Transforming Trees

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.

 

1

Auxin (technically, indole-3-acetic acid)

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.

 

2

Auxin produced in the leaf blade flows through the petiole to the twig.

 

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.

3

Close-up of the bud and abscission layer location on a thimbleberry plant.

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.]

4

Thimbleberry bud and twig after leaf abscission

 

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.

5

Thimbleberry petiole after cutting off the leaf blade

 

I checked on the plant once a week. In a couple of weeks, I found what you see in the picture below.

 

6

Same twig, showing the loss of the petiole with no leaf blade

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;

7

A bare branch

8

a bunch of fallen leaves

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.

9

The leaves are still attached

 

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.

10


Brown curled dead leaves hanging on a salmonberry

 

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.

 

The Verdict

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.

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Posted on November 29, 2015, in Trees, Tryon Creek and tagged , , , , , , , , , , . Bookmark the permalink. 7 Comments.

  1. Dear Dr Natural-ist

    This comment is off-topic but then, it’s me- Carl Axelsen. Here is a story you might enjoy and even pass on.

    While working to cage a cedar against marauding beaver, a lady walker stopped and asked “What are you doing?” I told her “Caging these cedars.” “Why? Do they try to escape?”

    Try story. I am fairly certain it was said tongue-in-cheek and was quite witty as such. But….you never know.

  2. Thanks for the lesson – I’m delighted to learn more about nature.

  3. In order for the tree (a pin oak perhaps which weirdly keeps its leaves until new buds begin to open in spring) to keep its leaves all winter, there must be an advantage in nourishment still coming from the leaf.

  4. Hi, Ginny: Thanks for your comment. GOOD FOR YOU, in asking what benefit this is to the tree. First, let me say that the retained leaves are, for purposes of photosynthesis, dead. They do NOT continue to contribute sugars to the tree’s stem, and because they are already functionally sealed off from the tree, they are not recycling nutrients back to the twigs either.

    The only study I have found on the benefits of marcesence is one where deer were offered twigs of species that normally retain their leaves over winter. For several species, the deer preferred twigs from which the leaves had been stripped off by hand over the twigs that still had the dead leaves on them. (Oaks were unfortunately not one of these species.) So perhaps this is a browsing protection for some species. Other “browsing” protection *speculations* I encountered were that the dead leaves were protecting the buds from hungry squirrels.

    Other speculations are that the dead leaves help shield the buds from the winter environment. Another theory is that by retaining the leaves all winter, the leaves then drop to the ground and start decaying and releasing nutrients right at the time when the tree is starting to grow and needs nutrients. Neither of these hypotheses have been tested yet as far as I can determine. Potential downside – in snowy areas, the retention of leaves might lead to a greater percentage of limbs being broken off.

    None of the studies I reviewed directly referenced the fact that marcesence *tends* to be associated with the physiologically juvenile parts of the plant (see an earlier Naturalist’s Note of Adolescent Antics of Ash).

    Thanks for your thoughtful question, Ginny.

    — Bruce Rottink

  5. Ginny’s question has been rolling around in my mind all day. At the risk of getting too geeky, I decided to bring up another *possible* reason why some trees might hold onto their dead leaves all winter.

    There is a phenomenon in genetics called “pleiotropy.” This means that there is one gene that impacts two or more traits in an organism. One example of pleiotropy is the disease of humans called “phenylketonuria.” This is caused by a single gene mutation. This one gene mutation causes both mental retardation as well as reduced hair and skin pigmentation. So one gene is controlling more than one characteristic.

    Thus it is *possible* (let me emphasize *possible*) that a gene which causes marcesence also controls some other trait in the plant that is very important, possibly highly advantageous. The marcesence itself may be neither positive nor negative, but rather just a side-effect of some gene that is doing something very important for the tree. What is this important thing? I have no idea!

    Let me emphasize that pleiotropy MAY be the answer to this question, but I am aware of absolutely no scientific results that either support or negate this hypothesis!

  6. Jennifer Godfrey

    Fascinating Post! And great discussion! Do you know, is there a similar hormone trigger and abscission process in the “self pruning” nature of Douglas Fir trees? I always notice the discarded lower branches when there is not enough light .

  7. Jennifer, I’m glad you like the post and discussion. Great question, but rather than answer it briefly, I’ve decided to answer it with a whole new Naturalist Note. Probably early next month (being realistic) but it will give you a much better and broader perspective than a quickie comment would.

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