I’ve noticed an interesting trend with the oaks in my neighborhood this year. I know these photos are all dreary and grey, but it’s really the only time of year to capture this phenomenon. Check it out:

Lots of folks know oaks hold onto their leaves for a bit longer than everyone else. But in the photo above, only the lower stems are retaining their leaves. The upper crown has lost its leaves.

This oak is doing the same.

It’s pretty consistent in my neighborhood where there are a lot of mature oaks. What’s up with this?

After discussion with a handful of arborist friends, we think we have an answer. It has to do with the age of the stems. See the oak below? It’s a pretty young oak.

And its entire crown is holding onto its leaves. Holding onto their leaves is called ‘marcescence’. Lots of plants do this, and currently it is believed this behavior deters herbivores from grazing on their stems.

Seeing a young oak hold onto all of its leaves makes sense because where oaks evolved, herbivores throughout time have been able to reach all of their leaves. And so only oaks that developed this trait survived coexisting with big herbivores.

As oaks age and become taller, the need to hang onto their leaves diminishes, since big herbivores can’t reach their leaves. But, sometimes the lower stems of some oaks still retain their leaves, despite being high up.

This suggests it isn’t height alone that is responsible for marcescence fading away over time.

Instead, it is due to the age of the stem. The lower, primary stems of the oaks in my neighborhood have all been removed by tree cutters. Primary stems are ones that are the first to develop with the tree’s initial maturing growth phase.

Following that, they tend to develop ‘epicormic’ stems, stems that begin growth in response to some damage. Epicormic stems aren’t primary stems, and so have a “younger age”, since they weren’t present when the tree was initially maturing.

My friend Alex said it best:

As the tree ages and grows higher, the need for it disappears. Since epicormic growth can be seen as a younger iteration of the parent stem, they retain the trait they had when they were small.

The epicormic, younger stems “think” they are a small tree, and so behave like one.

What we claim is that the marcescent behavior is an indicator of if a stem is a primary stem or a young epicormic stem. Epicormic stems can develop to an age where they too grow out of this trait.

People often think they are unsightly and will prune them off. But you might want to think twice about doing that. Epicormic stems are an awesome method that trees can reiterate, and their presence is largely credited for keeping trees alive during extremely stressful events as they live for hundreds of years.

That’s all for now. Turns out I couldn’t vacation without writing :)

If you want to read some more, I just published a long article about the relationships of gymnosperms. Trees like pines and firs.

Talk to ya next time,

TF