Two weeks ago Hurricane Sandy passed directly over this part of PA. While we didn’t suffer the devastation of the Jersey shore, we did have rain and wind. But even with all of the wind, the leaves stayed on a majority of the trees. That got me thinking about how leaves fall from a tree. I was reminded of one of the few things I remember from plant anatomy class and it all has to do with some special cells that connect the leaf to the twig.
As a tree gets ready for winter, it has to somehow deal with its leaves. If the leaves were to stay on the tree, there would be a lot of damage in the winter due to the weight of snow building up on the branches and leaves. Just think of the damage that occurs when we have an early snowfall before the leaves are gone. Somehow the tree needs to lose its leaves before the winter weather.
But the tree also has to accomplish this leaf loss in such a way that there isn’t a lot of exposed tissue. If leaves were just pulled off of a tree, the places were the leaves were connected to the twig would be an open wound. This would be an avenue that would allow bacteria and fungi to enter the tree. So the tree needs to not only lose its leaves but also do it in a way that protects it from pathogens.
Deciduous trees and bushes have evolved a way to lose their leaves in the fall (abscission) while doing so without creating open wounds. It all has to do with the abscission zone.
In the area where the petiole of the leaf (the small “stem” of the leaf) joins the twig, there’s an abscission zone made up of two kinds of tissue – the abscission layer and the protective layer. The protective layer is closest to the twig; the abscission layer is between the protective layer and the petiole. Both of these layers are only a few cells thick. During the growing season this layer of cells is healthy and keeps the leaves attached to the tree. But when the days become shorter, things begin to change in this zone.
The protective layer begins a process called suberization in which suberin, a waxy chemical found in cork, builds up in the cells. This provides an area of tissue that’s impervious to the outside elements. The suberized protective layer prevents leaf fall from leaving open wounds on the twigs.
While the protective layer is becoming filled with corky tissue, the abscission layer also transforms. Chemical changes in the tree cause this layer of cells to break down. When the cell walls have broken down sufficiently, the leaf will fall and the protective layer will show on the twig as a leaf scar.
It could be said that the leaves on a tree don’t fall so much as they are released by the tree. The tree breaks down the abscission layer and when it does, the leaves are sloughed off. Each kind of tree releases its leaves on its own schedule. In my yard the asian pear and redbud lose their leaves in early October while the maple and willow are just beginning to lose their leaves in mid November. When the abscission layer breaks down is species dependent but it eventually breaks down in all deciduous trees.
Sandy blew through the neighborhood but she couldn’t remove the leaves from the trees because they weren’t ready to be shed. But now the leaves are starting to fall in earnest. The protective layers have suberized and the abscission layers have broken down. The trees have done their part to prepare for winter. Now it’s time for me to do my part – rake the leaves!