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.
How Do Plants Get a Drink?
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
With summer comes a host of delicious fruits for us to enjoy. Cherries, peaches and apples top my list. They’re sweet and juicy. Have you ever stopped to think that the juiciness is primarily due to water the plant pulled from the soil? The same is true for all the fruits at Tryon Creek State Natural Area (TCSNA). How do the plants get the water out of the soil and move it up into their leaves and fruits? From the viewpoint of the plant, this is one of the most important things it does. No water means no leaves, no fruits and ultimately, no plants!
How does water get into the plant?
Most of the plant’s water comes in through the roots. The process of getting water into the roots relies on two things. First, a physical structure called a “semi-permeable membrane” and secondly, a physical process called “osmosis”.
Let’s start with the semi-permeable membrane. “Semi-permeable” means that some things can go through the membrane, and other things can’t! Every living cell in the whole world, plant and animal alike, is contained within a semi-permeable membrane. You have gazillions of them in your own body. Plants have semi-permeable membranes around the living core of each cell, although most are also encased in a rigid, non-living “cell wall” on the outside. You can think of the semi-permeable membrane as a thin film with small holes in it. In most cases, water can go through these holes, but other molecules, especially large molecules like plant sugars, can’t go through these holes.
The other key to getting water out of the soil is the process of “osmosis.” Osmosis simply means that molecules will move from areas of high concentration to areas of low concentration. For example, you can tell when someone wearing perfume walks into a room. The perfume molecules move from the area of their highest concentration (the perfume-wearer’s body) to areas of low concentration, namely the whole rest of the room. This movement of molecules is called diffusion. This is similar to osmosis, except that in osmosis the molecules have to go through the holes in the semi-permeable membrane.
So how does the osmosis work?
Remember that the water is moving from areas of high concentration of water to areas of low concentration of water. The water out in the soil is relatively pure. In contrast, inside the root are a host of molecules, such as sugars created by the plant. So the concentration of water inside the plant is much lower than the concentration of water outside of the plant. Thus the water moves from the soil into the plant via osmosis. The diagram below illustrates the key parts of osmosis.
Once the water is in the roots, how does it move up to the leaves?
You were hoping there was just one simple answer, weren’t you? Sorry, not this time! There are a couple of different processes that plants use to move the water from the roots up to the leaves. For this posting, I’ll just talk about one of them which is called “root pressure.” Caution: Root pressure is a technique which is important for some plants, but scientists have found other species of plants that don’t use root pressure at all.
Root pressure is something which is most important for short plants that are growing in relatively moist soils. As the roots suck up more and more water, the roots can’t just keep swelling up bigger and bigger, the water has to go somewhere else. The plant solves this problem by pushing the water up through the stems to the leaves. Two species at TCSNA which clearly use root pressure are jewelweed (Impatiens capensis) and Pacific waterleaf (Hydrophyllum tenuipes). Check out their unique flowers, a good reference when trying to identify these plants.
For plants that use root pressure, something very interesting happens when the soil is fairly wet and the weather is humid. Normally a fair amount of water is lost through tiny pores on the leaves by the process called “transpiration.” In humid weather very little water escapes from the plants through transpiration. Soon the leaves are wondering what to do with all this water that’s being pushed up to them by the roots.
Naturally, the leaves have a solution. The leaves force the water out in a process called “guttation.” The leaves force the water out through special structures called “hydathodes.” These hydathodes are most frequently found on the end of the teeth on the edge of the leaves. You can see the results of guttation in the photo below.
Another plant at TCSNA that also does this is the Pacific waterleaf, which you can see below.
I’ll bet you’ve wondered why they call this plant “waterleaf” haven’t you? Well now… [Stop right here! This is Bruce’s conscience speaking. Bruce has found not one shred of evidence in the scientific literature that guttation is why they call this plant “waterleaf.” I have to watch him like a hawk!]
The pattern of exuded water on the leaf is much different from if the leaf has been rained on. Below is a picture of a Pacific waterleaf leaf after a rainstorm. The water in this case is just a thin shiny layer across the entire leaf, as you can see (or not so much) in the photo below. This shows that rainwater is not the cause of the droplets of water on the teeth of the leaf.
What other evidence is there of root pressure?
The ultimate evidence of root pressure is shown below. I chose a Pacific waterleaf plant less than 2 feet tall. It was growing in fairly wet soil. Then, after getting the permission of the Park Rangers, I carefully cut off the top of the stem. I went on a hike for about an hour and then returned and took the photo below.
The droplet of water at the cut tip of the stem has been forced up by root pressure. It clearly hasn’t been pulled up by the leaves, because there are no leaves to pull up this water.
If you want to see guttation for yourself, come to the park in the early morning after a big rainstorm. Look carefully at the jewelweed near either Obie’s Bridge or Beaver Bridge. You can also check out the Pacific waterleaf on Old Main, or other trails. You’ll be able to see for yourself that the plants have done a good job of getting a nice big drink of the water from the soil.
Seeds: Then and Now
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Almost every plant at Tryon Creek State Natural Area (TCSNA) started from a seed. Back “Then”, in the mid-1800s, Socrates Hotchkiss Tryon staked his land claim at the mouth of Tryon Creek. “Then” logging started in the nearby forests. From that time until “Now” we have basically the same kinds of seeds at TCSNA.
So what has changed?
Well, the first thing is the abundance of seeds. The plant’s most important activity on a day-to-day basis is making sugar using just sunshine, water and carbon dioxide from the air. This sugar is in essence the “money” of the plant. Just like you have to budget your money, the plant has to decide where to invest its resources too. And, just like you, the plant only has so much “money.”
The plant’s options are to invest in roots, shoots, leaves or seeds. The roots are pretty important since they absorb the water and minerals the plant needs to stay alive. The leaves are important, because they make the sugar the plant lives on. And the stems are pretty important because they are what holds the leaves up in the air so they can catch some decent sunlight.
But what about the seeds?
Nature’s only measure of success is whether or not an organism leaves offspring. So the plant better invest in seeds. Well, maybe. If it is one of the few plants at TCSNA that dies at the end of each season, like our jewelweed (Impatiens capensis) it really needs to put some priority on seeds, or the family line is finished! On the other hand, if it is a perennial plant like Douglas-fir (Pseudotsuga menziesii) that might live 500 years, it doesn’t need to produce seed every year, but it can’t put it off forever.
Scientists have found that when sugar is in short supply, perennial plants will invest their sugar in leaves, stems and roots before they will invest in seeds.
Why would sugar be in short supply?
There are many reasons why a plant’s sugar supply would be low. For many plants here at TCSNA the most important issue is the amount of light the plant gets. With too little light the plant makes less sugar. With less sugar, the plant invests it in leaves, stems and roots, but not seeds. For example, you’ve probably seen salal (Gaultheria shallon) plants (see photo below) in the forest at TCSNA.
There is one patch of salal growing under a dense canopy of trees that I’ve been watching for 4 years. Struggling along in heavy shade, not once has it flowered, much less produced fruit and seeds! In contrast, the salal growing in my yard in the full sun produces abundant delicious berries every year.
The dominant trees always have lots of sunlight, but for the other plants, light is an important contrast between “Then” and “Now.” From a plant’s perspective, logging can be either good, or bad! For the straight, tall trees, it’s bad news because they are going to be cut down. But, for many (but not all) of the smaller plants and bushes like thimbleberry (Rubus parviflorus) struggling to stay alive under those big nasty, light-hogging Douglas-fir, logging means “Happy Days are Here Again!” (Assuming of course that the plant doesn’t get ripped out of the ground when a log is dragged over it.) Post-logging, the surviving plants are basking in the full sun and its leaves are pumping out sugar like crazy! With lots of sugar, there’s enough to invest in leaves, stems, roots, and lots of seeds! It was also a great time for the few trees the loggers left behind because the trees were too small, growing in inaccessible areas, or were in some way defective.
Lots of seeds back “then,” what else changed?
Logging is also an excellent way to churn up the ground, exposing mineral soil. That’s a good thing, because in the undisturbed forest, much of the ground is covered with “litter.” No, not litter like candy wrappers and dog-poop bags, but the litter of fallen leaves, small twigs, old cone scales, and such, as shown in the photo below.
Scientists know that litter can form a barrier to the successful germination and establishment of new plants, particularly for species with small seeds. In addition, this litter layer, also known as “duff”, has a tendency to dry out quite quickly, depriving new seeds and seedlings of optimum moisture conditions. For these smaller-seeded species, there is nothing like exposed mineral soil to provide the perfect place to start a new life.
What kind of seeds are there are Tryon Creek SNA?
The diversity of seeds at TCSNA is amazing. Some of the major differences are how the seeds are dispersed and the size of the seeds. These are very important decisions. The best chance for a young perennial plant is to get away from the mother plant, so it doesn’t have to compete with it. So plants have developed a wide variety of mechanisms for getting their seed spread around. Then too, the plant has to decide on seed size. Putting a lot of resources into a few large seeds would give the resulting seedlings a real leg up when they are first getting established. But on the flip side, if the plant produces very few seeds, and they just happen to land on a rock, or get eaten, too bad! Alternatively, the plant could produce millions of smaller seeds, so that at least some would surely land in favorable sites and avoid being eaten, but then they don’t have much energy with which to get started.
How do seeds get spread around?
At Tryon Creek, seeds mainly are dispersed by either the wind, or by various animals. Two good examples of wind-dispersed seeds are our black cottonwoods (Populus balsamifera ssp. trichocarpa) and our bigleaf maples (Acer macrophyllum). Although both are wind dispersed, they choose radically different ways to do that. The cottonwoods have tiny seeds with each seed sporting a large fluffy mass of fine hairs that completely obscures the actual seed (see the picture below.) This fuzz suspends them in the air as they drift around the forest. The maple has gone with a more traditional idea, having a wing (as you can see in the picture below). This wing arrangement helps the seed drift away from the parent tree.
Other plants use a different type of wing to disperse their seeds, a bird’s wings. All they have to do is produce a nice attractive fruit. The fruit attracts birds that digest the fleshy part of the fruit, and excrete the seeds at some distant location. Two examples of this from TCSNA are the western wahoo (Euonymus occidentalis) and red huckleberry (Vaccinium parvifolium), as seen in the pictures below. (In both cases, the blue lines represent a length of half a centimeter, about two tenths of an inch.)
The third, and most unusual way that plants are dispersed is used by the jewelweed that grows abundantly in the wet areas near the creek. When the jewelweed seed pods are ripe, there is hydraulic tension in the walls of the pod. When the pod is touched, or just naturally dries out, the pod “explodes.” The walls of the pod curl backwards with amazing speed and force. This sends the seeds flying through the air to a new location. Before and after photos of a seed pod are shown below.
How important is seed size?
Seed size varies a lot. The monster seed of TCSNA is the beaked hazel (Corylus cornuta var. californica). There are lots of medium sized seeds like Douglas–fir and western wahoo, while the “tiny” end of the spectrum is represented by red huckleberry. Find them all in the photos below, which are all to the same scale; the blue line represents half a centimeter (about 2 tenths of an inch).
It is interesting to note that there is no clear relationship between fruit size and seed size. Look at the vastly different sizes of the red huckleberry and western wahoo seed above. Then look back earlier in this note at the fruits of these two species. The fruits are nearly identical in size.
Clearly, seed size has little or no relationship to the ultimate size of the plant. Douglas-fir grows phenomenally larger than either beaked hazel or western wahoo, and yet the Douglas-fir has the smaller seed. Scientists in California studying seed size of over 2,500 native plant species have come to an interesting conclusion. First of all, shrubs and trees have seeds adapted to their environments in totally different ways. For shrubs, those species with the largest seeds are best adapted to areas with heavy shade and fierce competition. But for trees, the species with the largest seeds are the ones best adapted to survive on drier sites. The trees of TCSNA follow that rule quite nicely. Both black cottonwood and red alder (Alnus rubra) are commonly found in moist sites, and have relatively small seeds. Douglas-fir, with seeds dramatically larger than either of those two species, are commonly found on somewhat drier sites.
So the bottom line is that compared to the post logging “Then”, we probably “Now” have a lot fewer seeds produced by the shrubs and ground cover plants at TCSNA due to heavier shading. And especially for the smaller-seeded plants, the presence of an undisturbed litter layer is holding back the success of the seeds that are produced.