City Trees

in our modern world. It is released by the burning of fossil-fuels, such as from coal-burning power plants, auto- mobile emissions, and some industrial processes. Sulfur dioxide combines with atmospheric moisture to produce sulfurous acid (H 2 SO 3 ) or sulfuric acid (H 2 SO 4 ), a compo- nent of acid rain. When absorbed into lichen thalli by air or raindrops, these compounds are quite harmful. Sulfur dioxide's primary injury to lichen is its ability to degrade or destroy chloro- phyll, weakening the union and often leading to death for the species that cannot avoid or tolerate exposure. The damage caused by sulfur dioxide is most severe where moisture levels are high and pH is low. Low pH caused by acidifying acid rain is an exacerbating feedback mecha- nism. Trees have different average pH levels of their bark, and pH sensitive epiphytic lichens will be injured at dif- ferent rates depending on the trees on which they grow. Lichens growing on birches and conifers, which have more acidic bark, generally decline faster than lichen growing on oaks and sycamore, with intermediate acidity, or the more alkaline elms. Sulfur dioxide can also inhibit cellular respiration for both the fungal and algal partners and can reduce both sexual and asexual reproduction rates, lead- ing to population declines. Since some varieties of lichen are more sensitive to the effects of sulfur dioxide than others, these differences can offer greater precision in the use of lichens as bio- monitors. Surveys of lichen distribution in industrial or urban areas have helped to generate maps of air pollution concentration, especially when compared with historical lichen records or surveys from less polluted regions. At the heart of a polluted area, where toxins concentra- tions are high, lichen will be absent. Just outside of these centers, pollution tolerant species will be found, and fur- ther still from the centers, decreased levels of atmospher- ic contaminants can be deduced from increased diversity of lichen species. A point scale system for sulfur dioxide contamination based on this aspect of lichen diversity, devised by Hawksworth and Rose in 1970, has been used to map pollution levels in the U.K. with high accura- cy. England, like many densely-populated, industrialized nations, has lost many of its epiphytic lichen due to acid rain and atmospheric sulfur dioxide. Other pollutants, such as carbon monoxide, fluoride, agricul- tural chemicals such as copper-containing anti-fungal sprays, and radiation, are similarly injurious to lichens and decrease both their species populations and overall biodiversity. These humble creatures are excellent indicators of an eco- system's atmospheric health, and it would be wise for us to pay them more attention. Wherever you are in the world, the next time you step outside, see how long it takes before you notice one, and then another, and another over there. Consider their age, their strength, the tiny partner- ship within, and what other secrets they may hold. 35

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