The organisms commonly seen growing on tree bark, often mistaken for moss or a disease, are actually lichens. These fascinating life forms appear as crusty patches, leafy structures, or miniature shrub-like growths clinging to the surface of trunks and branches. Lichens are complex entities that thrive in a wide variety of environments, from arctic tundras to tropical forests. Understanding why they appear on trees requires looking closely at their unique biological composition and the specific requirements for their survival.
The Symbiotic Nature of Lichen
Lichen is not a single organism, but rather a successful partnership between two or more distinct life forms existing in a close, mutually beneficial relationship called symbiosis. The body of the lichen, known as the thallus, is primarily composed of the fungal partner, referred to as the mycobiont. The mycobiont provides the physical structure, offering a stable and protective shelter for its partner organism.
The second component is the photosynthetic partner, the photobiont, which is typically a green alga or a cyanobacterium. This partner is responsible for generating food for the entire composite organism through photosynthesis, converting light energy into carbohydrates utilized by the mycobiont.
This arrangement allows lichens to colonize habitats where neither partner could survive alone. The fungus actively absorbs moisture and minerals from the air and rainwater, while shielding the delicate photobiont cells from intense sunlight and desiccation.
How Lichen Colonizes Tree Bark
Lichen colonization begins when reproductive structures successfully settle on a stable section of tree bark. Dispersal can occur through two primary methods: sexual and asexual reproduction. The fungal partner can reproduce sexually by releasing microscopic spores from specialized cup-like or disk-like structures on the thallus.
However, for a new lichen to form via sexual spores, the newly germinated fungus must quickly encounter a compatible free-living photobiont in the environment, which is often a low-probability event. A more efficient and common mode of dispersal is asexual reproduction, which ensures both partners are distributed together. This involves the formation of tiny, dust-like fragments called soredia or small, fragile outgrowths called isidia.
Soredia are minute clusters of algal cells wrapped in fungal hyphae, easily carried by wind or water to new locations. Isidia are larger, specialized thallus fragments that break off for mechanical dispersal. Colonization requires only a firm, non-growing surface for attachment and access to moisture.
Lichen’s Relationship with the Tree
A frequent concern among those who notice lichens on their trees is whether the growth indicates or causes disease or decline. Lichens are classified as epiphytes, meaning they utilize the tree purely for physical support, much like a vine uses a trellis. They do not possess roots that penetrate the inner bark or wood tissues to extract water or nutrients, which distinguishes them from parasitic organisms.
The attachment structures, known as rhizines in some species, are superficial and serve only as anchors to the outer layer of the bark. Lichens obtain all necessary minerals and water directly from the atmosphere, absorbing moisture from fog, dew, and rain that washes over the bark surface. Their non-parasitic nature means they do not drain the tree’s resources or interfere with the transport of water and sugars within the tree.
Heavy lichen growth is often an indicator of a tree that is already under stress, rather than the cause of the problem. A tree with a thinning canopy due to age, disease, or root issues allows more sunlight to reach the trunk and branches, creating a more favorable environment for the light-requiring lichens.
Environmental Factors Driving Lichen Growth
The distribution and abundance of lichen on trees are strongly influenced by specific environmental conditions. Moisture and humidity are perhaps the most influential factors, as lichens lack a protective outer layer to control water loss and must absorb water directly whenever it is available. They flourish in areas with frequent rainfall, high humidity, or persistent fog, which allows them to quickly become metabolically active.
Light exposure is another significant determinant; while lichens are slow-growing, the photobiont component requires sufficient light for photosynthesis. This explains why lichen growth is often densest on the upper sides of branches, the sunny side of the trunk, or on trees located in open areas. The physical characteristics of the bark also play a role, as rough, stable, or slow-shedding bark provides a better, long-lasting surface for attachment compared to smooth, rapidly exfoliating bark.
Lichen sensitivity to air quality is a well-documented ecological phenomenon. They draw everything they need from the atmosphere, which makes them highly vulnerable to gaseous pollutants, particularly sulfur dioxide. In areas with high levels of industrial pollution, lichens are often absent or represented only by a few highly tolerant species. Therefore, a diverse and thriving lichen community on tree bark is often a reassuring indicator of clean, unpolluted air quality.