Highway pollution effects on microhabitat community structure of corticolous lichens

We studied lichen communities on bole and base tree trunk segments along forest edge-to-interior gradients on opposite sides of a major highway and a control site in central North Carolina, U.S.A., to investigate if these two communities differ and if so, do they differ in response to highway pollution. At each site we measured various environmental parameters including ambient air NO2 concentrations, and sampled lichens on 5-7 trees along each of five parallel transects established at the forest edge and at 25, 60, 100 and 150 m into the forest. We compared lichen communities between the two trunk segments via species richness and composition by habit, photobiont type, and reproductive strategy. We then ran dual (bole and base) NMS ordinations with subsequent correlation/regression analyses to explore/test relationships of lichen parameters with environmental variables among the 15 sample transects combined. Species richness was similar between trunk segments at transect and site levels as well as overall. Bole and base communities were more compositionally similar to each other at the highway sites than they were at the control site, based on Bray-Curtis similarity indices (BC). Tree base communities differed in terms of functional groupings, with greater proportions of squamulose, cyanolichen and sterile species than found in tree bole communities, but varyingly so among sites. Patterns of bole-base BC values with distance from the forest edge were not apparent in any of the sites. Ordination analyses resulted in Axis 1 representing most of the variation for each trunk segment. Along this axis, correlations were similar between boles and bases, with the strongest ones involving lichen species richness (negative) and NO2 concentrations (positive); notably weak correlations involved tree species number, canopy cover and DBH. Similar patterns were found when lichen species number was correlated with environmental parameters directly, with NO2 concentration correlating strongest at each trunk segment. Among functional groups, % crustose and % fertile species on bases correlated significantly with NO2. Lichen species-NO2 relationships on boles and bases were both found to be highly significant quadratic relationships with base lichen richness being stronger.