Lichens – A Fascinating Partnership

Here is a question that we get occasionally …

There are lichens growing on my tree. Is this going to harm the tree? Does this mean that my tree is dying?

I’ve been thinking about this lately as I’ve been noticing some of the interesting lichens growing on our own trees, including some of our mature fruit trees.

The simple answer to the question is no, lichens do not harm your trees – they are just using them as a substrate to grow on. They also grow on rocks, gravestones, soil, and the outside walls and roofs of buildings.

Lichens often grow denser on the
north side of a tree where conditions
are typically moister.

Lichens are rather complex life forms. They are not a single organism but actually a combination of two separate organisms; the mycobiont which is a fungus and the photobiont which is either a species of green algae, a cyanobacterium (blue-green bacteria, previously called blue-green algae), or sometimes both of them together. These two organisms, the mycobiont and the photobiont, coexist in a symbiotic relationship each benefiting from the other in order to survive.

The fungus, which surrounds the algae cells, provides protection to the cells and keeps them in a moist environment (most of the time).

The algal/cyanobacterial component which contains chlorophyll produces food for the fungus through photosynthesis. If a cyanobacterium is part of the “lichen package”, it can also fix atmospheric nitrogen converting it to a form that can be used by the lichen. When it rains or when the lichen dies, this important nutrient leaches into the soil and becomes available to other plant life in the surrounding area.

Lichen can often be found growing on
rocks, rock walls, even gravestones.

So, as you can see, lichens are pretty self-sufficient. They are not parasitic organisms; they do not “feed” upon the substrate they are attached to. They don’t have roots; they attach to the substrate via nonvascular fungal filaments. Lichens derive everything they need to survive from the atmosphere. The fungal component of the lichen is by far the more prominent partner and gives the lichen its shape and structure. With its larger surface area, the fungus is able to “collect” water and mineral elements from the air. Some species that grow on rocks can eventually over a very long time slowly through both mechanical and chemical means break the rock substrate down, gradually turning it into a mineral soil.

A ground cover of reindeer “moss”.
This species is an important food
source for many animals.

Lichens are famous for their ability to survive in very harsh environments; from the frigid arctic tundra to blistering hot, arid deserts. They are able to withstand prolonged periods of desiccation during which they dry out completely and become very brittle. If pieces of these brittle lichens break off and land in a favorable location, they can create new lichens. When moisture becomes available again, through rain, mist, fog, or even just humidity, the fungal partner quickly absorbs water, rehydrates, and becomes chemically functional again.

Fascinating!

Foliose lichen on a tree branch

There are tens of thousands of different species of lichen but most can be grouped into four basic growth forms; foliose, crustose, fruticose, and squamulose. The first three of these are the ones I have seen most. Foliose lichen, as the name implies, is rather leafy in appearance. They grow flat on the substrate but are not tightly attached. These are the ones I mainly find on my trees. Crustose lichen is very flat and is tightly attached to its substrate. These are often found on rocks.

Reindeer moss is a fruticose lichen

Fruticose lichens are more three dimensional and are usually attached to the substrate at a single point. Reindeer “moss” (which grows on the ground) and “old man’s beard” (which hangs from trees) are familiar types of fruticose lichens.

Many species of lichen are important food sources for a variety of different animals from large mammals like reindeer and caribou to smaller mammals like voles and flying squirrels. In addition, some species of the more upright fruticose forms provide both shelter and nesting material for many birds, small mammals, and a variety of insects.

This fir tree supports a diversity of lichens
including both foliose and fruticose forms.

Lichens are very sensitive to air pollution and grow well only when the air is clean and unpolluted. If air quality declines due to pollution, especially from sulfur dioxide, lichen populations will begin to decline. In fact, many studies use lichen growth as a bio-indicator of pollution levels and trends.

So if you have lichen growing on your trees, rock walls, and other places in your garden, consider yourself lucky! This probably means you have nice clean air to breathe and you can rest assured that these fascinating symbiotic organisms are not harming your plants or causing their demise – they are just “hanging out”, basking in your pure air and soaking up a bit of your sunshine and rainwater!

Until next time – Happy Gardening!

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