Bark, soil and lichens are effective indicators of dust from limestone industries in Thailand

Cement production, quarrying and stone crushing are major emission sources of alkaline dust that can affect human health and vegetation. The main objectives of this study were to evaluate the potential of using bark pH, soil pH and lichen community for indicating alkaline dust pollution. Twelve polluted sites were in a limestone industrial area. Bark pH and the lichen community were observed on Alstonia scholaris trees, and soil pH was obtained from topsoil samples. The bark pH at all polluted sites was significantly higher (5.5 to 7.3) than that at the unpolluted site (4.3). Among the polluted sites, the highest bark pH value was observed at the nearest site to the center of the industrial area, while the lowest value was discovered at the farthest site. Bark pH showed a strongly negative correlation with the distance from the center. Soil pH at the unpolluted site (6.3) was also significantly lower than that at the polluted sites (7.6 to 8.1), except at the farthest site (6.5). The soil pH also tended to increase closer to the center. Seven lichen species were observed on the trunks of investigated trees in all polluted sites and were observed only at sites more than 4.7 km away from the center, where bark pH ranged from 5.5 to 6.3. The extent of dust impact on vegetation seemed to be within 6–7 km from the center. The results of this study confirm the potential of the bark pH of A. scholaris , soil pH and lichen community as long-term indicators of alkaline dust pollution.