The Falling Leaves
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The forest at Tryon Creek State Natural Area (TCSNA) is currently completing one of its most dramatic transformations. The leaves of many plants die and fall to the ground. But wait – do they just die, or is it closer to “murder most foul?” Read the facts, and you can be the judge!
Why do some plants shed their leaves?
Many plants lose their leaves each fall, all the way from bigleaf maple (Acer macrophyllum) to thimbleberry (Rubus parviflorus). These plants have leaves which function best at warm temperatures and long days; in other words, during the summer. With summer conditions, they manufacture lots of sugar for the whole plant.
However, as leaf activity slows down in late summer less and less sugar is produced by the leaf. The plant as a whole operates on the philosophy of Vladimir Lenin, a founding father of the Soviet Union: “He who does not work, neither shall he eat.” In other words, if a leaf is not contributing to the whole plant, the whole plant will not support the leaf.
How does the plant know when it’s time to shed a leaf?
The plant’s leaves produce not just sugar but several plant hormones as well. One of these hormones is auxin. The structure of the most common auxin is shown below.
Healthy, active leaves produce lots of auxin. The auxin produced by the leaf moves from the leaf, down through the petiole (the stalk that attaches the leaf blade to the stem) into the twigs and branches, as shown in the thimbleberry leaf below.
The plant tissues use the amount of auxin moving from the leaf as an indicator of leaf activity. When there’s lots of auxin flowing through the petiole, the plant knows the leaf is being productive. Low auxin levels coming out of the leaf is a signal to the plant that the leaf’s activity is slowing down, and it’s time to ditch that leaf.
So what happens to the leaf?
At the base of each leaf, where the petiole joins the twig, there are two things: a bud, and an abscission layer. By mid-summer, the buds become quite prominent, as can be seen in the close-up of a thimbleberry below. The abscission layer is a very thin layer of cells near the base of the petiole.
Below is a picture of a thimbleberry twig and bud just after the leaf has abscissed. [Note to Nature Nerds: For most deciduous plants, the abscission zone is right next to the twig, and there is no “base of the petiole” left after leaf fall. Eventually the base falls off too.]
How does the abscission layer work?
The abscission layer is very sensitive to the amount of auxin flowing through the petiole. When the level of auxin drops in the fall, the cells of the abscission layer become active. Those cells nearest the twig start to seal off the twig from the leaf. They are in essence creating a scab on the twig, even before there is a wound. Meanwhile the abscission layer cells nearer the leaf blade start to become very fragile. When the “scab” is complete, the fragile cells at the base of the petiole are so weak the leaf will break off in the slightest breeze.
To show how this works, I did a little demonstration on a thimbleberry plant growing on the side of the road at TCSNA. I cut off one leaf blade, leaving only the petiole attached to the stem of the plant. The result is pictured below.
I checked on the plant once a week. In a couple of weeks, I found what you see in the picture below.
The petiole from which I had removed the leaf blade had fallen off the twig, in spite of the fact that the leaves and their petioles above and below it on the stem were perfectly green and healthy. Since the petiole without the leaf was producing very little auxin, the cells in the abscission layer got busy, and isolated the petiole from the rest of the plant. This caused the petiole to die, and drop to the ground. One of the lessons here is that it takes a while for the abscission layer to kick into gear and isolate the petiole and leaf from the rest of the plant.
To demonstrate the activity of the abscission layer, I set up a small demonstration. One summery day, I collected two small branches of vine maple (Acer circinatum). I put one of the branches in a vase of water. With the other branch, I did what any normal person would do, I microwaved it for one minute, and then put it in a vase of water. (Note: My wife is never surprised by this sort of thing going on at our house. She is a saint! And you only read about the stuff that worked. But I digress….)
The results with these two branches are shown below:
Results of putting a fresh vine maple branch in a vase of water for 2 weeks;
The result of the vine maple branch I microwaved, and then put in a vase of water for a couple of weeks is shown below.
So what happened here anyway? The first picture is not a surprise to those who have kept flowers in a vase on the table. The leaves stay alive, but slow down tremendously, lowering the level of auxin production. The cells of the abscission layer sense this lower auxin level, and begin the process of isolating the leaf tissue from the rest of the plant and becoming fragile. The leaves then fall off.
In the second case, the microwaving kills both the cells in the leaf, and the cells in the abscission layer. Once the abscission layers cells are killed, they will never be able to either seal off the leaf from the branch, or become fragile. Hence the leaves never fall off.
Conversely, when scientists have removed the leaf blade from the petiole, but artificially supplied the petiole with auxin, the petiole remained attached to the branch indefinitely.
Okay, weird; but is it relevant to nature?
Yes! This explains something that you occasionally see in the forest. Sometimes you will see some brown, curled leafs which are obviously dead, still hanging on a plant. For example, the dead leaves hanging onto this salmonberry (Rubus spectabilis) plant along the Red Fox Trail.
Why didn’t the abscission layer kick in and isolate these leaves, and cause them to fall off the plant? The answer in this case is that this whole branch, including all of the cells in the abscission layer, died rather quickly, due to the supporting branch having been broken. These abscission layer cells weren’t alive long enough to seal off the leaves and cause them to drop off.
So did the leaves die all by themselves, or were they murdered by the plant’s abscission layer when they stopped being productive? You can decide for yourself, but for me, I call it “murder most foul.” The forest as a place of peace and tranquility? Not hardly!
Why can’t Nature be simple?
Just be aware that a few deciduous plants, including some oak trees, have abscission layers that partially form in the fall (enough to kill the leaves) but finish developing in the spring, so the trees hold onto their dead leaves all winter. These trees are referred to as being marcescent. What’s worse, in a few of these marcesent species, only the lower (juvenile) parts of the tree are marcesent, while the upper (mature) parts aren’t. I should stop now!
Why do you think some trees hold on to their leaves? We’d love to know your thoughts, leave us a comment with your guess.
Posted on November 29, 2015, in Trees, Tryon Creek and tagged abscission, Auxin, Fallen leaves, forest, Maple, marcesent, Nature, Thimbleberry, Transformation, Trees, Tryon Creek State Natural Area. Bookmark the permalink. 7 Comments.