Is Tree Lichen a Problem?

[Mark Lewis]

I received this question, and wanted to post it here so the answer can be shared.

I was once told that if a tree has lichen growing on the bark it was a indication that the tree was stressed and was in decline and destined to die. Is this true? I ask because last week I went to a tree farm to buy a crimson maple and all the trees the fellow had had lichen on the bark. I asked him the question and he said he had never heard that it meant a dieing tree. I figure a guy who owns a tree farm should know, but again he really didn't convince me with his answer.

[Peter Smallidge]

Lichens are a symbiotic result of merging (not quite the right word) a fungus and algae or bacterium (http://en.wikipedia.org/wiki/Lichen ).  There are often rumors of the ill-effects that lichens convey to trees, but they are not true that I have known.  I have heard of one experience where a woodlot owner liquidated his nice, not quite mature, sugar maples because he thought the lichens were an indication of "sick trees."  I have never seen, nor heard or read, of any connection between the presence of lichens and a decline in tree health.  Rather, on a health perspective, the presence of lichens indicates good air quality (because lichens can’t grow under air pollution).  The converse, the absence of lichens means bad air quality, doesn’t hold true.

 It is important to differentiate between lichens which are rather dry, thin and coarse, and mosses.  I’ve seen mosses growing in areas with poor drainage and on trees that are dying.  The moss is an indicator of a problem, and an increase in sunlight, and not the causal mechanism of the tree’s decline.

Perhaps others have more to share.

Peter

[Mark Whitmore]

Thanks for the great reply Pete.

I agree with Pete that I have never heard or read anything that would indicate lichens are in any way harmful to trees or an indicator of poor health. Nothing, period. Indeed, there are some lichens that have symbiotic  blue-green algae and actually have been documented to contribute nitrogen to the forest. Enjoy lichens for their beauty and the biodiversity they promote by providing habitat for beasts you don't normally think about like predatory insects that eat the insects that might harm trees.

I grew up in Washington state and seeing moss on a hardwood makes me shrug my shoulders but out east (i've only been here for 25 years) I think Pete has a point. The presence of moss on a tree can indicate where there may be water seeping from the interior of the tree to the outside via a crack or wound. That would indicate there is a water storage reservoir (read heartrot) in the tree that is supplying water to the mosses through dry spells. Be careful with this as a diagnostic. You would need to find the crack or wound that might be seeping. Often I've seen perfectly healthy trees with moss around the base.

One thing is certain about forest health, nothing is simple. There are usually multiple causes for decline that can involve abiotic influences, like water relations, and biotic influences that sometimes cannot be easily seen, like root rots. Even declining ash are not necessarily infested with Emerald Ash Borer! Watch for woodpecks on those ash, that's when you likely have EAB, but not necessarily.  

[Lewis E. Ward]

A good question and a great answer. Will have to remember this for the RNYW-Woodswalk on Wild Mushroom and Forest Health Woods Walk Saturday May 11th at the Arnot Forest 1-4pm.