One question I get asked routinely is whether lichen kills trees. The question may pop into your head when you notice pale green or gray patches spreading across the bark and branches of a tree in your yard. I completely understand why someone would think that these lichens are harmful. At first glance, they can look like a disease or some weird fungus eating away at the wood, and since lichens are often seen attached to dead or dying branches, it’s natural to assume the worst — that your tree is being slowly smothered to death! But the truth is much less alarming, and once you understand what lichens are and why they grow where they do, you might even come to appreciate their presence and even their sublime beauty.
First of all, lichens are not fungi, though they do contain fungal elements. The most amazing thing about lichens is that they are actually, dare I say, a miraculous partnership between a fungus and either algae or cyanobacteria. The fungus provides the structure and shelter, while the algae or bacteria use sunlight to produce food through photosynthesis. Together they form something entirely unique: an organism that doesn’t behave like plants or fungi alone. Because they don’t have roots and don’t pull nutrients from their host, lichens are not parasitic. They don’t burrow into the bark or siphon off resources from the tree. Instead, they live on the surface, taking what they need from sunlight, air, and rain.
So, if lichens don’t harm trees, why are we so quick to blame them for their decline? The answer lies in underlying circumstances. Lichens require light and moisture for photosynthesis. They thrive in places where conditions are stable and light is ample, which often prevail on older trees or on branches that have lost some of their leaf cover. A thinning crown or declining tree lets more sunlight reach the trunk and branches, creating an inviting environment for lichens to spread. In other words, lichens are not the cause of poor health — they are more like indicators. Their sudden appearance may be a sign that the tree’s canopy is thinning due to stress, disease, damage, or even age, not that the lichens themselves are causing those problems.
Now that we know that lichens are not killing your tree, let’s consider the role of lichens beyond their misunderstood relationship with trees. They are actually a sign of good air quality! Lichens are highly sensitive to pollution, and you will never find them thriving in smoggy or industrial areas. Their presence means you can breathe easy, the air is clean. They are also miniature ecosystem unto themselves. Tiny insects and invertebrates use lichens as shelter, and birds scour them to find these tasty morsels and sometimes harvest bits of them to line their nests. Far from being a detrimental nuisance, lichens are a quiet contributor to the biodiversity of our landscape.
For those still uneasy about the look of lichens on their prized ornamental trees, it’s important to remember that removing them isn’t necessary for the tree’s health. Scraping them off can even cause harm by damaging the bark. If their presence seems to coincide with a decline in vigor, the better approach is to look for underlying issues — soil compaction, drought stress, root disturbance, or pests and diseases that may be weakening the tree. Addressing those factors will do far more for your tree’s health than worrying about the lichens clinging harmlessly to its surface.
Their appearance may raise questions, and now you can answer those questions for others. This is the answer I always give, and I find it reassuring: Lichens are not killing your tree. If anything, they are simply along for the ride, adding texture, color, and a reminder that even the smallest details in nature are part of a much bigger story. This is just the tip of the iceberg for lichens; there are over 18,000 species of lichen, and there could be as many as 250,000; they are a fascinating life form that I heartily recommend learning more about. You can start by reading this article on Ten Things You Might Not Know About Lichens, But Wish You Did. I already told you one of them.
If you would like to learn more about the lichens living in your yard, Natural Neighbors can help you. Find out more about our program by clicking HERE, and while you are there, sign up for a visit. We will come by to help get you started on your healthy habitat journey! We happily share our knowledge and resources at no cost to you, and while we are at it, we find many other ways for you to support biodiversity in your yard.
Photos: by Rich Couse
