If you saw Part I of this blog entry, then I know you have been waiting, patiently, I might add, for the second part. Well here it is: still no. They have no ability to cause disease or death of plants – no pathogenicity genes, no teeth, no stingers, no destructive enzymes. They are simply using the branch or tree on which they occur kind of like we use a sofa – as place to get comfy and hang out.
Lichens are not a single creature, but are wonderfully cooperative composites of multiple organisms that together make something completely different than the individual components. For many years it was known that lichens were dual organisms of a green alga and a fungus. More recently, it was discovered that some lichens are made of a cyanobacterium with a fungus, and a few lichens are composed of both the green alga, a cyanobacterial species, and a fungus. However, in the past decade, additional habitants have been found in the lichen body: nitrogen-fixing bacteria. These are not green, like cyanobacteria, but are similar to bacteria that live in soils and help plants to use nitrogen from the atmosphere for growth.
Who gets what out of this cooperative arrangement? The algae and cyanobacteria are capable of photosynthesis. The fungi provide environmental protection from the elements for the oxygen-producing smaller green partners, and the ability to capture mineral elements. The bacteria, in addition to providing nitrogen, also are capable of contributing other minerals, help with photosynthesis by providing essential compounds, support both fungal and algal growth by producing hormones, and help to degrade older parts of the lichen body.
Lichens can grow on any surface: stone, concrete, cars, sides of houses, windows, on the desert soil, and just about any surface that doesn’t get up and walk away.
Lichens are important in the Pacific Northwest as food, shelter, and nesting material for wildlife. They are a critical part of the forest ecosystem, providing, in part, nitrogen to old-growth forests as the lichen body degrades. Desert lichens form crusts that help to stabilize the soil and help to prevent surface water run-off when it does rain. Humans have traditionally used lichens as survival food, tanning agents, bedding, diapers, decorations, and for dyeing cloth and wool.
So don’t assume, if you see lichens on a plant that is dying, that the lichens had anything to do with it. They are just trying to find a place in which to manufacture their own food and hang for a while.
Key to lichens: Macrolichens of the Pacific Northwest. Second Edition 2009, by Bruce McCune and Linda Geiser, OSU Press, Corvallis, OR.