Analysis of metal content and vertical stratification of epiphytic mosses along a Karst Mountain highway

Abstract

Road-based transport emissions are a major source of atmospheric metal pollution. However, there have been few studies on emissions from road traffic in mountainous areas. In this study, epiphytic mosses from trees at different elevations of a highway, a typical road with extraordinary elevation change in a mountainous area of karst in Guizhou, China, were analyzed for metal content as well as the spatial distribution pattern of metals. Mosses were sampled from three sections of highway at different elevations, from 1292–1357, 1394–1441, to 1481–1548 m. Principal component analysis and heat-map clustering were used to identify the principal factors affecting metal deposition. The results show that the metals of mosses from different elevations were divided into four factors. Group 1 which included Ni, Fe, Mg, Ba, and Al was attributed to a dominantly geogenic source. Group 2 included Zn, Cu, Mn, and Cr, from vehicle-related materials including tires and brakes. Group 3, Cd, can be attributed to high Cd background levels from local origins and traffic emissions, particularly tire wear. Group 4, Pb, is associated with brake wear and historical deposition. The epiphytic moss widely distributed in the study area, Ectropothecium aneitense Broth. & Watts, was used to analyze the spatial distribution pattern of the metals. Metal content gradually decreased with increase in elevation. Levels of Ni, Fe, Mn, Ba, and Cd were all significantly correlated with elevation (p < 0.05), simultaneously affected by terrain and vertically stratified. We highlighted the vertical distribution characteristics of metal in epiphytic mosses in this study, which could improve moss application for ecological monitoring due to road-based transport emissions with elevation changes.