Keep 'em or Drop 'em?

Posted on November 18, 2022

Normally, as the days get shorter and air turns colder in the Fall, the leaves on deciduous trees stop producing food for the tree (aka photosynthesis), thus losing their green color. Eventually the tree expels its leaves using a sort of cellular “scissors,” since they aren’t really of use any longer. But a few trees like beeches and some oaks tend to hold onto their leaves. This is called marcescence.

There are good reasons why trees shed their leaves, mainly to reduce water loss and protect against frost damage during their dormant season. So what’s the advantage for some to hang onto them? Ecologists don’t really know exactly, but they have some theories, based on the fact that marcescence most often shows up in young trees, or on lower branches. Take note the next time you’re on a hike, you’ll see that it’s the little beech trees in the understory that still have their leaves, not so much the big mature ones.

So, Theory #1: When leaves drop, they decompose on the ground around the tree, providing essential nutrients to the tree later on. Beeches and oaks tend to grow in drier places, so by holding the leaves through the winter, they ensure they drop in the spring when the new buds finally push them off. This means they will decompose at the time the tree needs those nutrients the most.

Theory #2: Leaves on young, shorter trees or on low branches trap snow, providing the tree more moisture come spring.

Theory #3: Marcescent leaves provide protection for buds over the winter, both from frost, and hiding them from browsing deer.

Or, Theory #4: Trees that keep their leaves are just old-school! At one time, all trees were evergreens, in other words, not deciduous. Over time, some adapted to changing conditions as their ranges increased, and slowly evolved into deciduous trees. Maybe beeches and oaks are just a little behind other species, and are still evolving into fully deciduous trees.

We are guessing it’s a combination of all these theories that is the answer. What do you think?