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.
By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
This Naturalist Note is dedicated to Phil Hamilton who passed away on June 12. Phil devoted more than 24,000 volunteer hours helping make Tryon Creek State Natural Area the wonderful place it is today. He played many leadership and resource roles, but is perhaps best known as the ‘King of the Ivy Pullers.’ He not only pulled a lot of ivy himself, but led hundreds of groups of ivy pullers out into the park. Every time I went out with him, he told me something brand new about the forest that I did not know before. He was a great role model, and helped many people get started on the track of volunteering at Tryon Creek. I, and I’m sure many others, will remember this dedicated, knowledgeable and hard-working man forever. Thank you Phil!
If there was a vote on the most despised plant at Tryon Creek State Natural Area (TCSNA), ivy would win hands down. This aggressive, invasive plant outcompetes and displaces many native plants. In an area near the Red Fox Trail where the ivy completely covered the forest floor, I removed and measured the ivy in a three-foot by three-foot plot. In that plot there were 285.8 linear feet of the ivy vine. (Yes, it was really thick, in multiple layers!) If (and thankfully, it doesn’t) this density of ivy covered all of TCSNA there would be enough ivy to wrap around the earth at the equator more than 6 times!
Ivy’s habit of climbing up tree trunks makes it difficult to ignore. Not surprisingly, ivy has many special features that make it so successful.
Ivy: The ingenious climber
Ivy needs sunlight to grow. How does a plant get close to the sun? Most trees, like our Douglas-fir (Pseudotsuga menziesii) develop a thick trunk that lifts their leaves up toward the sun. Building the thick stem takes a lot of resources. A study in western Washington showed that for 47-year-old Douglas-fir, 87% of the above ground biomass was in the trunk of the tree. The major purpose of the tree trunk is to get the needles up into the sunlight where they can photosynthesize. The ivy developed the habit of just climbing the tree trunks that were already there. It saved itself all the energy required to develop a self-supporting stem.
I pulled down an ivy vine that was growing up the side of a tree. The diameter of the ivy’s stem at ground level was 3/4 of an inch. Twenty-one feet up the tree, it was not much smaller, as you can see below:
Since ivy doesn’t need a thick stem to hold itself erect, it uses its energy to grow taller.
In contrast, a western redcedar (Thuja plicata) only 10 feet tall growing along Old Main Trail had a basal diameter of 1.59 inches. The redcedar needs this thick stem to hold itself up, while the ivy doesn’t.
Not that ivy vines don’t grow large, especially when two or more vines merge together.
Ivy only had to develop a method of holding onto the tree. Voilà! The aerial rootlet, which adheres to the tree’s bark:
Using these aerial rootlets, the ivy manages to climb up the trunk of trees into the light without having to expend the energy to develop a big, supportive stem.
Creating a Home for Others
While we rarely envision ivy as a benevolent plant, other organisms may have a different view. As you can see in the photo below, sometimes the mass of ivy stems creeping up a tree is part of a community complex most frequently involving moss or licorice fern (Polypodium glycyrrhiza). While removing ivy from a tree near the Red Fox Trail, I collected the sample (in cross section) shown below. It is a combination of primarily ivy and licorice fern, with a hint of moss.
The mass of roots, stems and miscellaneous dirt measured about 7 cm (~2-1/2 inches) thick.
How much water might this hold? I cut a 2-1/2” by 3-1/4” sample from the tree. I soaked it overnight in water. I weighed the wet sample and then let it air dry completely and weighed it again. I calculated that a square foot of this material would hold slightly more than 2-1/4 quarts of water. This is a mixed blessing. While some of this is a nice reservoir of water for the licorice fern growing in this mass, it is also a significant weight burden for the tree.
To find out how much water might be stored in the mass of ivy roots and licorice fern, I did some calculations. I measured the diameter of the trunk of a large fallen alder tree near the Middle Creek Trail at 10 foot intervals, up to 63 feet above ground, where it started branching out. Based on this data, I calculated the surface area of the tree trunk. If the entire surface of this tree trunk were covered like the sample above, the ivy/moss/licorice fern could potentially contain up to 1,520 lbs. of water. That’s three-quarters of a ton of water. Yikes!
Ivy: It’s Tough
Every species of plant contains nutritious chemicals like sugar, cellulose and dozens of others. This naturally attracts other species that don’t have the ability to capture solar energy to sustain themselves. One of the keys to a plant’s survival is to protect itself from these organisms, which range from molds and insects, all the way to humans. In the picture below, you can see the surviving remnants of leaves on one of TCSNA’s common shrubs.
To find out how effective ivy is in protecting itself, I conducted a survey in the fall of 2016. I examined the leaves of three species of plants, and counted the number of leaves (or leaflets) that were damaged. To minimize the possible effects of humans, I examined sites more than 10 feet from a trail. (Confession: I don’t actually know what caused the damage; it might have been insects, diseases, a hailstorm or whatever.) For each species, I examined leaves in two different places (for example, near Red Fox Trail and near Old Main Trail), to get an “average” value.
The results are presented below:
Number of Total leaves Percent of
Species damaged leaves examined leaves damaged
Red Alder 181 199 90.0%
Oregon grape 375 559 67.1%
Ivy 93 279 33.3%
Ivy has less leaf damage, whatever the cause, than either the red alder or the Oregon grape. Good for the ivy!
Ivy is a Persistent Grower
Every plant has a growing season, and for ivy, it’s long. To determine how long into the fall/winter this plant might grow, I measured the growth of an individual ivy stem along the Red Fox Trail. The data shows that ivy continues growing quite late in the year.
In contrast to the ivy, on September 28, one of the Indian plum (Oemleria cerasiformis) plants I was monitoring in that area was completely bare of leaves, while the other Indian plum in that area had dropped about 98% of its leaves.
Ivy’s Secret Strategy
One of ivy’s secret strategies is that virtually every place along the stem where there is a leaf, there is the potential to grow roots. That is seen in the photo below:
Should the stem of this ivy plant be broken, no sweat! Every part of the stem has its own root system and can stay alive. This is in contrast to most woody plants which only produce roots at a single point in the plant.
English Ivy really isn’t that bad (a tidbit for geeks!)
It turns out that much, if not most, of the ivy that we have at the park really isn’t English ivy (Hedera helix); it’s Irish ivy (Hedera hibernica). Not that the other plants care!
The key reliable morphological feature that discriminates between the two species are the miniature hairs that grow in clusters on the plant. The Irish ivy hairs are in small clusters lying flat on the plant’s surface, while English ivy hairs are in larger clusters and stand erect. The microphotographs below of plants collected at TCSNA shows the difference.
To further complicate things, hybrids of English and Irish ivy have been discovered and…. Okay, I’ll quit now!
The Ivies: Green Success Stories
The ivies in the genus Hedera are very successful plants. They can grow tall without having to use their own stem to support themselves. When hacked into pieces, many of the pieces are able to stay alive and become a whole new plant. They also appear more resistant to disease and predation than many of TCSNA’s other plants. They have a longer growing season than many of our native plants. All of this spells success for the plant, and lots of work for our ivy pullers who are trying to encourage the growth of native plants by reducing the resource competition from the ivy!