Every couple of weeks I get a call to go out and look at a terrifying disease that is threatening someones prize specimen tree. It is easy to make light of this, but the sense alarm felt by the home owner is genuine, and one should never make fun of someone who is genuinely concerned with the health of trees. Typically these calls come after a soaking rain when the bark of the tree erupts into a verdant green carpet, or possibly a gray-green bouquet of fronds. The culprits are two ancient inhabitants of the forest, that may even predated the arrival of trees. Lichen have been around for at least 400 million years and their neighbors the moss are found in the fossil record back to the beginning of the Permian period, 300 million years B.P.
Moss, a fuzzy or sometimes minutely leafy, non vascular plant, is classified in the group of plants know as Bryophyta. Moss plants will grow on most moist shady surfaces like rocks, trees and soil. They prefer not to be exposed to direct light and therefore are most often found on the north side of tree trunks. Moss plants need free moisture to complete there life cycle but will survive for long period of time in a dry desiccated state. If they receive sufficient moisture they will “green up” and come to life with in a short period of time, seemingly overnight. Moss does not have the waxy outer leaf layer known as the cuticle that prevents most higher plants from drying out. Moss are able to survive on what amounts to essentially dust and rain drops. They do not harm trees and do not digest the bark they are attached to.
Lichen on the other hand are not really considered plants at all but are a curious symbiotic relationship between a fungus and an algae. The fungal partner provides nutrients and a physical structure for the algae. The algal partner is able to photosynthesize and create sugars to feed both partners. Both partners present in lichen gain moisture and nutrients from the atmosphere. It is rare for them to take any nutrition from a tree that they are growing on. Lichen are usually larger than moss and have a blueish green color. The algae partner may be a blue-green algae which would explain the lichens color. Lichen can have several different forms from crusty or crustose , or foliate forms that resemble leaves, called foliose. Lichens simple structure means they are unable to avoid the accumulation of damaging pollutants. In fact they can be used as a measure of air quality. A decline in lichen populations is indicative of high levels of pollution. The presence of moss and lichen usual indicate moist conditions, and reasonable air quality. The presence of mushrooms and other fungi may be more serious.
I.S.A.Certified Arborists are knowledgeable in identifying moss, lichen and dangerous tree fungi. Take the time to consult with your Certified Arborist if you suspect your tree has something more serious than moss .