly limited. This study aims to determine the levels of heavy metals present in the tree bark layers and to test the factors affecting heavy metal accumulation in the tree bark. Cassia fistula tree was selected for this study because it is a common tree species in Southeast Asia and the Indian subcontinent. In Thailand, this tree is widely found along roadsides and is often planted for the purposes of shading and decoration. The principle benefit of this study is to provide useful information regarding the sampling criteria for the use of native tree species as bioindicators of atmospheric heavy metal accumulation in tropical zones. II. MATERIAL AND METHOD A. Site Location The study area is in the City of Chiang Mai, a metropolitan city in the northern region of Thailand. Chiang Mai is considered a tourist city and has attracted over 7 million visitors annually [9].This urban area is rapidly growing, as is the number of registered vehicles in the urban center. The samples were collected from 20 roadside trees along two main roads (Manee Nopparat Road and Huay kaew Road) in the city of Chiang Mai. B. Sampling Tree and Method The trees with a measured diameter at breast height (DBH) ranging from 21 – 30 cm were selected. The samples were selected from roadsides with less than 3 m distance from the Levels of Road Traffic Heavy Metals in Tree Bark Layers of Cassia fistula Tree Rungruang Janta, Somporn Chantara, Angkhana Inta, Munetsugu Kawashima, and Kenichi Satake International Journal of Environmental Science and Development, Vol. 7, No. 5, May 2016 385 DOI: 10.7763/IJESD.2016.V7.805 Manuscript received April 20, 2015; revised June 26, 2015. Rungruang Janta and Munetsugu Kawashima are with Environmental Science Program, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand (e-mail: rrjanta@yahoo.com). Somporn Chantara is with Environmental Science Program, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. Somporn Chantara is also with Chemistry Department, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand (e-mail: somporn.chantara@cmu.ac.th). Angkhana Inta is with the Department of Biology, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. Kenichi Satake is with the Faculty of Geo-Environmental Science, Rissho University, Saitama 3600194, Japan.