Over the course of many years working with trees, hundreds of people have asked me about “that  stuff” growing on the bark, or hanging from the branches of trees.  What is it?  Is it a disease?  Is it hurting the tree?

Trees often have crusty, leafy, or stringy growths of material that may be gray, blue-gray, or gray-green in color.  In some trees it completely envelopes the branches and trunks.  Most often, it is seen as scattered patches on the bark.  In some parts of the Napa countryside, it gracefully drapes on the oaks like Spanish moss, a similar looking, but unrelated plant that grows on trees in the southern states.

These colorful and diverse tree associates are lichens.  As far as I know, no research has ever proven that lichens are parasitic.

Simply put, a lichen is a fungus that grows symbiotically with algae, resulting in a composite organism.

The fungus is adept at absorbing water and minerals, but unable to synthesize carbohydrates.  It  joins with an alga, a green plant that has the ability to produce carbohydrates through photosynthesis. When they team up, both benefit from the relationship. The fungus takes advantage of the alga’s ability to create food using sunlight’s energy. Water and simple minerals are absorbed directly from the air. No nutrients are taken from the “substrate” — the tree branch, fence post, or rock on which it grows.  Personally, I saw lichens growing on my brother-in-law’s old VW minibus when he left it parked in one place for several months.

Lichens are not pathogens.  They do not attack plants.  They only need a place to attach themselves where they can receive light and clean air. Yes, clean air.  Lichens are sensitive to air pollution, especially sulfur dioxide.  In Arboriculture the authors state that lichens “seldom grow down-wind from large cities or industrial complexes”.

Though they are not  pathogens, lichens may indicate that a tree is declining.  On rapidly growing trees, the canopy casts dense shade, limiting the amount of light available to the lichens. In a declining tree with less foliage, more light reaches the inner canopy and branch surfaces, thereby favoring the lichens. In addition, fast-growing trees don’t have static bark.  It expands, splits, cracks and peels off, thereby disturbing the lichens.  In contrast, a declining tree tends to have a more open canopy and inactive bark.  It makes a better home for the lichens.

In an indirect way, lichens may contribute to the decline of trees that have begun to lose vitality.  In Tree Maintenance, the author, P.P. Pirone, states that when extensive, lichens interfere with “gaseous exchange” in parts they cover.  Trees breathe through bark lenticels.  Interference can harm the plant. Pirone mentions that any copper spray, like those used for disease control in trees, will eradicate lichens.

To get  a better handle on the subject, I recently ordered a great little booklet, California Lichens (Doell & Doell, California Lichen Society, 2000). Along with dozens of color photographs, it provides some of the lichen basics.  Here are a few.

Lichens take 3 general forms:

Foliose lichens have leaf-like forms with distinct upper and lower surfaces.

Fruticose lichens are shrubby or pendulous.  The hanging one often confused with Spanish moss is  Ramalina menziesii.  A close look reveals its netted structure.  (The true Spanish moss is actually Tillandsia usneoides, an epiphytic green plant, more closely related to pineapples than lichens!)

Crustose lichens are firmly attached to the substrate, forming a crust.

Lichens are not “vascular” plants i.e. they do not have a circulatory system.  Neither are they seed-producing plants.  They reproduce largely by vegetative means, breaking apart and falling or drifting onto a new substrate.  They do have structures that aid in reproduction: Soredia, granular collections of a few algal cells surrounded by fungal threads.  They break off easily, making a basic unit that can grow on any suitable surface.  

Lichens are not unique as “composite organisms”.

Another example is the fungus-root: mycorrhiza, a distinct organ formed by the intimate association of fungi and plant roots.  Most plants need this association for normal growth.

Another, even broader in scope, is the association of plant and animal cells with mitochondria.  These organisms are genetically distinct from their hosts, yet they live inside of the cells of all plants and animals and make it possible to convert food to usable energy.

Remember that famous quote from John Muir in My First summer in the Sierra:  "When we try to pick out anything by itself, we find it hitched to everything else in the Universe."

That is the case, in a tangible, practical way, with composite organisms.  Lichens are a perfect example.

Bill Pramuk is a Registered Consulting Arborist. Visit his website, www.billpramuk.com, email questions to info@billpramuk.com, or call him at 707-226-2884