First Published in the Edmonton Journal 10/6/2005

As a kid, I was a bit worried the first time I saw masses of small twigs and branches scattered all over the ground beneath a row of massive laurel leaf willow trees on our farm. I thought there must be something seriously wrong with them. But over the years I saw that the willows consistently dropped twigs, yet the tree remained vibrant and healthy. Later I learned that all trees will drop branches from time to time as part of their normal life cycle; this particular process is called cladoptosis.

Culling the Herd
Cladoptosis is derived from the Greek words clados, meaning branch, and ptosis, meaning fall. As far as scientists have been able to determine, cladoptosis seems to be a response to adverse growing conditions that affect branch and twig vigour.

When it comes to falling branches, there’s a vast difference between cladoptosis and the damage caused by windstorms, lightning strikes, snowfalls and the like. The damage from weather is pretty indiscriminate; true, the weak branches are usually the first to go, but even strong, healthy branches can be sheared off if the weather is severe enough.

Cladoptosis is a more orchestrated event. The process typically begins when a branch is stressed by lack of light, shaded out by nearby buildings, other trees, or even other branches on the same tree. Instead of being a net producer of energy for the tree, it becomes a liability, consuming more energy than it gives back to the tree.

When a tree detects a branch that has become an energy liability, it uses abscission zones to shed the unproductive branch. These are zones of specialized cells that exist at specific points on twigs or branches that are designed to separate somewhat like an opening zipper when conditions demand it. Leaves also have abscission zones, which explains how leaves drop off trees so effortlessly in the fall. The advantage of abscission zones is that they can provide a clean break between tree parts, which encourages quick healing at the point where the twigs drop off. The abscission zone is always on standby, ready to sever a branch whenever it becomes a net energy liability.

Not all trees are natural shedders. Some are extremely reluctant to give up branches under any circumstances, while others will drop branches quickly and easily. The willows on our farm fall into the latter category, as do poplars, maples, ashes and oaks.

On the other hand, trees such as white pine lack abscission zones and are poor natural shedders. If the branch of a white pine dies, it clings to the trunk until outside forces knock it off the tree. That force could be an animal, inclement weather, or even the presence of fungi growing on the dead tissue, which can weaken the connection between branch and trunk to the point that its own weight snaps it free.

Lumbering towards Perfection
Cladoptosis isn’t just an issue for gardeners – it actually has a measurable impact on the lumber industry. Trees that naturally shed have fewer and smaller knots in their lumber, whereas trees that don’t undergo cladoptosis develop so-called “loose” knots that reduce the grade of the lumber. Loose knots indicate that the branches have died and shrunk, while the healthy tissue around the knot has continued to grow, leaving a distinctive zone of empty space around the knot’s periphery.

Another Fall Issue
When the twigs or branches of your trees start tumbling down, it can be a little alarming. But if the tree is healthy and well-maintained, such shedding is completely normal – just think of it as your tree’s way of getting rid of the freeloaders. It’s actually saving you from a little pruning work!