What Lichen Is
Lichens represent a unique biological partnership. They are a symbiotic association between a fungus (mycobiont) and a photosynthetic partner (photobiont), typically green algae or cyanobacteria. The fungal component provides the structural body, or thallus, offering protection from environmental factors like high light levels and desiccation.
The photosynthetic partner, residing within this fungal structure, produces carbohydrates through photosynthesis, which serves as a food source for the fungus. Lichens are distinct from plants, fungi, or algae, relying on this specialized arrangement for their existence. Their composite nature means identification relies on recognizing their forms and characteristics.
Visual Characteristics
Identifying lichen often begins with recognizing its growth form. Crustose lichens, for instance, form a flat, paint-like layer that adheres tightly to the surface, making them difficult to remove without damaging the substrate. Their texture can be smooth, cracked (rimose), or divided into distinct areas (areolate).
Foliose lichens, on the other hand, appear leafy, with flattened lobes that can be loosely or tightly attached, often having a distinct upper and lower surface. They may be anchored by root-like structures called rhizines.
Fruticose lichens are three-dimensional, resembling small shrubs, threads, or branches that can stand erect or hang. These typically have a uniform color throughout their cylindrical or flattened branches, lacking distinct upper and lower surfaces.
Color is another visual characteristic, though it can vary with moisture levels and light exposure. Lichens display a wide spectrum of colors, including shades of gray, green, yellow, orange, and brown. For example, the presence of usnic acid often gives lichens a pale yellow-green hue. Surface textures can range from powdery or granular, as seen in soredia, to warty or bumpy.
Reproductive structures also aid in identification. Apothecia are common sexual fruiting bodies that appear as disc- or cup-shaped structures. Perithecia are another type of sexual structure, typically flask-shaped and partially embedded within the thallus. Asexual reproductive structures include soredia, which are powdery or granular bundles of fungal hyphae and algal cells that can disperse to form new lichens. Isidia are finger-like or branched outgrowths from the thallus surface that can break off and establish new growths.
Habitat and Environment Clues
The environment and specific substrate where a lichen grows provide context for identification. Lichens can colonize a wide array of surfaces, including tree bark, rocks, soil, and decaying wood. Some species prefer specific substrates, such as alkaline or acidic surfaces. The presence of certain lichens can even indicate the substrate’s chemical composition.
Environmental factors such as light exposure and moisture levels also influence lichen presence and appearance. Lichens require light for the photosynthetic activity of their algal or cyanobacterial partners, leading to their presence on exposed surfaces. They can absorb water directly from the air, and many species flourish in humid or foggy areas. Air quality is another environmental indicator, as some lichens are highly sensitive to pollutants like sulfur dioxide, and their presence or absence can signal the health of the surrounding air. Observing these habitat details can narrow down identification possibilities.
A Step-by-Step Approach
Identifying lichen involves a systematic observation process. Begin by carefully observing the lichen using a magnifying glass to discern fine details of its growth form, color, and surface features. Pay close attention to the shape of the thallus, its attachment to the substrate, and any visible reproductive structures.
Document your observations thoroughly by taking clear photographs and detailed notes, including the specific substrate, location, and appearance of the lichen. Note any variations in color or texture, especially when wet versus dry, as moisture can alter a lichen’s appearance. Once detailed observations are recorded, compare them with descriptions and images found in specialized field guides, online databases, or identification applications.
It is helpful to learn how to distinguish lichens from other organisms they might resemble, such as mosses, algae, or fungi. Mosses are plants with distinct stems and leaves, unlike lichens, which are a symbiotic association and lack true leaves. Algae typically appear as slimy mats, especially when wet, while fungi often form more loosely woven patches.