Green Gardening Matters: Lichen: a three-way symbiotic organism

Last week, a neighbor knocked on our door. He was holding a twig covered with lichens. He wanted to know what heck they were and how could he get rid of them, because they were killing his tree. I explained that no, these were not killing his tree, but because the twigs were bare, their spores had germinated there to take advantage of the good light (without the leaves). He's not alone in thinking the the lichens are causing harm to trees, so I thought I should provide some needed background.

First, some bark botany: The function of bark on a tree is to protect the living tissue, the cambium layer of phloem and xylem, that lies just inside the bark. It's this living tissue that produces the annual rings of the wood in the tree. The bark contains complex, often resinous, chemicals that provide a waterproof layer of protection. The trees do not lose or absorb water or water soluble chemicals through the bark. They could ooze resins and in fact rubber trees are good example of how the resins are waterproof. When rubber trees are tapped, it's done very carefully so that some of the bark remains intact so the cambium layer is not disturbed.

So, as I explained to my neighbor, lichens are opportunistic organisms and occurred on that branch because the tree had lost its leaves. It had not caused the tree to lose its leaves and that these organisms are not parasites, but are epiphytes and do not take nutrients or moisture from their host plants. The lichens actually provide additional protection for the cambium layer so they do no damage. In fact lichens often occur on rock surfaces, which would not provide any nutrients or water. Lichens are pioneer organisms and quite often are the first life after volcanic eruptions and lava flow.

How do they withstand such harsh conditions?

Lichens are a result of a symbiotic relationship of a fungus, a photosynthetic partner, either green algae or cyanobacteria, or both, and as recently discovered, bacteria. The algae are green plants in this partnership and photosynthesize to provide nutrients for its fungal host. In turn, fungi provide shelter for the algae so it can survive in harsh conditions. We’re not really sure how the bacteria fit into the partnership, but maybe they help to break up the rock or fix nitrogen from the air.

The evolutionary result of the self-sustaining partnership is a unique joint structure, the lichen thallus, which is indispensable for fungal sexual reproduction. The lichen spores include those for all three symbionts: the fungi, the algae, and the bacteria.

Lichens come in many colors, sizes, and forms and sometimes look like plants, but they're not plants. They are generally classified by the fungal shape:
- Fruticose when they have tiny, leafless branches.
- Foliose when they have flat leaf-like structures.
- Crustose when they have flakes that lie on the surface like peeling paint.
- Leprose when they have a powder-like appearance
- and other growth forms.