Category Archives: 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.
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.
Fungi in the Forest
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The fungi (singular = fungus) at Tryon Creek State Natural Area (TCSNA) are endlessly interesting. They’re more than just the cute mushrooms along the side of the trail. They take on many forms, and have more diverse lifestyles and habitat preferences than we might imagine.
What do fungi look like?
When you say “fungus” most people think of either mushrooms, or that fuzzy stuff on the tomato you left on the counter too long. In truth, they can look like almost anything.
A hypha (plural = hyphae) is a single living “thread” of a fungus. Hyphae are the building blocks of every part of the fungus. Even mushrooms, the fruiting body of some fungi, are just a tightly packed bunch of hyphae. The fungus pictured below grew underneath the loose bark of a dead western redcedar (Thuja plicata) tree on the Big Fir Trail. The fuzzy white strands surrounding the solid central tan fungal mass are a good example of fungal hyphae.
Some fungi do something really different. The honey fungus (Armillaria mellea), for example, sometimes uses a huge number of hyphae to create a structure called a “rhizomorph”. Rhizomorph roughly means “root-like thing.” These rhizomorphs are essentially hollow tubes made up of many, many hyphae. They are typically 3 or 4 mm wide (about 1/8 of an inch). Rhizomorphs are created by the fungus to move water and nutrients from one place to another. It has been suggested that when the fungus starts running out of nutrients in one place, it uses the rhizomorph to “explore” for another good location, and then transports some nutrients to that location to jump start the next fungal infection. The rhizomorphs of the honey fungus are black and about the same dimensions as a shoelace. In fact, foresters refer to this species as “the shoelace fungus.” The rhizomorph frequently grows between the bark and the woody tissue of the tree. The photo below shows a log lying alongside Old Main Trail. Several rhizomorphs are clustered side-by-side to form the big black splotch in the middle. This rhizomorph became visible when the bark fell off the log.
What else might fungi look like?
One form of fungus that is common to the bigleaf maple (Acer macrophyllum), is powdery mildew that grows on the surface of the leaf. Mostly it just looks like a light coating of white fuzz or dust, but it is a fungus. Sometimes you may see black dots which are the spore-producing bodies of this fungus.
What do fungi like to eat?
You might think that since fungi are mostly just “rotting stuff” that they would eat anything. However, certain fungi have definite food preferences. Take for example wood decaying fungi. In medium or larger woody plants, there are two distinct kinds of wood. In the cross-section of a Douglas-fir (Pseudotsuga menziesii) below you can see two different colors of wood. The peach-colored wood in the middle is the heartwood, while the pale wood around the edge is the sapwood.
For fungi, the heartwood and the sapwood are two different types of food.
The bad news: It is loaded with chemicals (generically called “extractives”) which inhibit the growth of fungi. It contains very little easily digestible nutrients like stored starch.
The good news: There are no living cells to “fight back” against a fungal invasion.
The bad news: There are lots of living cells which sometimes fight back against the fungi.
The good news: There are lots of easily digestible stored foods like starch and sugars.
As you might suspect, some fungi have evolved to favor the heartwood, and some fungi have evolved to favor the sapwood. One manifestation of these preferences was found on the end of a log at TCSNA on the ground near the Nature Center. The fungal fruiting bodies indicated by the red arrows are from a species growing in the sapwood. The fungal fruiting bodies indicated by the white arrows are of a species growing in the heartwood. The third fungus, indicated by the yellow arrow, is also growing in the sapwood. I have seen other examples of this kind of distribution, but this was the most dramatic. Please note that the lower part of the log is hidden by the dead leaves on the ground.
What parts of the wood are the fungi eating?
Not only do different fungi attack different parts of the tree, they also eat different chemicals in the wood. Wood is made up of two main chemicals. The first is cellulose, and the second is lignin. Cellulose is a simple molecule made up of a long chain of glucose (a type of sugar) molecules, and only glucose molecules. Each glucose molecule is bonded to the next glucose in the same way. Lignin, in comparison, is a very complex molecule. It is made up of a variety of molecules, which are linked together into a network, not a nice straight chain. In addition, the bonds between the molecules which make up lignin are extremely variable.
Fungi use enzymes to break down large molecules. Digestive enzymes usually have one very specific bond that they break. The bottom line is that cellulose is easier to digest than lignin. The diagrams below convey the idea of lignin being complex compared to simplicity of cellulose. The colored shapes are the component molecules of cellulose and lignin, and the black lines (or chains, or spirals) represent the diversity of bonds between those molecules.
In the real world, most wood appears to be light brown. Cellulose is pure white. Lignin is brown. So if a fungus eats all the lignin, the residue looks white. (Perversely, these fungi are called “white rot”, although the part of the wood they are eating is brown. Oh well!) My experience is that examples of white rot are less common than examples of brown rot. Pictured below is an example of white rot from a log near the Red Fox Trail.
The fibrous cellulose left behind by the fungus is chemically identical to cotton fibers. Just think, your next tee shirt could be made out of a tree!
The brown rot fungi that only eat cellulose, leave behind the lignin. This is quite common. A stump with the cellulose eaten out of it is pictured below.
After the wood is attacked by brown rot fungi, it tends to break into cubes. This leads to another name for this type of fungi, “brown cubical rot.” The lignin is not fibrous at all. This is seen in the picture below.
White rot, brown rot, heart rot, powdery mildew and more, the fungi of TCSNA are a diverse and interesting bunch. They play many important roles in our forest’s ecosystem. While their small size frequently makes them hard to find, keep your eyes peeled! The rains of fall have already started to bring out their fruiting bodies in all their glory.
Discover more about our Fantastic Fungi here!
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.