Category Archives: Trees

Our Dynamic Forest

By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester


This year January brought us an unusually wet, heavy snow.  In my Lake Oswego backyard, it amounted to just over 7-1/2 inches of the white stuff.  The snow at Tryon Creek State Natural Area (TCSNA) was roughly similar.  As with so many other unusual events, it was a great opportunity to learn more about our forest.


The wet, heavy snow brought many changes.  Some that we humans, entranced with the visual wonder that is our forest, tend to regard as tragic.  But Nature may have a different view.  Let’s take a look at some of the things that happened.


Look out below! 

All kinds of trees fell down.  As shown in the photo below, the top snapped off from this red alder (Alnus rubra) growing near Red Fox Bridge.  You can see the top lying on the ground.  For the alder, this is a horrific setback, if not death.

Photo 1

Top of alder broken off near Red Fox Bridge.


However, the plants growing on the ground under this alder may have a different perspective.  I stood right over the alder trunk lying on the ground, pointed my camera upwards and took this picture of a significant hole in the canopy.

Photo 2

View of the sky where the alder fell down.


Do you suppose the plants growing on the ground are looking up and thinking, “Oh what a tragedy.  Now we’re going to be growing in full, life-giving sunlight, and we won’t have competition from the alder.”  No matter what kind of tragedy it was for the tree that fell down, many of the neighboring plants will be celebrating because of the extra sunlight they will be receiving.


And if the existing plants already on the ground aren’t able to jump in and take advantage of the newly sunny spot, rest assured that some new plants will.  The photo below shows numerous red alder seeds (two are marked with red arrows) on the Middle Creek Trail the very same day I photographed the broken alder.  Finding these tiny seeds in the forested area would be very difficult, but have no doubt, they are there!

Photo 3

Red alder seeds on the trail (green Douglas-fir needle at bottom provides perspective).


Death Cleanses the Forest

Perhaps you mourn the loss of so many good trees.  In at least some cases, your tears are wasted.  A storm like the one we had can be viewed in part as Nature cleaning up the forest.  For example, as part of a human cleanup effort, I spent some time cutting through the trunk of a western redcedar (Thuja plicata) that was lying across the Cedar Trail so the trail would become passable (see photo below).

Photo 4

Western redcedar stem lying across the trail (note pen for scale).


It was sad because it was a young tree, with potential to become one of the esteemed elders of the forest.  Or so I thought.  As I dragged some of the branches off the trail, I noticed the top of this tree (pictured below).

Photo 5

Dead top of the fallen western redcedar tree.


The top four to five feet of this tree had already been dead for some time.  So the real story was that this tree was already having problems of one kind or another, and the storm just ended its struggle.  Since it already had a dead top, its long term potential was not as great as I originally thought.


In another case, a very tall (about 115 foot) Douglas-fir (Pseudotsuga menziesii) fell down across the Old Main Trail.  This is another tree that I cleared off the trail (Note:  The clean-up work I did after the storm proved very educational.  You might want to give it a try!)  The top was forked due to some damage many years ago, as indicated in the picture below.

Photo 6

Fork-topped Douglas-fir on the ground after a heavy snowstorm.


But this is another example of a tree that was already in trouble.  The smaller branch on the right side of the picture shown above had been damaged many years before this year’s storm, as you can see below.

Photo 7

Broken, semi-rotten top end of one of the major stems on a Douglas-fir.


I sawed off the top 12” of this stub, and inserted a pencil into the soft rotten area in the center of the stem.  The results are shown below.

Photo 8

Pencil stuck in stub of tree trunk.


Photo 9

This is how far I could stick the pencil in.


I could easily stick the pencil a couple inches into the rotten wood.  I cut 2 more feet off the end of this stub, and was still able to stick the pencil about ½” into the rotten center of the branch.  Once the fungus gains this much of a foothold in a tree, it’s only a matter of time before it seriously weakens the tree.

So once again, the storm felled a tree that was already in trouble.


Dead Trees Can be Useful

And if you mourn for the dying trees, rest assured that not all of the forest inhabitants share your grief.  Bark beetles lay eggs under the bark, and their larvae start burrowing through and eating the soft nutritious tissues that are right under the bark.  Of the hundreds of species of bark beetles, at least some attack after the tree is dead.  These beetles leave the kind of tracks like those you can see after the bark has been removed from this branch collected at TCSNA.

Photo 10

Tracks left by bark beetles eating the soft tissues of the branches.


And of course, once insects get into a tree, can woodpeckers be far behind?  The photo below shows a heavily “wood-peckered” long-dead tree along Old Main Trail.

Photo 11

Heavily woodpecker-ed dead tree along Old Main Trail.


And Some Weird Stuff…

The snow also brought at least one unique observational opportunity!  Down near the creek in one area, I noticed that the snow had patches of yellow color.  (No, it’s not THAT!)  There were no animal tracks in this area, so I seriously doubt the yellow patches were from dogs or coyotes.  According to reports on the internet, yellow snow in this context is frequently the result of pollen getting mixed in with the snow.  Sadly, I got a picture, but never collected a snow sample for microscopic examination.  The storm was roughly at the time that some hazel (Corylus spp.) would be shedding its pollen, but I have no proof that’s what it is.

Photo 12

Yellow patches of pollen (?) on fresh snow near Beaver Bridge.


Assuming this is pollen, I have no doubt that pollen is shed like this on the ground every year.  However, it takes a snow covered forest floor before we will ever notice it.


Our Ever Changing Forest

Our forest is an ever changing ecosystem.  If we could see this forest in 400 years, much of it would look unfamiliar.  Most often the change is very slow, but a catastrophic event like a dramatic storm puts the changes in a time context we humans can relate to.  Enjoy our forest today, because when you come back tomorrow, it will be different.




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.



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.



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.


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.]


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.


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.



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;


A bare branch


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.


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.


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.

Fungus Among Us

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.



Hyphae of this fungus are reaching out along the edges

 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.


Old black rhizomorphs that grew under the bark of a tree


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.


Powdery mildew on a bigleaf maple leaf.


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.


Cross section of Douglas-fir tree trunk


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.


Log showing different fungi growing in heartwood and sapwood areas.


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.


Interior of this log was attacked by white rot fungi, leaving pure cellulose.


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!


Close up of cellulose fibers remaining after attack by white rot fungi.


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.


Stump which was attacked by brown rot fungi which digested all the cellulose.


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.


Wood residue after attack by brown cubical rot


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!


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