Fungi and Forests

Mushrooms are the above-ground fruiting bodies of certain types of fungi. Fungi are diverse and adaptable, and they live in many different environments. Many fungi live in the soil, where their threadlike filaments fan out and tangle together into cords through the dirt, creating paths for interspecies interactions (interactions between different species). Many people think fungi are plants, but they are actually closer to animals. Fungi do not make their food from sunlight, as plants do. Like animals, fungi must find something to eat. Yet fungi obtain not only food for themselves but also make nutrients for other organisms. This occurs because fungi have digestion that is extracellular (taking place outside the cells). They excrete digestive acids outside their bodies to break down organic materials into nutrients. It is as if they had everted (inside out) stomachs, digesting food outside instead of inside their bodies. Nutrients are then absorbed into their cells, allowing the fungal body to grow-but also other species' bodies. The reason there are plants growing on dry land rather than just in water is that over the course of Earth's history, fungi have released digestive chemicals that break down rocks, making mineral nutrients available for plants. Fungi (together with bacteria) make the soil in which plants grow. Fungi also break down wood. Otherwise, dead trees would stack up in the forest forever. Fungi break them down into nutrients that can be recycled into new life. Fungi are thus world builders, shaping environments for themselves and others.

Some fungi have learned to live in intimate associations with plants, and given enough time to adjust to the interspecies relations of a place, most plants enter into associations with fungi. Endophytic and endomycorrhizal fungi live inside plants. Many do not have fruiting bodies. We are likely never to see these fungi unless we peer inside plants with microscopes, yet most plants are thick with them. Ectomycorrhizal fungi, on the other hand, wrap themselves around the outsides of roots as well as penetrate between their cells. Many of the favorite mushrooms of people around the world-porcini, chanterelles, truffles, and matsutake-are the fruiting bodies of ectomycorrhizal fungi associated with plants. They are so delicious, and so difficult for humans to manipulate, because they thrive together with host trees. They come into being only through interspecies relations.

The term "mycorrhiza" is assembled from Greek words for "fungus" and "root"; fungi and plant roots become intimately entangled in mycorrhizal relations. Neither the fungus nor the plant can flourish without the activity of the other. From the fungal perspective, the goal is to get a good meal. The fungus extends its body into the host's roots to remove some of the plant's carbohydrates through specialized structures, made in the encounter. The fungus depends on this food, yet it is not entirely selfish. Fungi stimulate plant growth in two ways: first by getting plants more water, and second by making the nutrients of extracellular digestion available to plants. Plants get calcium, nitrogen, potassium, phosphorus, and other minerals through mycorrhiza. Forests, according to researcher Lisa Curran, occur only because of ectomycorrhizal fungi. By leaning on fungal companions, trees grow strong and numerous, making forests.

Mutual benefits do not lead to perfect harmony. Sometimes the fungus parasitizes the root in one phase of the plant's life cycle, exploiting the root for nutrients but giving nothing in return. Or if the plant has lots of nutrients, it may reject the fungus. A mycorrhizal fungus without a plant collaborator will die. But many ectomycorrhizas are not limited to one collaboration; the fungus forms a network across plants. In a forest, fungi connect not just trees of the same species, but often many species. If you cover a tree in the forest, depriving its leaves of light and thus food, its mycorrhizal associates may feed it from the carbohydrates of other trees in the network. Some commentators compare mycorrhizal networks to the Internet, writing of the "woodwide web," as mycorrhizas form an infrastructure of interspecies interconnection, carrying information across the forest. They also have some of the characteristics of a highway system. Soil microbes that would otherwise stay in the same place are able to travel in the channels and linkages of mycorrhizal interconnection. Some of these microbes are important for the removal of contaminants from soil. Mycorrhizal networks allow forests to respond to threats by, for example, mobilizing bacteria that degrade pollutants.