Six years ago, several folks asked me about the yellow tree that can be seen from 25E on Cumberland Mountain near the tunnel entrance on the Tennessee side. As you approach the northbound portal, you can see it (carefully if you’re driving) over to the left, about a third of the way up the mountain. It’s bright yellow green foliage stands out distinctly amid a mountainside of normal colored green trees, especially in bright sunlight.

I tracked down what I thought was the tree back in September of 2015, but wasn’t totally convinced I’d found it because leaves had started changing color a little. Also finding a single tree amongst a mountain side full of them is the proverbial needle in a haystack. Two friends asked me about the tree recently, which provided incentive to revisit the tree and verify its identity. I now have better GPS equipment and mapping software to help me get in the right vicinity, and I really studied the shape of the tree’s canopy closely with binoculars to assure that when I saw the tree up close I’d know it. The better technology helped some, but it still took quite a bit of zig-zag walking to find it. But found it I did, and confident it was the one. It’s a white oak with a very large canopy growing at the base of a rocky bluff. This is a very common tree in our area, and there were in fact other white oaks nearby, but they were their normal dark green color. So even though it’s identified, there is still some mystery to the tree: why is it yellow in the middle of summer?

Normally when I see an off-color tree like that, I assume it’s unhealthy and not long for the world, but this one has hung in there for six years that I know of. 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 is either biotic (bugs and disease organisms) or 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.

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 a dominant canopy position, meaning it’s competed with surrounding trees well enough to have grown tall, so that 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 a retired area forester from Tazewell, Tennessee.