Ailing trees often acquire a blue-green growth on their bark that appears to be a fungus. Although they may look menacing, they are actually harmless organisms called lichens. Lichens usually appear on trees in declining health, and their appearance indicates that the tree is damaged or suffering from various environmental stresses. Lichens may not be the cause of a tree's ailments, but without proper care they can contribute to its death.

Lichen is growing on the bark of tree trunks in a redwood forest.

What are Lichens?

Lichen covers a large stone on the forest floor.

A lichens is an organism made up of an alga and a fugus that work together to help each other grow and thrive, since neither partner can survive alone. The fungus is largely responsible for the hard structure of the lichen, called the thallus, which may be flat (crustose), leafy (folicose), or hair-like (fruiticose). The lichen's thallus protects the alga from drying out while it produces food and vitamins for the organism through photosynthesis. In return, the fungus collects minerals and water for the organism from the surrounding air. Lichens come in different forms and colors and can live on trees, shrubs, soil, rocks and other matter. The organism created through this symbiotic relationship is hearty and resilient and can survive in almost any environment.

How Lichens Grow

A bird is perched on a lichen covered tree branch.

Lichens are spread in a couple of ways. Pieces of the thallus can be broken off and carried by wind or water or passing animals. Once it is re-deposited, the organism will continue to grow and spread in its new environment if the conditions are right. Because part of the organism is a fungus, it has the ability to create and release spores. Although some spores pick up cells from surrounding algae as they are released, most spores must find an algal partner after they land. If the spores land near algae that is compatible, they will likely form lichens. Lichens live on surfaces that are fully exposed to the sun. Though they are highly adaptable, they need good air quality to grow and will not thrive in areas with heavy smog or pollutants.

Common Tree Ailments

Sunlight shines through the canopy of trees and casts shadows on the ground.

Lichens usually begin growing because the tree canopy starts to thin and let sunlight through. Trees that have a thinning canopy are usually suffering from poor environmental conditions. This could be caused by poor soil quality or water drainage, lack of sun or nutrients, pollution, or over-pruning. A thinning canopy hinders photosynthesis, which can cause the tree to starve and prevent new growth from forming. Although lichens do not cause tree ailments alone, an overabundance of lichens can produce excessive shading that stunts new growth. They can also cover large areas of tree bark and hinder gas transference, which can lead to the further decline of a tree.

Protecting Trees from Lichen

A tril meanders through the woods.

The best way to prevent lichen is to boost the health of the tree. Soil and water management, along with proper fertilization, can promote a thick, healthy canopy. This is usually all that is needed to deter lichen growth. If trees have trouble developing new growth, light pruning can sometimes promote the development of fresh shoots. Over-pruning can damage trees and hinder development, so care should be taken to prune trees only as needed.