Dead but Not Gone

Ever wonder why at this time of year a lot of deciduous trees look like this:
Instead of this:

How come the leaves died but didn’t fall off the tree? Do we need more wind?

There’s actually a word for it. It’s called marcescence. (I know. I just had to assure my spellchecker that it’s a real word.) Even though the word doesn’t exactly roll off the tongue, the phenomenon is interesting.

Normally, when deciduous trees sense that it’s time to close up shop for the season, they drain all the leaves of fluids (like water and chlorophyll), and form a scab where the stem of each leaf meets the branch. When the scab is complete, there is nothing holding the old leaf to the tree, and the slightest breeze blows it away.

The bare tree stands ready to tough out freezing temperatures and heavy precipitation (and I mean heavy in weight as well as volume).

But marcescent trees don’t finish that last part of the scab until winter is over, so the trees stand holding their dead leaves for three whole months. Why?

It’s a mystery.

It seems like holding all your leaves would be a liability if you were a tree in a climate prone to heavy snows and/or frequent ice storms, because the leaves would hold the precipitation and weigh down your branches until they broke off.

Marcescence occurs more frequently with young trees than old ones, and it is typical of only some species. Though no one is sure exactly why it occurs, there are a few interesting ideas…

The bare winter tree may look like a pile of sticks to us, but to an animal that just had to shift its diet from juicy green leaves to nuts, berries, and TWIGS, it looks like something else entirely. Leaf buds have formed on the branches. The buds and the branches are full of sugary sap (aka FOOD). A hungry deer may eat a small tree all the way to the ground.

But – to you, me, and the deer – a tree that holds its dead leaves looks like a dead tree, and dead trees are brittle and dry. The deer will skip over the dead-looking trees in search of juicier treats.

Another possibility is that the retained leaves act like thousands of blankets insulating the baby leaf buds from the cold. This may work out in climates where it sometimes gets cold enough to freeze water but not wet enough to weigh down branches.

Still another possibility is that the dead leaves function to keep moisture from evaporating out of the branches and buds. (Did you know that there are only two ways for water to get out of a plant? An animal takes it out, or it transpires through the leaves and stems. In essence, plants sweat.)

All of these ideas seem plausible. In fact, the marsescent tree may benefit in all of these ways.

Then again, maybe there’s still more to discover about the phenomenon. The forest is full of questions waiting to be asked.