Monthly Archives: November 2015
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.
Mother Nature: Master Recycler
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Recycling has become a “big deal” for humans in the years since I was born! As a kid, I remember burning newspapers in the burn-barrel by our garden, and throwing a massive number of tin cans in the garbage which doubtless wound up in some landfill. Now, the Oregon Department of Environmental Quality (DEQ) proudly reports that in 2013, Oregonians recycled 53.9% of our post-consumer waste. Nice try, but we don’t even come close to Mother Nature’s record of recycling: 100%!
Here at Tryon Creek State Natural Area (TCSNA) the forest constantly recycles biomass like leaves, logs and dead animals. Sometimes it’s fast, and sometimes it’s slow, but it’s always thorough!
We need look no further than some of the TCSNA’s old logs and stumps to see that sometimes recycling takes quite a while. This rotting log is approximately 18” in diameter. It is on the side of Old Main Trail and hasn’t changed much in the last 5 years, and I don’t expect it to completely disintegrate any time soon. In fact, research foresters report that fallen Douglas-fir (Pseudotsuga menziesii) logs sometimes take almost 200 years to completely decay!
But other stuff “recycles” faster, doesn’t it?
Absolutely! In order to see how fast things are recycling (“decaying”) at TCSNA, I set up a small study. Scientists who want to study recycling in the forest often use things called “litter bags.” (The term “litter” here refers to the fallen leaves, twigs and branches on the ground, not to candy wrappers and used Kleenex!) I collected the plant material for this study off the ground, so this material was ready to start decaying.
I cut square pieces of window screen to make my bags. I placed the plant material on half of each piece of screen, and then folded the other half over the top, and stapled it shut. I fastened each bag to the ground using four big nails, one in each corner. I set up the bags in an area of TCSNA where they wouldn’t be disturbed. I put out some bags on September 11 and others on September 13, 2014.
In each bag, I put one of five things; Alder (Alnus rubra) leaves, bigleaf maple (Acer macrophyllum) leaves, western redcedar (Thuja plicata) twigs with their green scaly leaves, Douglas-fir twigs with needles, and finally, the scales from a Douglas-fir cone. I had two bags of each type of material. Then I fastened the bags to the ground.
So what happened?
This is what it looked like on September 13, 2014, after the full study was installed. You can see some of the green leaves inside the bags.
I took pictures of every bag each month. When I took pictures I brushed off the top of the bag, loosened two of the nails holding it to the ground, and slipped a piece of white plastic underneath the bag to provide contrast to the material inside the bag. I refastened the bags and replaced the litter following each photo. Below are some highlights.
By October 3, 2014 some natural forest litter had fallen on the bags. This is totally realistic. There were times when the litter bags were almost completely covered with natural litter from the trees.
Having the litter inside bags did create a certain amount of unrealism. This point was made dramatically during my March 2015 visit to the litter bags, when the little critter pictured below was crawling over them. To the extent that snails might accelerate litter decomposition, my study was only an approximation of reality.
So let’s see the decay process!
The decay rates for the samples in my litter bags varied a lot between species, and sometimes between particular leaves of the same species.
The alder leaves decayed dramatically over the course of a year. After 79 days, the leaves had lost their color, but had only just started to disintegrate. By the end of March, the leaf in the upper half of the photos was pretty much reduced to the mid-rib (the tough “vein” going from the base of the leaf right through to the tip) and the lateral veins. In contrast, the leaf in the lower right hand corner still had a lot of the leaf blade left.
Once again, the maple leaves were significantly decayed after the first year, but the petiole (the stalk that attaches the leaf to the branch) being more “woody” than the leaf blade is still largely intact.
Western redcedar is loaded with hydrocarbon molecules that impart decay resistance. The most amazing thing was that in November 2015, after more than two months on the ground, most of the redcedar branch was still green! (Confession time: The other redcedar branch had turned completely brown at this point.) After over a year on the forest floor, this branch, and its needles, was still largely intact.
Unlike the alder and maple samples, this bag includes the woody twig in addition to the foliage. The Douglas-fir twig rapidly shed all its needles, producing an un-photogenic combination of a bare twig, and clumps of dead needles. The slight movement of the bags in preparation for taking photos is what caused the needles to gather in clumps. The needles, though brown and scattered, are individually maintaining their structural integrity. As with the western redcedar discussed earlier, the presence of hydrocarbon molecules in the needles and stem are helping resist decay.
Douglas-fir cone scales
The Douglas-fir cone scales are tough and woody. To tell the story of their decay in the first year, we only need two photos. In the 12+ months in the litter bag, there was no perceptible change in the Douglas-fir cone scales, except they are now slightly darker! Again, shifting the bag for photos results in shifting the scales around within the bag.
The Cycle: Life > Death > Life
As organic matter decays, important chemicals like nitrogen and phosphorus are slowly released to soil for growing plants. The partially decayed organic matter in the soil dramatically increases its moisture holding capacity, and water infiltration rates, among other things. Better than most of us, Mother Nature knows that the rotting leaves and stems of today are the key to the towering trees of tomorrow! Without recycling, there would be no forest as we know it.