Dr. Stephen Clayden, Curator of Botany, New Brunswick Museum
Lichens as indicators of air quality in the greater`Saint John region
Lichens occur on tree trunks, rocks, soil and other substrates in many kinds of ecosystems, including the simplified or constructed ones found within cities. As one approaches an industrialized city like Saint John from the surrounding countryside, a decrease in the diversity and coverage of lichens, especially of tree-inhabiting species, is evident. Many ecological and physiological studies have shown that such changes are due primarily to the sensitivity of lichens to air quality. The differential sensitivity of various lichens to sulphur dioxide (SO2), a harmful air pollutant originating from the burning of fossil fuels and from other industrial processes, has made it possible to develop lichen-based assessments of the severity of SO2 pollution in other regions.
This project is assessing the usefulness of lichens as indicators of air quality in Saint John and the surrounding area. Sites surveyed for lichens in this region by H. R. Hinds in 1975 and 1976 are being reexamined, and changes that have occurred over the intervening 30 years are being recorded and interpreted. Additional surveys have been undertaken with the goal of improving the baseline of data against which future changes in the diversity and distribution of lichens in the region can be gauged.
Work on the project began in 2004, and has so far involved three undergraduate summer students, Matto Mildenberger (University of Toronto), Aaron Granger (UNB), and Sarah Little (UNB). Among the key findings to date are:
1. The regional distribution of Lecanora conizaeoides, a non-native lichen relatively tolerant of air pollution, appears to reflect downwind dispersal of SO2 emissions from major point sources in and near Saint John. This lichen occurs upwards of 40 km downwind of the city centre in the direction of the prevailing winds (toward the northeast), but no more than 5 km upwind. There appears to be a pollution threshold, probably involving a critical level of sulfate deposition, which determines the regional presence or absence of L. conizaeoides on the naturally acidic bark of tree species such as red spruce, balsam fir, and heartleaf paper birch. Below this inferred level of deposition, the lichen is absent. The bark pHs of red spruce and heartleaf paper birch on either side of the threshold do not differ significantly from one another. That is, higher levels of sulfate deposition do not further lower the bark pH of trees with naturally acidic bark. Thus, these trees become locally suitable for colonization by L. conizaeoides not because of increased acidity, but because of sulfate deposition.
2. Elm trees outside the inferred area of higher sulfate deposition have a bark pH > 5.4, and a relatively high diversity of epiphytic lichens, including “cyanolichen” species known to be very sensitive to air pollution. Within the area of higher deposition, the pH range of elm bark is 3.6 to 4.4, and the diversity of epiphytic lichens is considerably lower. Elm bark appears to be poorly buffered against acidification resulting, plausibly, from sulfate deposition.
3. The lichen Hypocenomyce caradocensis is present sporadically on elm trees with acidified bark in Saint John and Halifax, NS. This species has not been recorded previously in North America, but is known to have invaded urban areas in part of northwestern Europe, where it occurs with Lecanora conizaeoides.