By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
While mists, drizzles, showers and rains are common events at Tryon Creek State Natural Area (TCSNA), a real “gully washer” is relatively rare. But last December we had one. If you were smart, you stayed inside. If you were a naturalist, you went out into the forest to see what you could learn. It was darn wet, but it helped me understand how dramatically a heavy rain can change the forest.
November 2015 delivered 4.49 inches of rain, and the first six days of December had rainfall of 2.75 inches. So the forest was already soggy when a big storm dumped 2.67 inches of rain on December 7th. This was followed by 1.66 inches on December 8th. Yikes!
What happens during a rainstorm?
First of all, water falls out of the sky with stunning intensity. The intense impact on the earth breaks tiny soil particles loose and throws them up into the air. To document this effect I went to two different areas of the forest. First I went to an area where there was a significant number of small evergreen shrubs. I placed a stiff sheet of white plastic vertically under these shrubs, with the bottom edge resting on the ground. The photo below shows the sheet nestled amongst a dense clump of Oregon grape (Mahonia nervosa). I left the board in position for two minutes while it rained heavily.
Then I went to an area with no shrubs present, and very little tree canopy due to the dominance of deciduous trees like bigleaf maple (Acer macrocarpa) and red alder (Alnus rubra). When I looked up, I saw mainly sky. I took another stiff sheet of white plastic board and held it vertically with the lower edge pressed down on the soil surface. I held it in place for two minutes. I took photos of the two sheets. As you can see in the photos below, there were almost no soil particles splashed up on the board in the area with lots of shrubs. However, a lot of small soil particles got splashed up on the board located in the non-shrub area. Some of the particles were splashed up more than 12 inches in the air.
The tiny soil particles splashed up into the air can have one of two negative fates. First, they might get splashed into a stream of water moving across the soil surface, and get carried far away. This is erosion. The other negative thing, as soil scientist have found, is that these particles can plop back down on the ground blocking the tiny soil pores that water uses to enter into the subsurface soil. Eventually, this pore blockage leads to more water runoff and erosion.
Storms can cause soil erosion (and soil deposition!)
Intense rainstorms, especially falling onto soil that is already very wet, cause lots of erosion. Erosion is when soil is carried by flowing water from one place to another. There was a classic example near Beaver Bridge. Soil washed down from the hillsides either wound up in the creek itself or was deposited in the bottomlands of the Tryon Creek canyon. Example deposits are pictured below. The shiny areas that look like water are actually deposits of clay soil that have eroded from higher ground, and have been deposited in the flat bottomlands near the creek.
The deposited soil contains so much clay that it is in nearly impervious to water. On April 9, 2016, I performed the classic water infiltration study on these deposits. I pushed a can that had no ends about 2 inches into the soil, and then poured some water into the can. In the eroded soil deposited near the creek, I waited for 1 hour and 5 minutes, and there was no change in the water level inside the can.
I repeated this experiment in a nearby forested spot about 10 feet higher in elevation which had no eroded sediments deposited on it. The water totally sank into the soil in just 5 seconds.
One of the main reasons for this slower infiltration rate is that the eroded soil deposited near the creek is made up of finer particles than the soil typically found in the TCSNA forest. This finer structure results in the deposited soil being less porous than the typical forest soil. Below are detailed pictures of particles of soil found at a typical forest site, and eroded soil deposited in the floodplains near the creek. To get these pictures, I put soil samples in a jar, and then filled the jar with water. Then I vigorously shook the jar, and instantly extracted a sample with a turkey baster. I deposited a single drop on a piece of white paper. I let the drop dry for about 2 minutes, and took the following pictures through a microscope. The blue scale on the side was included in the pictures to ensure that the pictures were sized identically.
The many dams of Tryon Creek State Natural Area
Beavers are the most famous dam builders at TCSNA, but they’re not alone! People (like us) have built a vast network of dams at TCSNA. We call them trails, but to water moving underground, they’re dams. A heavy rain is all it takes to demonstrate how effective these dams are.
As you walk along many trails on a sunny day, you see some holes in the ground at the side of the trail. Lots of students on school nature hikes have asked me what lives in those holes. Well, it could be lots of things! But I really didn’t understand those trailside holes until I was out in the rainstorm. It turns out that no matter how those trailside holes got created, at least some of them now function as drainage pipes.
The picture below was taken pointing uphill. At the bottom is Cedar Trail, with a hole right at the edge of the trail. Further uphill is the forest. When you look up into the forest, there is no water running on the surface of the ground down towards the trail. However, when the underground water flow gets to the trail, it is blocked by the compacted earth under the trail. It has no choice but to surface and flow over the top of the trail. Thus the trail is acting like a dam for underground water flow. The red arrow identifies the water emerging from a pre-existing hole.
The tendency of water to “pile up” on the uphill side of a trail, like we’ve seen above, probably contributed to the mess shown in the next picture. As the soil on the uphill side of a trail gets more saturated, it is less supportive of the trees growing there. For certain trees, especially those which are leaning, or growing on a slope, the soil is no longer firm enough to hold up the trees. Below is a picture of a red alder (Alnus rubra) growing alongside the Cedar Trail. The tree came fell over during a big rainstorm.
Rainstorms have many victims
The intensity of the rainstorm affects more than just the soil and trees! These two young rodents pictured below were probably blown or washed out of a tree where they lived during one of our heavy November 2015 rainstorms. For a size comparison, the red arrow in the upper left of the photo points to a green Douglas-fir needle. It appears the rodents have not yet opened their eyes, which mice and rats normally do when about 2 weeks old.
Rainstorms are rare but important events that can play an important role in the forest. As we have seen, they can reshape the landscape by moving large amounts of soil from one place to another. They can topple trees opening up new opportunities for other plants. They can kill animals which can impact the forest in a number of ways. As we travel through the forest, it’s good to remember that rare events like massive rainstorms can have dramatic effects on the forest we all love.