Have You Ever Seen a Cradle Knoll?

Sep 24, 2018

Have you ever seen a cradle knoll? Do you know how they form, or their importance on the landscape? That is the subject of this week’s episode of Wildlife Matters.

On a recent field trip I led, we came across a large red pine that had blown over in a summer storm. We were in a recently completed timber harvest area, and this tree was one that had been left to continue to grow, what we sometimes refer to as a sentinel tree or a super-canopy tree. This tree would never be either, thanks to a gust of wind. Now it will be a log, fifty feet long, maybe thirty inches in diameter, until it eventually crumbles into the earth. The group that was on the tour saw this as a bit of a waste. They were forest landowners who grow trees for profit, for income, as well as for wildlife, aesthetics, or resource protection.

Some landowners might think you need to get after that downed tree right away and have a logger salvage it for whatever it’s worth. As a wildlife biologist, that tree is worth as much to me lying down as it was standing up, maybe even a little bit more. Now that it is on the ground, it will attract insects that will live beneath the bark and feed on the cambium. Those insects will feed all kinds of wildlife, like birds, salamanders, rodents, even black bears that will tear the bark away to get at the grubs. Lying on the ground, without its natural defenses, the tree will decompose a little faster, especially with insect action and fungal growth. It will house rodents, which will feed hawks, foxes, maybe bobcats or coyotes. When it finally does crumble completely, it will return valuable nutrients to the ground and contribute to the crucial organic layer that helps our sandy soils remain as productive as they can be. And that’s just the log – the root is a different matter entirely.

As we walked past the downed tree, I simply said that the tree was valuable for wildlife on the ground, and that windthrown or rootsprung trees were the only way we would ever get the natural feature known as a cradle knoll. Well, that stopped the group in its tracks. These forest landowners were unfamiliar with the concept of a cradle knoll.

If you have walked more than a couple hundred feet into the woods of North Central Wisconsin, you have probably been within sight of a cradle knoll, or even walked across or stood on it. In its simplest form, it is a hole or depression in the ground with a lump or pile of dirt next to it. They tend to form on glacial drumlins, where you have a layer of soil atop a deeper layer of glacial till or gravel. As a child, growing up with a drill sergeant for a father, we called them foxholes because they looked like someone dug a hole and threw all the dirt in a pile on one side to form a small berm. In fact, they are formed by trees tipping over, and their root ball flipping from horizontal underground to perpendicular or vertical above ground. The root ball brings a large volume of dirt along with it, creating an equally large hole next to it. In the case of pine trees, they have a tap root which makes a quite deep hole which sometimes will fill with water. Before our modern fire suppression era, these trees would lie on the ground until a typical periodic fire would come along and consume it. The root would be the last to dry, and typically being covered with dirt it would burn poorly if at all. The dirt clod would collapse overtop of what was left of the stump or roots, and erode back into the hole. So those cradle knolls you see in the woods could well have been made hundreds of years ago.

An old forester once told me that he did some research that indicated that these cradle knolls played an important role in microtopography, or the ground relief on a small local scale. Not only that, but they also help turn over the soil, burying some things on top and exposing some things that are buried to help with nutrient cycles and impacting plant growth and succession. Yet, with our current timber sale techniques, we have no mechanism to imitate the natural cradle knoll process unless we purposely leave trees standing to tip over. And even then, we likely won’t see the level of fire on the landscape that would consume such a large tree and complete the formation of this interesting topographical feature.

Striving to make new things familiar and familiar things new, this is the masked biologist coming to you from the heart of Wisconsin’s great Northwoods.