We all know correlation does not equal causation, and that it’s unfair to convict someone based on circumstantial evidence. But when every appearance points to a culprit, it’s hard to resist jumping to conclusions, which by the way is my favorite athletic endeavor. After all, the kid out in your yard holding a baseball bat might not be responsible for the ball that just smashed through your window.
A landscape tree has a rough life, by definition beset with hardships not faced by its forest-dwelling peers. When chronic stress catches up to one and it declines and dies, I often hear from the homeowner about beetles, pillbugs, mushrooms or what-not (mostly what-not) found near the crime scene that must be to blame. It’s understandable – it’s like the kid with the bat.
The truth is it’s very rare for an insect or pathogen to be the sole reason for a tree’s demise, and unusual for a single agent to be even a primary cause. Those guys are just there cleaning up or possibly just passing through. But dang, they sure look guilty as heck, don’t they? Unfortunately the “wrong place at the wrong time” defense is useless most of the time.
One common observation I hear about a declining tree is that as branches die, tufts of “moss” can be seen accumulating on them. A dead spruce will be festooned tip to root with the greenish fuzz. Maple trunks may sport a crusty version of this stuff. Obviously, “moss” is evil and it kills trees, right?
Though it can look like moss, the “guilty by association” organism is called a lichen. It’ll thrive on just about anything left out in the weather long enough. Lichens are as happy living on rocks and old lumber piles as on a tree branch, and they don’t kill those things. I’m fairly sure anyway.
Mistletoe and dodder are plant parasites that freeload off trees and other plants by siphoning off sugars made by their hosts. But believe it or not, lichens are harmless. The reason they doesn’t hurt trees is that they make their own food. It’s not a plant, yet it photosynthesizes – how does that work?
It seems that lichen is a kind of mutt. I’m not sure if it’s on its mother’s or dad’s side, but lichen is half algae. As we know, algae has chlorophyll and makes its own food from sunlight, but needs a moist place to hang out. It won’t last long on a rock or tree branch.
The other half of lichen’s family, um, tree, is a fungus, which is good at holding moisture but which can’t make its food. Somewhere along the line these two made a business deal. Alice Algae and Freddie Fungus took a lichen to each other, as my Biology teacher said. It’s one of those symbiotic relationships wherein both parties benefit equally. A handy arrangement, though not legally binding in most states.
So how come lichens appear as a tree dies back? Plants and lichens both need sunlight. A healthy branch packed with foliage will out-compete lichens, restricting their access to light and keeping them small. As a branch declines, foliage thins out and lets in more sun, and lichens grow tremendously. They’re victims of circumstance.
This dynamic can be seen on a small scale each fall after deciduous trees shed their leaves. Over the winter lichens grow slightly, expanding when daytime temperatures get above freezing. In the spring they shrink back as leaves again overtake them.
When something looks obvious you may feel silly looking for other causes. I mean it’s pretty far-fetched to think the kid with the bat isn’t the guilty one. What are the chances it’s a ploy to get you out in the open so aliens can abduct you ? (I’m guessing small, but that’s not my area of expertise.)
But when a favorite tree appears to be consumed by “moss” don’t be fooled. Lichen or not, things are not always as they appear.
Photo: A lichen covered tree (courtesy Wikimedia user).
Interesting article, Paul. It is also important to note that lichen are extremely sensitive to air pollution. So a tree covered in lichen is actually a sign of a well functioning and health environment (at least in terms of air pollution).
Lichen are fascinating. About two years ago I hiked the new Moxham Mountain trail and found some of the most amazing lichen stands I’ve seen. These were on the ground, not in trees, and I also was saddened to see that they already were taking a serious beating from hikers who just have to get out on every ledge with a view.
My mother used to eat Lichens, said she learned to eat them during the depression. However, after about three spoonfulls of elephant ear lichen soup she would get the crazies, start convulsing, talking in tongues, and then standing straight in the corner and screaming. When she would finally awake from this she just thought she had a good meal. To this day she still has it from time to time.
Hi Paul. I enjoyed reading your essay on lichens. A friend and I climbed Giant mountain on the ridge trail yesterday. Lots of ice. I stopped frequently to appreciate beard lichen, reindeer moss, and many others on rocks and trees. In other locales British soldier lichen is my favorite.
I am in Maine and my home is surrounded with mostly deciduous trees. A few of them had what I am assuming is bearded lichen…looks like Spanish moss I’ve seen in the South.
This year almost every tree in hanging with it. Why? and will it harm the trees.
can you email a picture? This spring most tree species have a distress crop triggered by last year’s drought, and some of the flowers look like Spanish moss… email@example.com