Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Wolf Trees

Tuesday, December 18, 2012

I am not and never have been a farmer. I do not know the open spaces that farmers know. I have never walked the fields and pasturelands that are so familiar to farmers. In New England where I live, most of the farmland has reverted to forests. It is in these forests, these converted farmlands, where I spend my time hiking. All I know of these old farms are the stone walls left behind. The New England landscape is famous for these stone walls.
The story goes like this: About 19,000 years ago, a great ice mass grew out of Canada and covered the land - the last ice age. As the ice moved south, it scoured the landscape, carrying rocks, boulders and stone with it. When the ice melted, this material was left behind, littering the landscape. When our ancestors farmed the land, they needed to clear the land of these stones, so they would line them up at the edge of the field, out of the way of their plows. These stone walls were the boundaries of the fields and, for the farmers with livestock, natural stone fences.
Anyone hiking through the forests in New England has experienced these stone walls among the trees, all that remains of the farms that once covered this region. I have hiked these forests and seen these walls my whole life. So imagine my surprise when, at 60 years old, I first learned about Wolf Trees.

Wolf Tree. What a wonderful name for a tree.

Here is what I learned about wolf trees.
Trees, like most plants, need sunlight to produce their food. The more sun that shines on their leaves, the more food they produce and the larger they grow. So sunlight is a very important commodity for trees. As such, one would expect the competition for sunlight to vertical treebe fierce. And it is, especially in a forest where all of the surrounding trees are competing for the same light.
If a tree becomes shaded by other trees, it can’t pick up its roots and move to another, sunnier location. Its only option is to send its leaves up above that of the surrounding trees, or it will suffer the consequences. Look at any tree in a forest and you will see most of its branches pointing upwards in an effort to send its leaves toward the sunlight. There are very few horizontal, or sideways-pointing branches. Any such branch would produce leaves that would be shaded by other, higher leaves, and would receive very little sunlight for food production. This clearly would be a waste of time and energy for the tree.

wolf treeIf, however, a tree were to grow in an open field, the leaves on a horizontal branch would receive just as much sunlight as those on a vertical branch. These trees, with so much more leaf surface to catch the suns rays, would grow larger and faster than other trees, and their overall shape would be very different from their forest-dwelling cousins.
As the tree grows, the leaves of a horizontal branch would eventually be shaded by the leaves on the branches above it, so this horizontal branch would continue to grow out away from the trunk to stay in the sun. To support its weight, this branch would grow thick as well as long. Such a tree has a very distinctive shape and is easily recognized. This is a wolf tree.

In a forest, a wolf tree is most often found growing along an old rock wall. The theory is that when a farmer cleared a field for pasture, one tree was left to grow along the wall. Since this tree had no competition for sunlight from other trees, it would grow large and fast, spreading its branches horizontally as well as vertically. It would grow into a wolf tree.
There is some debate as to the origin of the name. Some say that the name refers to its growing all alone with no other trees around it - like the lone wolf. Others say that, like a wolf preying on all of the smaller animals around it, the wolf tree preys on the plants underneath it by producing so much shade that the other plants can’t grow. Whatever the true derivation of the name, I am glad that the name stuck.

There is also some controversy as why these trees were allowed to grow in the first place. After all, the farmers have cleared all other trees from the fields. Why leave one tree to grow. The most common explanation is that the tree was left to provide shade for the farm animals. Cattle could often be found gathered around these trees on a hot sunny day.
Another explanation is that the trees were so big and it took so much energy to cut them down, that it wasn’t worth the effort. They were just left to grow.
Of course, not being able to ask any old-time farmers that let these trees grow in their pastures, it is hard to know exactly why they were allowed to grow. However, it is possible to look at both of these explanations with a critical eye and see if they make sense. This is a skill all good scientists have. Always look at any information from all angles and see if it makes sense.
Explanation one: One would think that the shade provided by a wolf tree would benefit cattle on a hot sunny day. In fact, cattle are often found under these trees on such a day. However, gathering under these trees on another type of day might not be such a good idea for the cattle. A wolf tree, being the only tall object around, is the most likely object in the field to be struck by lightning. Any livestock gathering under the tree may be killed by the lightning. In fact, the U.S. Department of Agriculture says that 80% of all accidental livestock deaths are caused by lighting. Seems like it would behoove a farmer to remove all trees from the pasture to protect the livestock from lightning.
Explanation two: Trees begin life as fairly small entities. It takes a long time for them to grow to a full sized wolf tree. Once fully grown, it would take a lot of effort to cut them down. So if a farmer doesn’t want the trees in their pasture and only lets them survive because it takes too much effort and cost to remove them, why let them grow in the first place. After all, the farmer has removed all of the other trees from the field. Wolf trees only grow in open areas. So the wolf tree must have started growing after all of the other trees were removed. Why not remove all of the trees at once, and not have to worry about removing a fully grown wolf tree in the future?

Whatever the reason for their existence, any walk through a New England forest may turn up a massive tree, with large, horizontal branches growing below other, mostly vertical branches. These wolf trees will often be found growing next to old stone walls. They are not only a delight to the eye, but to wildlife as well. Studies have shown that many birds and mammals frequent wolf trees much more often than other large trees. When walking through the forest, I am always on the lookout for wolf trees, not only for the wildlife they attract, but for the sheer magnificence of the tree itself.

Here are some pictures of large trees in the forest behind my house. Can you pick out the wolf trees?

trees 1

trees 2