The appearance of brightly colored or crusty growths on tree bark often raises a question: is this organism harming the tree? Lichen is a composite organism frequently found on trunks and branches across the world. Lichen does not kill trees or cause them to decline in health. It is a benign growth that uses the tree purely as a surface for attachment and stability.
Is Lichen a Parasite
Lichen is not a parasitic organism, which means it does not derive its sustenance from the living tissues of the tree. True parasites, like mistletoe, penetrate the host’s vascular system to steal water and nutrients directly, causing harm to the plant. Lichen lacks the specialized structures necessary to invade a tree’s bark or wood.
The organism functions as an epiphyte, a composite organism that grows harmlessly on another plant. Lichen only uses the tree for physical support, much like it grows on rocks, fences, or old tombstones. Its attachment to the tree’s outer bark is superficial, consisting of small fungal threads. These threads do not penetrate the inner bark where the tree transports water and sugars.
The relationship is considered commensal, meaning the lichen benefits from a stable, elevated substrate while the tree is neither helped nor harmed. The presence of lichen does not indicate a pest infestation or disease requiring chemical treatment or removal. Attempting to remove the growth by scraping or chemical application can damage the delicate outer bark. This damage creates entry points for true pathogens.
The Unique Biology of Lichen
Lichen is an example of symbiosis, representing a partnership between at least two different organisms that live together. This composite structure, known as a thallus, is formed by a fungus (the mycobiont) and a photosynthetic partner (the photobiont). The photobiont is typically a green alga or a cyanobacterium that produces food through photosynthesis.
The mycobiont, which makes up the bulk of the lichen structure, provides protection and shelter for the photobiont. It absorbs moisture and mineral nutrients directly from the air and rainwater, not from the host tree. The fungus benefits from the carbohydrates supplied by the photosynthetic partner, while the photobiont gains a stable environment protected from drying out.
The absence of roots or a vascular system means the lichen is entirely self-sufficient, relying on atmospheric sources for survival. This strategy allows lichen to colonize substrates that offer no nutritional value, such as bare rock. The growth is exceptionally slow, sometimes only 1 to 2 millimeters per year. This slow growth explains why it does not overwhelm a healthy tree.
What Lichen Presence Reveals About Tree Health
While lichen is harmless, a sudden or abundant proliferation can indicate underlying issues with the tree. Lichen requires ample sunlight to thrive due to the photosynthetic activity of its photobiont component. Healthy, vigorous trees typically have dense canopies that shade the trunk and lower branches. This shading creates an unsuitable, low-light environment for lichen establishment.
If a tree is stressed due to factors like drought, disease, or root damage, its canopy may thin out, allowing more light to reach the bark. This increased light exposure provides the conditions for lichen to colonize and grow more noticeably. The lichen itself is not the cause of the tree’s decline; rather, the tree’s poor health creates an opportunity for the lichen to flourish.
The presence of lichen serves an ecological function as a bio-indicator of air quality. Lichen absorbs everything from the atmosphere, and many species are sensitive to air pollutants such as sulfur dioxide. Finding a variety of healthy lichen species suggests the local air is relatively clean and free from contamination. Homeowners should view its abundance as a cue to assess the overall health of the tree for other stressors.