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.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The plants growing at Tryon Creek State Natural Area (TCSNA) require a supply of nutrients to stay healthy and keep growing. An important part of supplying those nutrients is the decay of dead organic matter like leaves and branches. This decay process releases the chemicals in the dead leaves and branches so those nutrients can be reused by plants that are still growing.
To study this process, I collected leaves, branches or cone scales of several species of trees found at TCSNA in September 2014. I only collected fresh materials which had recently fallen to the ground. Within two days I placed these in wire mesh (screen) “envelopes” and fastened the envelopes to the ground with nails. During the first part of the study I took photographs of the envelopes on a monthly or quarterly basis, but starting in September of 2016 I switched to once-a-year monitoring. The results after one and two years have already been published in earlier Naturalist Notes.
The one-, two- and three-year end results are presented here. While I had two envelopes for each of the materials, I only selected the most photogenic envelope for inclusion in this report.
Red Alder (Alnus rubra) Leaves
Red alder is a common tree at TCSNA, and is renowned for dropping its high nitrogen leaves onto the ground while they are still green. The results from this study are seen in the photos below.
One university research project studying the decay of dead red alder leaves indicated that 93% of the nitrogen in the alder leaves was released into the soil via the decay process the first year. The same study found that 91% of the calcium and 97% of the potassium (both key nutrients) were also released during the first year of leaf decay1. This relatively rapid decay of the red alder leaves was attributed to the fact that they generally contain higher levels of nutrients than most trees, thus nourishing the decay organisms.
Western Redcedar (Thuja plicata) Branchlets
In sharp contrast to the red alder leaves, the western redcedar foliage contains lower levels of many nutrients, thus making it a somewhat less attractive food source for microorganisms. In addition, redcedar foliage and twigs contain many terpenes. Terpenes not only give the tree it’s distinctive “cedar smell,” but terpenes also have anti-fungal properties. The major terpene in western redcedar (α-thujone ) is shown below.
No wonder that the cedar is decaying a little more slowly than some of the other samples.
Bigleaf Maple (Acer macrophyllum) Leaves
With no special chemical defenses against fungi, and a fairly high nutrient content, the maple leaves are another fast decaying material.
Douglas-fir (Pseudotsuga menziesii) cone scales
These Douglas-fir cone scales were already lying on the ground at the time of collection. The cones they came from had probably been chewed apart by a squirrel looking for the nutritious seeds.
Superficially, the cone scales after three years look almost exactly like they did the day I put them in the bag. Obviously the scales are made up of a very hard material, not too much different than the wood in the main stem of the tree. Mother Nature made the cone scales very durable to protect the precious young seeds, and as a result, they are being recycled very slowly.
Again, the Douglas-fir twigs and foliage contain some terpenes similar to the terpenes found in the redcedar, and thus, they decay more slowly than the alder and maple leaves. Notwithstanding the resistance to decay, all the needles have fallen off the twigs.
The End Result
The nutrients within each of the samples used in this study will be released and then absorbed by living plants and fungus, helping the forest to grow the next generation of life. This study shows the enormous differences in the decay times of different kinds of tree litter. The softer materials, like the alder and maple leaves also have the highest nutrient content, and lowest concentration of anti-fungal chemicals. But, as the old saying goes, “All in good time!” All these nutrients will once again join living organisms, ensuring the continuation of TCSNA’s forest!
1Radwan, M. A., Constance A. Harrington and J. M. Kraft. 1984. Litterfall and nutrient returns in red alder stands in western Washington. Plant and Soil 79(3):343-351.
The “Yucky Stuff” in the Creek
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
People come to Tryon Creek State Natural Area (TCSNA) to enjoy nature’s beauty. They want to enjoy a natural and pristine environment, largely unspoiled by humankind. Our free-flowing Tryon Creek is an object of particular attraction.
As beautiful as the creek can be, some visitors are alarmed when they see “pollution” (often referred to as “yucky stuff”) in the creek. The truth of the matter is, that what might look like “pollution” in the creek is oftentimes the result of purely natural processes, not pollution at all.
The two most common kinds of “yucky stuff” in the creek are white foam floating on the surface, and orange slime in the water. Once you understand these two kinds of “yucky stuff,” you can relax.
So what’s with the white foam?
Sometimes you can see large clumps of floating foam accumulate on the upstream side of logs or other barriers in the creek. A typical mass of foam, pictured below, spotted recently just upstream from Obie’s Bridge:
You might think it’s soapsuds that leaked into the creek, but it’s probably not human-made at all. What really happened is a longer, but more interesting story.
The edge of the creek is lined with all kinds of trees, especially red alder (Alnus rubra). Alders being alders, every fall dump huge numbers of leaves into the creek, as you can see in the photo below.
And leaves being leaves, they start to decay, oftentimes in the stream itself. The photo below shows a large clump of mostly alder leaves (now brown) that have sunk to the bottom of Tryon Creek. Numerous microorganisms are decomposing these leaves. Note the tiny clumps of white foam floating on the surface above these leaves.
So the leaves decay. Now what?
As microorganisms decay the leaves, DOC (dissolved organic carbon) is released into the water. DOC is a totally natural mesh-mash of different chemicals. One of the DOC chemicals is palmitic acid, which is shown below.
Palmitic acid is found in every red alder leaf. It is a major part of each cell’s membrane. Palmitic acid, or closely related chemicals, are found in every plant. Below is the chemical structure of a typical soap molecule manufactured by humans.
The resemblance is unmistakable. There are two more carbons in the soap, but the major difference is highlighted in red in the diagrams. It is easy to see how the palmitic acid might act a lot like soap.
Why do we get foam?
Normally, the water molecules at the surface strongly attract each other, and form what is basically a weak shield on top of the water. We call this shield “surface tension.” Small insects called water striders can be seen walking on this surface tension on top of the creek in the summer.
To see a demonstration of surface tension, please play the following video.
The DOC, just like soap, interferes with the natural bonding between water molecules in the surface tension layer. The end result of the soap or palmitic acid is that when air gets into the surface layers of the water, it isn’t squeezed out by the natural mutual attraction of the water molecules. Rather, the air enters the water and with a bit of agitation creates bubbly foam.
To demonstrate natural foam-making, I used a jar of Douglas-fir (Pseudotsuga menziesii) cones that had been sitting on my home office desk soaking in water for about two months. [Note: If you happen to see my wife at TCSNA, please casually mention that you also have a jar of Douglas-fir cones soaking on your home office desk. It will help me a lot!] I used this jar to demonstrate that you can create foam just from decaying vegetation. All you need is some agitation, like what you might get from “rapids” in the creek. Photos of this jar at various stages appear below. Note how much foam is floating on top of the water.
The foam persisted for a long time after the shaking. If this foam were produced in the creek, in 90 minutes it could travel a long way downstream.
So we’ve got white foam in the creek, what else?
The second type of “yucky stuff” we have in the creek is orange slime! This can be found in a couple of places at certain times of the year. I’ve seen some in the vicinity of Obie’s and Beaver Bridges. How this comes about is one of the most interesting and unexpected nature stories at TCSNA. The photo below shows a pool of orange slime in the creek.
Most of us have been told at one time or another that there are two types of organisms in the world. First there are those that use sunlight, water and carbon dioxide to produce sugar to provide energy for themselves. This is what salmonberries (Rubus spectabilis) and Douglas-fir trees do. They are called “autotrophs” from the Greek meaning “self- nourishing.” Second, there are organisms which eat and “burn” the carbon compounds produced by autotrophs to produce energy for themselves. These carbon compounds are as diverse as sugar and wood. This is how both banana slugs (Ariolimax columbianus) and people (Homo sapiens) survive. These organisms are called “heterotrophs,” from the Greek “other nourishing.”
Following this explanation, we see that the energy of the sun is the basis for all organisms. So far, so good, and if you are in 2nd grade, this is a decent way to start understanding life. However, the organism that makes the orange slime doesn’t fit into either of those categories. It’s weird!
I love weird stuff! Tell me more!
The weird organism is a special type of bacteria called “iron bacteria”. (I’ll skip lots of complex chemistry here! “You’re welcome!”) You and I eat carbohydrates like corn, donuts and potatoes, and oxidize it to get energy. In the process we give off carbon dioxide (each time we exhale) and water (through sweat, breath and urine).
Iron bacteria don’t do that. These bacteria “eat” a special iron compound (ferrous iron, if you must know). Ferrous iron is found underground where there is a deficiency of oxygen. As water carries it up towards the surface of the ground it encounters both more oxygen, and the iron bacteria. The iron bacteria oxidizes (“eats” to put it crudely) the ferrous iron, and “poops” regular old rust (ferric iron, for you geeks). These iron bacteria are classified as “chemoautotrophs” meaning roughly, “they feed themselves with chemicals.” That’s right, the orange slime you see in the creek is essentially rust excreted by these special bacteria. In eating this special iron, the bacteria get the energy they need to live. The photo below shows the ferric iron seeping out of the soil into the creek.
Given that TCSNA is just a few miles from an old iron mine, it’s not surprising that TCSNA’s soil contains the high levels of iron needed to support this kind of bacteria.
Unlike what you may have been led to believe many years ago, these bacteria represent a group of organisms which don’t rely on energy from the sun to stay alive. If the sun ever goes away, no more plants, no more bugs, no more birds, and no more people. Then the iron bacteria will have TCSNA all to themselves. It probably won’t be a very exciting place, but there will still be life here!
The next time you see some “yucky stuff” in the creek, pause for second before calling it pollution. You might just be seeing the end result of some very interesting natural processes. The floating white foam and orange slime are just as much a natural part of TCSNA as your favorite birds and flowers. They are another reminder that nature is endlessly fascinating.