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Northern forests have their own kind of epiphytes, plants which grow not on the ground but in the tops of trees. Most North Americans would quickly recognize the epiphytes of tropical rain forests: showy, exotic plants like orchids and bromeliads that gather water and create their own tiny pools high in the branches. But how many of us know what grows in the high branches of our maples and Douglas firs? They are among the most important organisms of the world's harshest environments. Ironically, they are among the most sensitive to disturbance. Scientists can tell how mature a forest is by studying them. And when pollution reduces air quality, they are among the first things to disappear. My first awareness of lichen came when I was three and my brother got store-bought packages of the tangly stuff to use as bushes in the landscape of his model train set. It seemed like plastic or rubber; I never realized it was a living organism. Over the years I noticed lichens growing on the bark of woodland trees, but I never recognized their complexity and diversity until a storm brought down some branches near my family's cottage. One of the red maples on the property displays pale grey and bluish patches all the way up its slender trunk. They do not parasitize the tree or penetrate its bark, but simply take advantage of the bright and harsh microclimate high above the dark, damp understorey. No soft-tissued plant could endure the harsh winter winds and plummeting temperature (I've seen the thermometer dip to 40 below) that blast the naked treetops. And lichen, in fact, is not a plant. One of the most appealing and readable web sites I've discovered in a while is devoted to lichens. The North American Lichen Project contains pages of beautiful images of this most lowly of life forms taken by Sylvia Durann Sharnoff and Stephen Sharnoff. It is one of the best places to learn about lichens and their importance to the world's ecosystems. Lichen is a symbiont: two distinct organisms which must come together, and rarely if ever can survive as individual species. The main structure is provided by a fungus, which provides protection for algae which live inside. Algae can perform photosynthesis, using sunlight to produce their own food in the form of sugars. The fungus gets up to 70 per cent of the sugar produced. Some species of lichen rely not on algae but on cyanobacteria, which
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