Lichens are moss like plants with a crusty, peeling or fibrous texture commonly seen growing on tree bark, especially the bark of conifers. The plants are nonparasitic and do not seek sustenance from the host trees. Lichens are actually composed of a certain algae and a fungus, which are closely associated in a symbiotic relationship where each benefits the other in growth. It is unlikely that the presence of lichens on a tree is the cause of tree decline or death.
The combination of the green or blue-green alga and fungus produces the lichen that bears little resemblance to either parent. There are three main types of lichens. The crustose lichens grow flat against the limbs, the folicose lichens have leaflike growths on their surface, and the fruiticose lichens are characterised by hair or fingerlike projections. Commonly grey green in colour, lichens may also be yellow, orange or dark brown. Though lichens stay firm under wet conditions, some brown or black lichen types tend to grow gelatinous when moist.
Lichens are hardy and adaptive plants that are mistakenly associated with tree decline because they thrive on damaged and stressed plants. The presence of lichens on poorly growing trees tends to be more pronounced since the exposed limbs on these plants provide lichens the sunlight they need for growth. Lichens also face reduced competition on weak trees, further aiding growth. Lichens are as common on nonliving objects like soil, rocks and fence posts as they are on trees and shrub limbs.
The growth of lichens is most prolific on larger, mature trees, where they are seen on limbs and trunks. The areas that are exposed to full sun are conducive to growth. Damaged or thinning canopies allow sun into inner tree areas, leading to extensive growth of lichens. The greater majority of lichens fail to thrive in shaded sites, and on healthy trees and shrubs. Lichens are also common on trees growing in polluted air with high levels of sulphur dioxide, acid rain or ozone.
Though outwardly it seems like the lichens are drawing their nourishment from the host tree, lichens only use the bark as a resting place by attaching to it shallowly. The plants do not penetrate the bark or tree wood, and hence do not obtain nourishment from nor damage the tree like parasites. Lichens continue to grow even after the trees enter their dormant state during fall and winter, obtaining water and minerals from the air by cell-to-cell absorption.
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