Lichens & Trees
Lichens are non-parasitic organisms that grow on rocks, gravestones, and woody plants, without causing any harm to them. They can have beautiful shapes and colors. If you take the time to look at them closely and study them, you will discover that they can have leaf-like or crust-like structures and may be attached loosely, very tightly, or even embedded. Homeowners often become concerned when they see large amounts of these growing on trees. They are worried that the lichen is affecting the health of the tree.
A lichen is a symbiotic relationship between two different organisms, a fungus, and an alga, where each contributes something in support of the other. The fungus is the dominant partner, providing the lichen it shape and fruiting bodies. The alga can be a green alga or a blue-green alga. These algae are both known as cyanobacteria and many lichens will have both types. The fungus does not contain chlorophyll or other means of producing its own food, so it relies on other organisms for nutrition. This is where the alga comes in. Because the alga can photosynthesize, it contributes sugars, starches, and other nutrients to the fungus and in return, the fungus collects water and minerals from the air and shares them with the alga.
When you see lichen growing on trees, it may be because the canopy is thin for some other reason, such as disease or insects, or abiotic stress from drought, compacted soil, or flooding. The Urban Forester in my office believes you see lichens growing on trees that are not thriving, again meaning the tree may be stressed in some way. Lichen prefers sunny areas with good air circulation, so plants with thin canopies provide excellent growing conditions. Consider that lichen could be an indicator to you that your tree is stressed and you should investigate to determine if there is a problem and then take steps to correct it if possible.