City Trees

are at a strong disadvantage—on cold coastal jetties, on bare Arctic rock, in the marginal extremes. Being small, slow, and physically fragile, how can they defend themselves? Like many creatures, lichen employ chemical warfare. Lichen fungi produce a unique phar- macopoeia of compounds, ones that they cannot produce without their photobionts, and these chemicals provide extra protection for the union. Some lichen manufacture their own antibiotics; some produce allelopathic com- pounds that can inhibit the growth of soil fungi and the seeds of vascular plants. Some, such as xanthones and pulvinic acid, are concentrated in the cortex of the thallus to offend the tastes of invertebrate browsers, and some compounds seem to act as sunscreens for the photobiont. In addition to these marvels, lichen can proudly wear the mantle of "rock-breakers." Many, particularly the crustose lichens, can produce weathering agents such as carbonic acid, which can interact with minerals or organic com- pounds in rock and wear them away. Lichen are powerful terraformers, but in deep time, weathering rock bit by tiny bit and over centuries, building soil. Many lichen species of all body forms, are epiphytes, or tree-dwellers. For some, the bark of trees is their exclu- sive habitat. Lichen can appear to be more abundant on dead or dying trees and are sometimes blamed for the decline of the tree, though they pose no threat to it. The hyphae of tree-dwelling lichen grow only into the outer, dead bark cells and do no damage to the trees on which they grow. Ailing or dead trees will have fewer leaves, thereby letting more light reach lichens on the trunk and branches, fueling their growth. Also, when trees are impaired, their bark grows more slowly and bark cells are sloughed off less frequently, allowing for larger, more con- tinuous lichen communities. Though the lichen union confers several benefits on its partners, it also makes the unit vulnerable if one of the partners is seriously impaired. Lichen have no true roots; they absorb water and dissolved minerals directly through their outer cortex, right through the walls of their thalli. Because of their minimal filtration mechanisms, they are very sensitive to environmental pollution. Air pollution has heavily impacted lichen diversity, particularly in cit- ies, though the effects have been seen in all parts of the globe. Lichen tend to accumulate minerals in their tissues, rather than excreting them, which has made them useful gauges of atmospheric and rain-bound pollutants, as well as radiation. Sulfur dioxide, in particular, has negatively affected lichen populations worldwide. Though found naturally in the atmosphere in low concentrations, sulfur dioxide is an environmental pollutant in the concentrations present (top) Common greenshield (Flavoparmelia caperata) from kindofcurious.com (right) Xanthoria sp. by Francis Gwyn Jones, Bugwood.org (bottom) Jason Sharman, Vitalitree, Bugwood.org 34

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