Category Archives: Trees

Roots and Soil

By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester

All photos by the author unless noted.

 

The forest at Tryon Creek State Natural Area (TCSNA) is literally rooted in the soil.  What happens below the surface of the ground is vitally important to the forest, but something which is generally hidden from us.  Rarely, Mother Nature provides us with a glimpse into her underground world, and it can be very enlightening.

 

The Root System Revealed

In late September of 2013, a little more than 5 years ago, a tree growing by the side of the road leading up to the “Horse Lot” at TCSNA blew over during a rainy, stormy period.  Seizing the opportunity, I started studying this event, with the idea of creating a series of Natural Notes possibly stretching over many years.

When the tree blew over, the root system was revealed.  Photos taken within 2 or 3 days of the blowdown are shown below.

photo 1

Bottom of the root wad of the fallen tree.

 

photo 2

Pool of water at the base of fallen tree a couple of days after the tree fell.

 

After the tree fell over, for at least a couple of days afterwards, there was a pool of water covering the clay layer exposed when the tree had fallen over.  It turns out that the soil under this tree, just like most of the soil at TCSNA, has only about a 2 to 3 foot layer of soil suitable for growing plants, underlain by a thick layer of clay.  This clay is very resistant to water flow and root growth.  For safety reasons, the Park Staff filled in this hole shortly after the tree fell over.  The photo below gives an indication of the size of the root wad.  The red arrows show moderate- to large-sized roots growing horizontally, not downwards, because they hit the clay layer.

 

photo 3

The author (5’10” tall) by root wad just after the hole it created had been filled in.

— Photo by Anonymous Park Visitor

 

I measured the thickness of the root wad within a week of when the tree fell over.  I did this by pounding a thick metal rod through the root wad, and measuring how much of the rod stuck out of the soil.  A picture of this method is shown below.  I pushed the rod through the root at 4 different locations, both 2 and 3 feet on either side of the trunk of the downed tree.  On average, the thickness of the root wad was 24-1/2 inches.

 

photo 4

Side view of fallen root wad, showing end of a metal rod I pushed through the root wad.

 

As you can see, there are no roots growing straight down out of the root mass.  The clay layer beneath this tree was not hospitable.

 

The Aging Root Wad

Five years after the tree blew down, I returned to the site, and measured the thickness of the root wad again, in the same way I had measured it when the tree first fell down.  In the 5 years since it fell over, the soil on the root wad was 4.9 inches thinner than it was when it first fell down.  Based on other trees that have fallen over in the forest, like this one along the Maple Ridge Trail shown below, I anticipate that sooner or later, all the soil will be washed off the skeletal root system.

 

photo 4a

Head-on view of fallen tree’s root system along Middle Creek Trail.

 

photo 5

Side view of fallen tree’s root system along Middle Creek Trail.

 

An Underground Dam

These root systems all raise questions about the depth of the “plant friendly” soil, which in much of the park, seems to be pretty shallow.  In many cases of fallen trees, the soil which is exposed is substantially clay.  Clay of course is resistant to water flow.  Just how resistant?  I collected a sample of clay from the root system of a tree that had recently fallen at TCSNA.

At home, I drilled holes in the bottom of a plastic cup, as shown below.  Water flowed easily through the holes as you can see in the picture below.

 

photo 6

Holes in bottom of plastic cup.

 

photo 7

Water flowing easily through holes in bottom of plastic cup.

 

For my test, I put about 1/3 of an inch of clay into the pot, and gently pressed it down into the pot.  Then I filled the pot with water.  There was some tiny amount of water that flowed through the holes but not much.  I let the pot sit with water in it for a couple of days.  Then I once again filled the pot with water and let it sit inside of a plastic tray.  I sat the pot on two pencil stubs to keep the bottom of the pot up off the tray, so water could easily run out of the holes.  This is illustrated below.  I left out the plastic tray in order that you could see the rest of the set-up more easily.

 

photo 8

Plastic cup with holes in bottom covered by soil composed mostly of clay.

 

In the course of 20 hours, not a single drop of water leaked out of the cup.  The clay used in this demonstration is clay that is found underground throughout much of TCSNA.

The nearly impenetrable layer of clay found at TCSNA means that the forest we love is dependent on approximately the top two feet of soil.  To put this in perspective, when leading hikes for students at the Park, I will oftentimes ask them this question:  What would they think if I went to Washington Square and dumped 2 feet of dirt on top of the asphalt parking lot, and declared that I was going to start growing a forest there?  Almost always the kids will say something like “You are crazy!”  But in fact, that is essentially the situation we have here at the Park.

The photo below shows me with a cardboard box the same height (24.5”) as the depth of soil supporting the trees at TCSNA.  This is the depth of soil I would pour onto the parking lot in order to create a forest at Washington Square Mall like that at TCSNA.

 

photo 9

The author demonstrating his plans for a forest on Washington Square’s Parking Lot.

 

The thin layer of soil supporting the forest at TCSNA is one reason that the trees need to shelter each other if they are going to resist being blown down by the wind.  They really do constitute a “Forest Community.”

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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.

 

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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