About three years ago I was asked by two different people about a yellow tree that can be seen on Cumberland Mountain near the tunnel entrance on the Tennessee side. As you approach the northbound portal, you can see it (careful if you’re driving) over to the left, about a third to halfway up the mountain, and indeed appears as a yellow tree amid a mountainside of normal colored green trees. In the right sunlight it really stands out.
Normally when I see off color trees like that, I assume it’s unhealthy and not long for the world, but this one has hung in there for three years that I know of, so I decided to try and locate it. It is no easy task to find one tree among many thousands on a large mountain. But from a distance I figured out where to hunt based on small ridges and drains that I could make out, and after two tries I think I found it. I was expecting to find some out of place exotic tree that normally has yellowish leaves, but was surprised to find that it was a big white oak, very common in our area.
So the tree created another mystery: “Why are you yellow?” So let’s delve into tree pathology a little.
Abnormal yellowing of leaf tissue is called chlorosis, and shows up when something messes with the all-important green pigmented chlorophyll, where photosynthesis takes place. Causes of poor tree health can be broken into two categories: Biotic (bugs and disease organisms), and abiotic (environmental).
Chlorosis is usually abiotic and caused by a lack of something the tree needs to grow. The most common cause of chloric leaves is an iron deficiency, but it could also be caused by a lack of some other nutrient such as boron or magnesium. Other reasons for leaf yellowing include excessively wet soil from overwatering, soil compaction, or poor drainage; all of which can result in root damage and impairing the trees ability of take up and transport soil nutrients. Extremely dry soil caused by drought is another possible cause. Chlorosis can occur in the entire tree canopy or may show up on just one branch. Health issues involving the tree’s immediate environment can be tough to correct, but adding a missing nutrient such as iron to the soil can be done. Compacted soils can be aerated to correct poor drainage issues.
So why is the mystery mountain tree yellow? I have no answer. The site around the tree is rocky, which suggests shallow and dry soil conditions. But the tree’s top is in what’s called a dominant canopy position, meaning it has competed with surrounding trees well enough to have grown tall, so its leafy top is sticking up into direct sunlight, critical for good tree growth and health. If the tree’s growing environment was persistently dry from shallow soil, it would never have become a dominant tree, but more likely a suppressed and slower growing tree, which is often a death sentence in the dog-eat-dog world of tree competition.
So the mystery tree remains a mystery.
Steve Roark is the area forester in Tazewell, Tennessee for the Tennessee Department of Agriculture, Forestry Division.