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.
Buds: A Bridge to the Future
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Winter is a tough time for the woody plants at Tryon Creek State Natural Area (TCSNA). The air gets both colder and, when the temperature dips below freezing, much drier. Most of the plants stop growing, and some shed their leaves. However, the plants have to be prepared for the next growing season. To prepare, they form buds as a “bridge” to the future. By September the buds are a conspicuous feature of woody plants at TCSNA .
A woody plant’s bud might merely look like a hard little blob on a branch, like this bud of a European hazel (Corylus avellana) growing near TCSNA’s main parking lot.
But the buds of TCSNA’s woody plants are actually quite interesting.
So what exactly is a “bud”?
At the tip of each branch is a small cluster of active cells called the apical meristem. At some point in the spring-summer-fall (it varies with different species), the apical meristem starts differentiating and forming a bud consisting of a variety of structures. These structures can be bud scales, leaves or flowers. These tiny structures rest over the winter, and come spring, they start growing. Even the tough-looking bud scales elongate a bit in the spring.
Below is a picture of a bigleaf maple (Acer macrophyllum) bud which is just starting to open. The different parts of the bud are labeled. The “scale to leaf” transition components have very, very tiny leaf parts at the tip of the scale, you’ll have to look closely.
Last spring I picked another newly opened bud of bigleaf maple which was slightly more advanced than the one pictured above. I took it apart to more clearly show the different components. In the picture below, the parts from the base of the bud are at the left hand side, and the other structures are arrayed in order, right up to the flower, which was at the tip. For completeness, at the base of the floral stem are two tiny meristems (not visible here) that will create next year’s buds.
Since the maples produce structures in pairs, one on each side of the stem, there are always an even number of scales and leaves. The flower is an exception to this rule. The “scale to leaf” transition phase is the most interesting. The leaves and flowers have a perfectly round “stem” connecting them to the branch of the plant. The “scale to leaf transition” structures are dwarf leaves supported by a flattened “stem” that resembles the bud scales in shape. These structures clearly demonstrate the plant’s flexibility when it comes to producing different parts. It’s not a clear “one thing or another” decision. (Note: Not all maple buds have these part-scale/part-leaf structures.)
Now it starts to get really interesting!
Okay that’s the basic pattern, but with dozens of different species of woody plants growing at TCSNA, we’ve got lots of variations in buds.
The first type of “weird bud” is the naked bud. This means a bud that has no bud scales. Our native cascara buckthorn (Frangula purshiana) produces naked buds as seen below. The leaves are fully exposed to the winter environment, but are very tough, and slightly hairy. If you want to see a cascara, go to Beaver Bridge. The cascara is about 5 feet upstream from the bridge on the side of the creek furthest from the Nature Center.
Flower Buds, Leaf Buds and Both of them!
As you saw with the bigleaf maple, some buds contain both leaves and flowers, but some contain only leaves and some contain only flowers.
Oftentimes you can tell if the bud contains flowers even before the buds open. In the picture below are two buds of Indian plum (Oemleria cerasiformis) collected from the same branch. They are just starting to expand in the spring. The big fat bud with the rounded end contains both leaves and flowers, while the skinny one contains only leaves.
Indian plum also teaches us that the term “bud break” is ambiguous at best. Below is a picture of a newly opened Indian plum “bud” containing both leaves and flowers. The young leaves including their tiny veins are clearly visible. The flowers are still contained with their own separate “buds.” So, with the Indian plum we have a bud within a bud.
The Indian plum plants are either male or female, and with rare exceptions, will have only functional male OR female flowers on a single plant.
Keeping it all together
In contrast, some plants have both male buds and female buds on the same plant. Red alder (Alnus rubra) is a good example. The alder tree has buds that only contain leaves, other buds that only contain female flowers, and other buds that only contain male flowers. The photo below shows the three different kinds of overwintering alder buds.
How else can the buds be different?
One of the most important ways that buds are different is that some species have determinate, buds and other species have indeterminate buds. Overwintering determinate buds contain all the organs (like leaves, needles, flowers, whatever) that will appear the following year. Indeterminate buds hold only a few of the organs that may appear next year.
A good example of a determinate bud is our Douglas-fir (Pseudotsuga menziesii). In the winter, all of the needles that will grow out of the Douglas-fir bud the next year are already present in primitive, miniature form call “primordia.” Pictured below is a Douglas-fir bud which I collected in late August and stripped off all its scales. Each little needle primordium will turn into an actual needle early next spring. Two of the primordia are indicated with black arrows. All of the needles destined for the 2016 branch are represented by a little bump of tissue. This entire green structure is approximately 2 mm (1/12”) in diameter. For plants with determinate buds, it is easy to see why the environment of one year is so important in influencing the growth of the plant in the following year.
In contrast, TCSNA’s black cottonwood (Populus balsamifera var. trichocarpa) has an indeterminate bud. For cottonwood this typically means that there are three or four relatively big pre-formed leaves that overwinter in the bud. Come spring, these leaves will expand very quickly, and start producing sugar for the plant. If the weather conditions are good, the apical meristem will create another leaf from scratch, and when that is done, the tree might produce a couple of more leaves, etc. This is why for indeterminate plants, shoot growth in any one year is profoundly affected by the environment in the current year, not the previous year!
Below is a whole cottonwood bud, and the same bud with the scales removed. In the second photo you can easily see two preformed leaves, two more are hidden on the backside of the bud.
The photo below shows a single, preformed cottonwood leaf in the bud. The light streak is the main vein which will go down the center of the mature leaf.
The photo below is of a black cottonwood shoot not too long after it emerged from the bud. You can see the 3 large leaves which were preformed, and the fourth, smaller, leaf which formed after the bud broke.
For the woody plants at TCSNA, the overwintering buds ARE the future. It is amazing that the fate of something as large as a tree rests within the tiny buds which bridge the gap between the growing seasons. All things considered, buds deserve a lot more attention than they receive.
Energizing the Forest
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Every living thing needs energy. Plants and some microorganisms use the process of photosynthesis to directly capture the energy of sunlight to meet this need. These organisms are called autotrophs, meaning “self-feeders.” Almost all other organisms rely on “eating” energy-containing organic matter produced by the autotrophs (see a previous Naturalist’s Note, “What’s the Yucky Stuff in the Creek” to read about one exception). These creatures are called heterotrophs, which roughly means “feeding on others.” Heterotrophs might eat autotrophs or other heterotrophs.
How does the energy move through the forest?
Ecologists have long been interested in studying how energy flows through the whole complex of autotrophs and heterotrophs in the forest. In “the old days” we were taught about food chains. In a food chain, trees captured the energy of the sun, and produced seeds, squirrels ate the seeds, and then hawks ate the squirrels. That’s correct as far as it goes, but is a bit too linear. Scientists now advocate a concept they call a “food web.” It’s pretty much the same idea, but recognizes that lots of heterotrophs have a varied diet and that a linear chain is too simplistic. For example, a tree produces seeds and a thimbleberry produces leaves. The seeds are eaten by squirrels. The leaves are eaten by insects. Birds eat the insects, and owls eat both the birds and the squirrels. For simplicity, in this posting I’ll stick with the food chain concept.
Each organism is the source of energy for the next organism up the food chain. However, there is one big factor to consider. Every organism uses up a certain amount of the energy it acquires to perform vital functions which might include staying warm or moving around. This “maintenance” energy is being used, or lost, at every level in the food chain. Thus, there is more energy available to the insects eating the plants than there is to birds eating the insects.
What effect does this maintenance energy loss have?
The most obvious effect of energy loss at each level in the food chain is that there is “more” of the organisms at the lower level of the food chain than at the upper level. Let’s look at a simple food chain at Tryon Creek State Natural Area (TCSNA). For this example I’ll use the chain of plant-worm-mole-owl.
At the very bottom, think of it as the “foundation,” are plants, which capture solar energy to make sugar. One example at TCSNA is the vine maple shown below.
The next step is the earthworms, pictured below, which feed on dead plant matter.
The next step in our example is a mole, which eats the earthworms.
The next link in the chain is an owl. This owl’s ready for a nice mole dinner!
To recap, our sample food chain is: plants capture energy from the sun, worms eat dead plant material, moles eat worms and owls eat moles. Owls are an example of what’s called an “apex predator,” meaning it is at the top of the food chain.
When you start thinking about food chains, you might develop a different perspective about the creatures you see in the forest. When you think of owls as eating moles you might view them differently. Take the great horned owl in the photo below:
Click on the owl’s picture to reveal a different perspective on owls.
An owl is just earthworms which have been reconfigured to fly.
How does the “maintenance energy” loss effect the food chain?
As mentioned above, each link in the food chain uses up some of the energy it captures just to stay alive. So each link up the food chain contains less biomass (weight) than the link just below it. It is useful to think in terms of biomass per unit area of land. For this discussion, the area of land is TCSNA.
Based on recent TCSNA citizen science project owl numbers (thanks to Matthew Collins), and the estimates of weights of the different species, there are approximately 20 lbs. of owls living at TCSNA.
They are represented by this red square with an area of 1 square inch.
Biomass numbers for some of the forest organisms aren’t easy to come by. Apparently everyone figures they had something better to do than find out how many pounds of moles there are in an acre of forest. So instead of moles, I’ll use some biomass numbers for mice found in a study at the H. J. Andrews Experimental Forest near Corvallis. Owls eat mice, and other things, as well as moles.
Using the same scale as the red square for the owls, the weight of mice at TCSNA is estimated at 506 lbs. It is represented by the gray square below. One caution should be noted. The biomass numbers always represent just one point in time (“the standing crop” in ecology-speak). In reality, these mice are constantly having babies and some of the babies and adults are constantly being eaten. Therefore, over the course of the year, the heterotrophs of TCSNA probably have the opportunity to eat a lot more than 506 lbs. of mice.
Numbers for the standing crop of worms that I could find were so wildly variable, that I decided to skip those.
Extrapolating from plant biomass data collected at several Pacific Northwest forests, there are approximately 197 million lbs. of trees and shrubs at TCSNA. Using the same scale as for the owls and mice, this would be a square approximately the area of 6 or 7 average city lots. Think about cutting out the 1 inch red square representing owls, and laying it out on your front yard, and comparing the size of that red square to the sum total of the sizes of your lot, your neighbor’s lots on both sides, and the three lots across the street. When comparing the biomass of the plants to the biomass of the owls, owls almost don’t exist.
But owls are the top of the food chain, right?
As I said before, owls are considered to be an apex predator in our forest. In an area the size of TCSNA they would provide very little food for the next link up the chain. In order for a predator to survive by eating owls, it would have to cover a huge territory to get enough food. Think of a good size hawk hunting over a big chunk of the Willamette Valley!
But there’s another option!
We’ve probably all watched too many movies of lions killing gazelles on the plains of Africa. This has created a sense of “prey” and “predator” that has limited usefulness when we think about the flow of energy in an ecosystem. Rather than think about a large owl-eating predator that covers huge areas, let’s look at an alternative.
Since there are only enough owls to support a small weight of the next level up the food chain, what if the organisms at the next level of the food chain were really small? When scientists studied owls in northern Idaho, they found something very interesting!
Click on the blood drop below to find out what the scientists discovered.
Owl’s blood containing red blood cells AND blood parasites (the horned demons)
That’s right, blood parasites were found in more than half of the northern saw-whet owls (a species we have at TCSNA). The parasites were getting energy from the owl’s blood. This isn’t the traditional view of “eating” something, but for energy flow purposes, it’s the same thing. The biomass of the blood parasites that are probably in owls at TCSNA is incredibly small, which fits into the pattern we have seen so far in the food chain. I am reluctant to declare that the blood parasites are the final link in the energy flow of our forest. However I will stop here, you get the idea!
All organisms in the forest need energy. As you can see, Mother Nature has been very creative in developing a variety of ways in which energy flows through our ecosystem. Enjoy the many manifestations of that energy as you, a child of the sun, hike through TCSNA.