The sight of green growths on tree trunks and branches is a common occurrence in many landscapes. While often broadly referred to as “moss,” these growths can actually encompass a variety of organisms, each with distinct biological characteristics. Understanding what these growths are and their relationship with the trees they inhabit can clarify whether they are a natural part of the ecosystem or a concern for tree health.
Moss, Lichens, and Algae
Many people confuse the different types of green, gray, or rust-colored growths found on tree bark. True mosses, lichens, and algae are distinct organisms with different biological compositions and growth habits. Distinguishing between them helps in understanding their interaction with trees.
Mosses are non-vascular plants, meaning they lack the specialized tissues found in higher plants for transporting water and nutrients. They typically form soft, green, carpet-like mats, absorbing water and nutrients directly from the air and rain through their leaves and stems. Mosses anchor themselves to surfaces using root-like structures called rhizoids.
Lichens represent a symbiotic partnership between a fungus and an alga or cyanobacterium. The fungus provides structure and protection from drying out, while the alga or cyanobacterium produces food through photosynthesis. Lichens exhibit various forms, including crusty patches (crustose), leafy shapes (foliose), and branched, shrub-like growths (fruticose), and come in diverse colors from green-gray to bright orange-red. They are non-parasitic.
Algae are simple, plant-like organisms that also perform photosynthesis. They often appear as thin, powdery, or slimy green films on bark, especially in damp conditions. Some types of algae, like Trentepohlia, can even create an orange or rusty-red appearance on tree trunks.
Common Tree Mosses
Numerous species of true mosses thrive on tree bark. These small, non-vascular plants contribute to the biodiversity of the tree’s surface. They are typically found in moist, shaded environments.
One common type is Cypress-leaved Plaitmoss (Hypnum cupressiforme), which forms dense, flat mats that can be dark green to yellowish-green. This species is widespread globally and grows on various surfaces, including tree trunks, logs, and rocks, often preferring acidic conditions. Its stems are prostrate and creeping, with overlapping, concave, sickle-shaped leaves that give it a resemblance to a cypress tree. Another example is Rough-stalked Feather-moss (Brachythecium rutabulum), which is also frequently found on tree trunks and branches. These mosses often present as loose tufts or rounded cushions.
Why Moss Grows on Trees
Mosses grow on trees due to a specific combination of environmental conditions and their unique biological characteristics. They are epiphytes, meaning they grow on other plants without drawing nutrients from them.
Consistent moisture is a primary factor for moss growth; they thrive in damp, shady conditions with high humidity, rain, or dew. Tree bark provides a stable surface that can retain moisture, creating an ideal microclimate for moss spores to germinate and grow. Mosses absorb water and nutrients directly through their leaves and stems, rather than through roots, making them reliant on atmospheric moisture.
Many mosses prefer shaded or partially shaded conditions, which are often found on tree trunks and the underside of branches. The rough texture of tree bark offers an excellent substrate for moss spores to attach and for their rhizoids to anchor. They obtain nutrients from rainwater, dust, and decaying organic matter that accumulates on the bark.
Impact on Tree Health
The presence of moss on trees is generally not harmful and often indicates a healthy, moist environment. Mosses are non-parasitic, using the tree as a surface for support without invading the bark or drawing nutrients.
While largely benign, dense layers of moss can retain moisture against the bark, which could, in rare instances, create conditions favorable for fungal growth or insect activity. This is typically a concern only for very young or already stressed trees that may have compromised bark. In extreme cases, heavy moss combined with absorbed water could add minor weight to branches, but this is usually negligible. Some view excessive moss as aesthetically undesirable.
Mosses can offer minor benefits, such as providing habitat for small invertebrates and contributing to overall biodiversity. Their presence can also be an indicator of good air quality, as they are sensitive to pollutants. Generally, removal is unnecessary unless there is an underlying tree health issue exacerbated by persistent dampness or for purely aesthetic reasons. Consulting an arborist can help determine if removal is warranted.