Tiny Rivers of Life
Stemflow: Tiny Rivers of Life
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
[Author’s note: Following a recent Naturalist’s Note on licorice fern, several questions arose concerning which trees licorice fern grows on and why it grows there. This note helps answer those questions.]
If you’re like me, when you think about rain, you think about drops of water falling out of the clouds, hitting your face and streaking those windows you’ve just washed. That’s part of the story, but there’s an important part of rain that’s easy to miss. It’s called “stemflow.”
Stemflow is water that starts out as a falling raindrop, but on the way down it hits a leaf or a twig. Much of this intercepted water starts to move over the surface of the leaf or twig to a branch and then to the stem. At that point it becomes part of a tiny river known as stemflow.
While stemflow is only marginally interesting to humans, both the quantity and quality of the stemflow can mean life or death to epiphytes, the small plants growing on the surface of larger plants like trees. The most obvious epiphytes at Tryon Creek State Natural Area (TCSNA) are moss, lichens and licorice ferns (Polypodium glycyrrhiza).
How is stemflow different from ordinary rain?
The chemical properties of stemflow are vastly different from those of free-falling rain. One study of stemflow on bigleaf maple (Acer macrophyllum) found that stemflow had a significantly higher concentration of the plant nutrients calcium, magnesium, sulfur and nitrogen than did raindrops. These chemicals apparently came either from dust that had been deposited on the branches since the last rainfall, or they were leached by the raindrops from the bark itself. In addition, stemflow was less acidic than raindrops.
In a study of stemflow on red alder (Alnus rubra), a nitrogen-fixing species, the stemflow contained 11 times as much life-sustaining nitrogen as raindrops. In contrast to bigleaf maple, the stemflow of alder was more acidic than the raindrops. Numerous studies show that different species of trees produce stemflow with different chemical properties. It seems reasonable that the chemical differences in stemflow between species of trees would influence the amount and kinds of epiphytes growing on those species.
So how much stemflow is there on a tree trunk?
Depending upon the kind of forest, researchers have found that the amount of stemflow is generally between 1% and 5% of the total rain that falls. The amount of stemflow on a tree of a given crown size varies depending upon how the branches are attached to the trunk, and how rough the bark is.
How do tree branches effect stemflow?
Tree branches can be attached to the main trunk of a tree at many different angles. If the branches point strongly upwards (as in the red alder pictured below) any rain hitting a branch will probably run right down to the main stem. However, if the branches are attached to the trunk horizontally, like they are in the western redcedar (Thuja plicata) pictured below, if a raindrop hits the branch, it probably won’t flow to the main stem and become stemflow.
Another factor which apparently hasn’t drawn the attention of researchers, is whether or not the foliage is droopy. Two of TCSNA’s conifers have extremely droopy small twigs and foliage. You can see that in the western redcedar pictured above. Droopy foliage is also characteristic of western hemlock (Tsuga heterophylla). As you can see in the picture below, with droopy foliage, intercepted water runs off the branch tips, and never becomes stemflow.
How does bark effect stemflow?
Researchers have discovered that trees with rough bark have less stemflow than trees with smooth bark. Having rough bark increases the surface area of the tree trunk. In the diagram below, the cross sections of two hypothetical trees are superimposed, each represented by a different color. While conventional measurements would say both of these trees are of equal diameter, clearly the orange tree, with the rougher bark, has a larger surface area.
Research suggests two factors are involved here. The first factor is that before stemflow can start, all of the bark surface area will need to be wet. Trees with rougher bark need more water to get all the bark wet, so stemflow, especially with a light rain, is less. The second factor is that bark itself absorbs some water, and with a larger surface area, more water can be absorbed by the bark, resulting in less stemflow. Less stemflow means fewer epiphytes.
I recently measured the “roughness” of the bark on the Douglas-fir pictured above. The roughness made the actual surface area of the trunk 10% greater than if the bark had been smooth.
One accepted method of determining bark roughness is to wrap a string around the tree, and measure how long the string is to determine the circumference. Then a wire is wrapped around the tree at the same height, but as the wire is wrapped around the tree, it is pushed into all the cracks and crevices on the tree. It stays “pushed in” and afterwards the length of wire needed to encircle the tree is measured. The ratio of the additional length of the wire divided by the length of the string is a measure of bark roughness. The process is illustrated below.
Okay, so where does licorice fern grow?
Some believe that bigleaf maple is the only significant host of licorice fern at TCSNA, while red alder will occasionally host a few fronds. To address those beliefs, I turned to the last resort of those truly desperate humans we call “scientists,” actual (No, no, don’t say it!) data!
First, looking around TCSNA I managed to find licorice fern growing as epiphytes on living plants of the following species: bigleaf maple (of course), red alder, Pacific yew (Taxus brevifolia), Oregon ash (Fraxinum oregona), Douglas-fir (Pseudotsuga menziesii), black cottonwood (Populus balsamifera spp. trichocarpa) and vine maple (Acer circinatum).
Next, to clarify the relationship between red alder and licorice fern, I grabbed a stick about 6 feet long, and walked along Center Trail/Big Fir Trail/Middle Creek Trail as far as Beaver Bridge. Any living alders along that path that I could touch with the stick while standing on the path became a part of my sample. Of the 36 alders in my sample, licorice fern grew on 35 of them (97+ %).
Finally, to address the issue of alders supporting only a few fronds of licorice fern, I stopped at three alders with “lots” of ferns and while standing still on one side of the tree, counted visible fronds. On one tree I stopped counting at 50, the next at 75 and at the third tree I stopped counting at 150 individual fronds. Thus it appears that licorice fern and alder can get along quite well. To be fair, there were a few good-sized alders which supported only 2 or 3 fronds high up in the tree.
Your mission, should you decide to accept it….
Search for licorice fern in unusual places. Will you be the first person at TCSNA to find it on western redcedar? Can you find it growing on a rock? Or maybe you can find it someplace totally unexpected like on an old signpost along a trail. Keep your eyes open, and if you find some licorice fern in a different kind of place, post a comment below. We’ll all be smarter. As someone once said, “Our current state of knowledge only represents the point at which we’ve decided to stop asking questions.”
Posted on February 19, 2015, in Plants & Wildlife, Trees and tagged Cottonwood, Douglas-fir, epiphytes, forest, lichens, Licorice fern, Maple, moss, Nitrogen, plants, Red Alder, stemflow, Trees, Tryon Creek State Natural Area, Western redcedar. Bookmark the permalink. 2 Comments.