Never say never.
The dogmatic answer to that frequently posed question is a comforting “no” echoed throughout garden publications. Here is a typical statement from the New Mexico State University Horticultural Guide H-167:
“Lichens may rest directly on the bark or be attached shallowly to it, but they do not enter the inner bark where food is transported, and hence do not rob the tree of nourishment. They are not parasites.”
A trouble with dogmas is that while they spread and simplify, gaining emphasis, occasionally somebody goes back and checks them out. Not that finding parasitism in lichens is new. One of the links below dates from the 80s. And, because lichens are not a single “thing,” but rather many unrelated species in diverse symbiotic relationships, ixnay on big sweeping generalizations.
I’m not posting this to shock treehuggers. Seems that under most circumstances most lichens are mostly okay. There’s no existential threat, arboreal apocalypse, or actionable garden info here. Just interest…
Assuming that lichens are never parasitic is actually too good to be true. To cite recent experience, would you leave the dog alone with an Italian sub? A fox with the henhouse? Me unsupervised in a Dunkin Donuts? Lichens are fungi bundled with algae or cyanobacteria. Fungi are tissue-invading machines. That is what they are good at. Their threadlike hyphae penetrate and digest organic matter. So why should a lichen fungus always “behave” and never help itself to a bit of the host tree’s substance? A little side-steppin’ shoud be expected.
What’s remarkable is that lichen fungi are not more parasitic to their hosts, although the hosts have defensive mechanisms, most conspicuously shedding bark and pesky pests thereon. Lichen fungi do invade tree tissues, probably mostly on young twigs. Researchers have long been aware that the lichen “oak moss” Evernia prunastri and species of similar Ramalina penetrate the bark, including the living sugar-conducting layers, and sometimes into the wood beneath. “Lichens are not parasites,” my ass.
Pause a moment. It may help to clarify that bark (phloem) is dead to the outside but alive and transporting sugar in its inner layers. Immediately interior to that is the water-conducting wood (xylem). Together the phloem and xylem are the circulatory system of the branch.
Botanists who have looked into lichen parasitism have found that host branches can lose their leaves outward to the leaf tip from the point where the lichen is attached. That is, water transported out the branch after passing the lichen seems to cause leafdrop. This could be due to toxicity and/or to tissue damage. Alternatively, and not an idea original with me, it could be adaptive for the lichen to defoliate the branch for increased light exposure. Weirder things are known.
It is not just oaks. Locally on Bald or Pond Cypress small tufted lichens* sometimes perch on young branchlets, which may become leafless or lifeless out past the lichens. In the photo below the Cypress branch is healthy below (to the right of) the largest lichen, sprouting side branches. The side branches at the point of lichen attachment, however, are dead, and those farther toward the branch tip (left of the lichen) are stunted or missing.
To see if the lichen affects the living inner bark and/or into the wood, I sliced branches lengthwise at the point of lichen attachment. Bark at the lichen attachment point is thickened and different from bark elsewhere.
Looking more deeply, the wood interior to the thickened bark is discolored as in the photo below:
To sum it up, these lichens are entering the living inner bark and penetrating (or at least damaging) the underlying wood, killing or stunting side-branches, and ultimately killing the host branch tip distal to the lichen. But fret not, the suffering twigs are few, and maturity brings toughness ….so the outcome seems merely a tiny bit of free pruning.
Dig in deeper:
Click for an older paper on this topic.
Click for some recent views from the West Coast, and sources.
The identity of the photographed lichen is not relevant to today’s article. I’m no lichen expert and have no interest in peripheral concern with species name. I’d be glad to share my tentative identification privately by e-mail upon appropriate inquiry.