Lichen are colorful, textured organisms commonly seen clinging to the bark of trees, surfaces of rocks, and bare soil. Their presence on a tree results from a highly specialized internal biology and precise external environmental needs. This article explains the unique biological partnership and the environmental factors that enable lichen to flourish on tree bark.
The Symbiotic Partnership Defining Lichen
Lichen is a stable, composite association between a fungus (the mycobiont) and one or more photosynthetic partners (the photobiont). The mycobiont, typically an ascomycete fungus, forms the vast majority of the structure, providing the physical body or thallus.
The photobiont is an alga or cyanobacterium that uses chlorophyll to produce sugars through photosynthesis. This food source is transferred to the fungus, which is heterotrophic and cannot produce its own nutrients. In return, the fungal filaments form a protective outer layer, shielding the photobiont cells from intense sunlight and desiccation.
This mutually beneficial arrangement allows lichen to colonize habitats where neither partner could survive independently. The fungus gains a steady food supply, while the photosynthetic partner gains a stable, moisture-retaining shelter and access to minerals. This biological resilience explains how lichen can thrive in nutrient-poor locations like bare tree bark, which does not provide sustenance but offers a stable platform.
Lichen’s Non-Parasitic Relationship with Trees
The growth of lichen on a tree often causes concern, as people mistakenly assume it is parasitic and harming the host plant. However, lichen is classified as an epiphyte, meaning it uses the tree solely for physical support. It secures itself to the outermost layer of the bark without penetrating or drawing resources from the tree’s internal tissues.
Lichen does not possess a vascular system, like the roots and stems of plants, and therefore cannot tap into the host tree’s xylem or phloem to steal water or nutrients. It secures its position using small, root-like structures called rhizines, which function only as anchors. These rhizines are superficial and do not penetrate the inner, living layers of the tree bark.
The presence of lichen indicates that the tree is simply providing a stable, upright surface that raises the lichen toward the sunlight. Lichen often appears more abundant on slow-growing or declining trees because a thinning canopy allows more light to reach the trunk and branches. The lichen is not the cause of the tree’s decline but is merely taking advantage of the increased light exposure.
Environmental Conditions Favoring Lichen Growth
Lichen’s primary reason for growing on trees is to gain height, but its ability to survive there is governed by external atmospheric conditions. Unlike plants, lichen cannot regulate its water content and absorbs moisture directly from the air, dew, fog, or rain. This makes high ambient moisture and consistent humidity a requirement for many species to be metabolically active.
The organisms acquire all necessary nutrients, including minerals, from the air and precipitation, which is why environmental quality is a major factor in their location. They are highly sensitive to air pollution, particularly sulfur dioxide, which can disrupt the delicate balance of the symbiotic relationship. The absence of diverse lichen species in an area often signals poor air quality, making them reliable environmental bio-indicators.
Lichen species display varying preferences for light exposure, which determines their specific placement on a tree. Some species prefer the bright, sunny conditions found on the upper branches or south-facing sides of a trunk, while others require the shade of the forest understory. Furthermore, the chemical composition of the bark, specifically its acidity or alkalinity (pH), can affect which species of lichen can successfully colonize a particular tree.
The Ecological Importance of Arboreal Lichen
Once established on a tree, lichen provides several benefits to the surrounding ecosystem, contributing to local biodiversity. Lichen serves as a food source for numerous animals, including deer, elk, and flying squirrels, particularly during winter months when other forage is scarce. Various invertebrates, such as mites and springtails, also consume the organisms.
The physical structure of arboreal lichen offers shelter and nesting materials for wildlife. Many bird species, including hummingbirds, utilize the soft, fibrous thalli to construct and camouflage their nests. For small insects and other invertebrates, the dense growth provides protective cover from predators and the elements.
Furthermore, some cyanolichens (which contain cyanobacteria as their photobiont) contribute significantly to nutrient cycling by fixing atmospheric nitrogen. When these lichens decompose or wash off the tree, this fixed nitrogen becomes available to other plants in the ecosystem. The growths also help retain water on the tree’s bark, slowing runoff and prolonging moisture availability.