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.
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 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.
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.
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.
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.
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.
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.”
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Following my recent Naturalist Note on water I was persuaded to write a follow-up note, and this is it!
Water is a key to life, for both plants and animals. When you think of water at Tryon Creek State Natural Area (TCSNA), you naturally think of the creek. But that’s a long way from the whole story. In a Naturalist Note1 published earlier this year, I included the following image of how much water was in the top 12” of soil at TCSNA in late April based on 22 soil samples.
Just as a reminder, I need to stress that this is only in the top 12” of soil. In much of the park, water available to the plants might be located as deep as 2.5 feet beneath the surface. At approximately 2.5 feet beneath the surface is a layer of clay which is relatively impermeable to plant roots (more on this in my next Naturalist Note.)
During the course of this summer, there was not much rainfall, as illustrated in the graph below, which is based upon rainfall at two local stations, one at the Lake Oswego City Hall, and the other at the Westlake Fire Station.
Rainfall April 23 to Aug 23, 2018
As documented in a recent Naturalist Note, light rainfalls such as the majority of those illustrated in this graph, may primarily end up in the crowns of trees and shrubs, and never even make it to the forest floor.
On August 22 and 23, 2018 I collected soil samples from approximately the same locations where I sampled the soil in April. I again took samples from only the top 12 inches. The results were stunningly different. Below is a diagram using the same number of pools which I described in the spring. This time, however, 42 of the pools are dry, and only 26 are full of water.
The number of Olympic-sized swimming pools that could be filled with water from the top 12 inches of soil found in late August 2018 at Tryon Creek State Natural Area.
Where did it go?
There are probably three main fates for the missing water in the top 12” of soil. First, any water at or close to the surface could simply have evaporated. I have no idea how much this would amount to, but in the top 1 to 2 inches of soil, I’m guessing this could be an important factor.
Subsurface Water Flow
Secondly, the water particularly in sloped areas, could have flowed down hill underground. Lateral underground movement of water is quite common. To illustrate this I poured several gallons of water onto the soil surface at a flat spot on the side of the Old Main Trail not too far from the Nature Center. After saturating the soil in this tiny area, I used a soil corer to create two 6” deep holes about an inch in diameter. I waited for several minutes until there was no freestanding water in either hole. Then I carefully poured water into the right hand hole, as seen in the picture below. Not surprisingly, the water flowed laterally underground, and appeared the other hole.
This subsurface water flow might be particularly important on the steep hillsides near the creek. To illustrate this effect, I found information online about two different watersheds. The first is the Tryon Creek watershed, which is the watershed in which the TCSNA is located. An analysis of this watershed by the City of Portland has shown that about 25% of it is made up of impermeable surfaces, like rooftops, sidewalks, driveways, streets, tennis courts and even swimming pools. An aerial photo of one small part of the watershed illustrates the extent of these impermeable surfaces.
Water falling on these impervious surfaces rapidly makes its way into Tryon Creek, thanks in part to storm sewer drains that are common in the city.
In contrast, the Fir Creek watershed, located in the vicinity of the Bull Run Reservoir in the foothills of the Cascades east of Portland is almost completely forested, with the only impermeable surfaces being a few roads in the area. These two watersheds are roughly similar in size.
The differences in the surfaces of these two watersheds creates an enormous difference in the water flow in the major creeks of the watershed. These differences are illustrated using data from the same “rain event” in both watersheds.
In this graph you can see that immediately after each large rainfall event that there is a sharp peak in the water flow in Tryon Creek. This sharp peak is followed by a slow decline in the water flow of the creek. It seems reasonable that the brief sharp peak in the creek depth is water running off the impermeable surfaces found in the watershed, and being quickly dumped into the creek by the storm sewer systems. The slower decline following the sharp peak, is, I assume, water actually slowly flowing through the soil and into the creek.
In contrast to the water flow in Tryon Creek, in the Fir Creek system, there is a slow but significant increase in the stream flow following the rains. I suspect this is because the water in this watershed all has to gradually seep through the soil, and slowly make its way down to the creek.
Water Usage by Plants
Thirdly, the water could have been extracted from the soil by the roots of plants, and subsequently evaporated from plant leaves. This would be one way in which soil water at some depths could be lost to the atmosphere. In studies2 of water usage by Douglas-fir (Pseudotsuga menziesii), for example, it was reported that a 60-foot tall tree with an 8 inch diameter used 5 gallons of water per day. A 91 foot tall Douglas-fir with a 14 inch diameter used 16 gallons per day. Our forest has numerous trees this big and bigger, so their water usage in summer for the forest as a whole could be more than we might first imagine.
Living as we do in an area with relatively dry summers and wet winters we could see dramatic changes from climate change. If we have a winter with subnormal amounts of rain, and warmer than average summers, there could be a large die off of moisture-loving plants which have lived in this area for some time. They will of course, be replaced with other plants, but the transition could be difficult.
2Wullschletter, Stan D., F. C. Meinzer and R. A. Vertessy. 1998. A review of whole-plant water use studies in trees. Tree Physiology.
Editor’s Note: Miss the Pacific Northwest rain? It’s been 48 days (June 21st) since measurable precipitation at Tryon Creek State Natural Area. Enjoy this post about rainfall in the forest!
Article by Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Mention “water” to anyone at Tryon Creek State Natural Area (TCSNA), and they will probably think of either the drinking fountain at the Nature Center, or Tryon Creek itself. However, we may need to consider other things in the park when someone brings up the topic of water. We can start by looking at the water cycle in the forest.
Here Comes the Rain
We are fortunate to be in an area with a pretty good rainfall. Sometimes it just drizzles, and sometimes it pours down. The first question is “where does the rain go?” Well, that depends on how heavy the rainfall is. This past April, I temporarily set up rain gauges at TCSNA when the forecast called for a rainy period for the next couple of days. I set up 2 rain gauges several feet apart under a large western hemlock (Tsuga heterophylla) and then placed a third rain gauge in a clearing less than 50 feet from the tree. I repeated this process with a large western redcedar (Thuja plicata). I checked the rain gauges after about a day of rain, and then again after 3 total days of rain. The results of both the redcedar and hemlock are combined and illustrated below:
It was astonishing to me that during the first 26 hours of rainfall totaling more than a third of an inch, that almost none of the rainfall penetrated the canopy of either tree. Okay, yeah, I know that when it starts to rain you head under a tree for shelter. But, I was surprised at how effective these under-tree shelters were. Even in the following two more days of rain, only a small portion of the water penetrated the canopy. For this three day event, only 18% of the total rainfall penetrated the canopy. No wonder there are few plants growing under mature trees of these two species.
I checked 2017 daily rainfall data collected by the City of Lake Oswego2 in downtown Lake Oswego, just a few miles from the park. The total annual rainfall was 53.13 inches. Based on my measurements during that one rain event, let’s assume that any daily rainfall of less than 0.35” will never hit the ground under these mature trees. In 2017, these light rains amounted to 25.9% of the total annual rainfall. Based on the information gathered in this study, none of that ever made it through the canopy. These means that plants growing under the canopy of redcedars and hemlocks experience a much different rainfall environment than other plants.
However, there can be lateral water movement in the soil once it hits the ground. To check that, I collected soil cores from beneath both the hemlock and the western redcedar. Under the redcedar the soil contained less water than in the surrounding areas beyond the redcedar’s canopy. For the hemlock, there was no difference between the under-the-canopy and outside-the-canopy soil water. This may have been due to the fact that the hemlock was growing on a significant slope, and the redcedar was growing in a flat area. Any rainfall uphill from the hemlock, probably traveled through the soil downhill to the hemlock.
And these aren’t the only species of plants that intercept the falling rain. Even our native Indian plum (Oemleria cerasiformis) seems to keep a lot of rain from ever hitting the ground, as seen in the picture below.
However, all is not lost. Numerous documents in the scientific literature point out that many plants can absorb water not just through their roots, but also through their leaves and needles.
An important function of the soil is to hold water for the plants to use. The forest at TCSNA is growing on soil that includes a significant layer of clay about 2-1/2 feet below the surface. Thus we see in some toppled over trees that the roots don’t go deep into the soil, but rather, tend to hit the clay layer and then begin to grow horizontally.
To determine how much water the soil holds, I used a soil corer to collect samples of only the top foot of soil at 21 locations at TCSNA. Thus this estimate of total water in the soil is VERY low, perhaps less than half of the water in the entire soil structure found at TCSNA. The approximate sampling locations are indicated on the map below.
I took the soil samples home and put them in plastic bowls to air dry. I weighed them periodically until they stopped losing weight. Then I calculated how much water was in the top 12 inches of soil at TCSNA. Then I carefully recalculated it 5 more times, because the answer astonished me. At the time I collected the soil samples, there was enough water in the top 12 inches of soil at TCSNA to fill 68 Olympic-sized swimming pools.
All plants need water to stay alive. As in humans, water is a key, and most often the dominant component of every plant. With the permission of TCSNA personnel, I collected the above ground parts of some plants, or parts of plants, and determined how much water they contained. The process was that I collected the plants in the forest, stuck them in a plastic bag, and immediately took them home and weighed them. Then I let them air dry in my garage. I periodically took the weights of each drying plant until the weight remained constant. Then I calculated the percent of water in the fresh plant. In a few cases the results were frankly surprising.
Latin Names not already noted: (Oregon grape, Mahonia nervosa; thimbleberry, Rubus parviflorus; swordfern, Polystichum munitum; horsetail, Equisetum sp.; red alder, Alnus rubra; English ivy, Hedera helix; waterleaf, Hydrophyllum tenuipes; jewelweed, Impatiens capensis;)
Plants contain a lot of water. Based on some samples I collected near the creek, if the entire park were covered in jewelweed about 4 feet tall (a typical mature height for this plant, the amount of water in the jewelweed would be more than enough to fill 1-1/4 Olympic sized swimming pools.
Both waterleaf and jewelweed will, under moist conditions, exude water from the edges of their leaves, especially on cool mornings. This is illustrated below (and no, it didn’t rain just before I took this picture).
The flip side of this is that waterleaf tends to wilt fairly easily on hot, dry days, as illustrated below.
In another spate of plant drying activity, I included the leaves of three species, and measured them on a schedule to compare how fast the leaves dried. The results are presented below.
The salal dried dramatically more slowly than either the elderberry or vine maple. This is not surprising because the salal leaves are much tougher than the other leaves. Salal is the only species of these three that holds its leaves over the winter.
It’s a wet, wet world
Water is unquestionably the dominant component of life on earth. The prominence of water in plants is documented above. Human beings, like me, and hopefully you, have been reported to contain somewhere between 55% and 60% water, with higher levels for infants. It is an amazing fluid that dissolves important nutrients, makes our cells turgid, and performs many other useful functions. Next time you see a rain cloud coming, be sure to step outside and say thanks.
1”Water, water everywhere,
And all the boards did shrink.
Water, water everywhere,
Nor any drop to drink.”
—- from The Ryme of the Ancient Mariner by Samuel Taylor Coleridge, 1797-1798
2 Thanks to Kevin McCaleb with the City of Lake Oswego for this data.
All photos by Bruce Rottink.