The original lichens could never thrive in unclean air, but with the changing environment, the subsequent migration of lichens has duly equipped them for survival. Previously, the absence of lichen in a certain area was a sure indicator of unclean air over that place.
But on moving away to new habitats, lichen on trees, has developed a certain resistance to unclean air. However, invasive lichen on trees can cause considerable damage to the host trees, negating tree lichen removal. This is best illustrated in 3 tree lichens: Common orange lichen, Rosette lichen, and Strigula lichen.
Lichen on trees, is, in general, perennial aerial plants, somewhat lowly organized. The distinguishing feature of lichens is their composite nature: they consist of two distinct and dissimilar organisms, a fungus and an alga. This symbiotic origin can be beneficial for their survival.
On the one hand, the fungus, is a saprophytic which derives its food from dead or decaying organic matter. but it is unable to survive in a bare, dry habitat. On the other hand, the alga, is an autotrophic capable of synthesizing its own food, but it cannot live without moisture. However, the two make quite a determined team, as together they are able to conquer very inhospitable environs where few other plants can grow.
This combined need for water is seen in that there is an abundance of dangling Old Man’s Beard lichen (Usnea) on trees in mountainous regions with high moisture content in the air. Further, the side, of trees, which is exposed to wind and rain is most luxuriantly covered with lichens.
The Common Orange lichen (Xanthoria parietina) on trees, has a flat, leafy and yellow-orange thallus or plant body. The bark of willows (Salix) is a favourite habitat of the Xanthoria parietina. Lichen damage is clearly evident when the orange lichen on trees takes possession of the willow branches, completely engulfs them, and seals off the stomata, obstructing the quick movement of water vapor, gases, and oxygen in and out of the tree leaves.
Indeed, the Xanthoria parietina is a species that is very tolerant to unclean air what with nitrates and ammonia from car exhausts and agricultural chemicals. Thus, tree lichen removal is quite impossible.
The structure of another aerial plant, Rosette lichen (Physcia adscendens), is well delineated against the Aspen (Populus tremula) bark; it is seen as a conspicuous greenish-grey color as a result of the photosynthesising Aspen bark. The Rosette lichen further appears as diamond-shaped marks on Aspen bark; these are in fact, tiny breathing holes or lenticels of the Aspen tree.
The Rosette lichen on trees, paves the way to lichen damage when it uses root-like filaments to gradually loosen the outer bark of willows and give access to fungi.
The Rosette lichen that grows in a habitat rich in nutrient components, will in turn be enriched in the underlying layer and this makes it a species which is neutral of a specified habitat and a species which requires a soil rich in nitrogen, a study suggests.
Strigula lichen (Strigula complanata) on trees, appears as gray-white crusts, on upper leaf surface, with supporting black fungal fruiting bodies. It is parasitic: it grows on the surface of thick leathery leaves of the Camillia (Camellia Japonica), penetrates the epidermis and burrows beneath the cuticle and outer cells. This lichen damage causes the leaves to turn brown.
Direct light, a moderate or cold temperature, constant moisture and high humidity are unconducive to tree lichen removal. This is because lichens are xerophytically structured to endure extreme conditions like low temperatures, and even unclean air.
Nevertheless, in tree lichen removal, good tree vigor will keep lichens at bay since unhealthy trees are usually infested with lichens and they soon succumb to lichen damage. A thick leafy canopy will keep shade-intolerant lichen species away. Also, light pruning of trees and new shoot growth may shade out lichens, thus eliminating the need for tree lichen removal.