Monthly Archives: July 2016
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.
By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
The bats at Tryon Creek State Natural Area (TCSNA) are one of our least seen, yet most interesting, animals. Their lifestyle is so different than ours that it’s no surprise our paths rarely cross. Interestingly, 25% of all species of mammals in the world are bats. The lifestyle of bats is very challenging, and they use some very unusual strategies to be successful.
Bats are the only flying mammals. As flying mammals, bats use enormous amounts of energy. Bats in some parts of the world eat fruit, drink nectar, eat pollen (and some even drink blood), but our TCSNA bats eat insects exclusively. This food is only seasonally available. Bats do not store food like some squirrels. And some of our bats are among the smallest mammals.
How do bats make it?
One of the challenges facing bats is that some of them are very small. Our little brown bat (Myotis lucifugus) weighs ¼ to ½ an ounce. Our native Douglas squirrel (Tamiasciurus douglasii) weighs approximately 20 times as much. The weight of an animal is very important because small bodies have a much larger surface area per unit of weight and lose heat a lot more rapidly. To demonstrate this, I ran an experiment.
Pictured below are two plastic containers which are filled with water dyed blue to make them more visible. The thickness of the walls of these two containers is identical.
I simultaneously filled them both with hot (132° Fahrenheit) water. I put on their lids and placed them in the freezer. After 40 minutes I took their temperatures, which are shown below.
The water temperature in the smaller container fell 70° F, while than the water temperature in the large container only fell 48° F. This is because the ratio of the surface area of the container to the volume of the container is 3.0 for the small container and 2.6 for the larger container. The larger relative surface area of the small container caused it to cool more quickly. Likewise, the small bats get colder much faster than larger mammals.
The way animals keep warm is to “burn” food. To save energy, when bats rest they let their body temperature drop to within a degree or two of their environment. When the bats let their body temperature drop they enter a state called “torpor.” This is an inactive state that is much deeper than human sleep. Some scientists believe torpor is different from hibernation only in depth of rest. Bats oftentimes enter torpor on a daily basis for a few hours. During torpor or hibernation, the bat uses less than 1/40 as much energy as it does when it is active. However, when the air temperature gets to just a few degrees above freezing, the bat will arouse itself and exercise muscles sufficiently to warm itself up.
Where do bats rest?
When not flying around feeding, the bats are in “roosts.” Most commonly we think of them roosting somewhere during the day while they’re waiting for their night-time feeding frenzy. These are called “day roosts.” These roosts keep them warm, and hide them from predators.
However, it turns out that bats only feed for a few hours each night. Bats can sleep up to 19 hours a day! The big feeding frenzy for bats is at dusk for a couple of hours, after which the bats rest a bit. At least for many bats, there is a second “meal” in the pre-dawn hours. During their inactive night hours they stay in a separate “night roost”. People who study bats think this might be to keep the day roosts more secret by reducing the fecal deposits near their day roost.
The day and night roosts can be as simple as hiding in an attic or under a bridge. In the more natural world, bats might roost behind a chunk of loose tree bark, or in a hollow tree, such as these examples at TCSNA. The hollow in the tree below is 10 inches deep, plenty of room for some little brown bats!
Bark crevices are also good roosting spots. Bats prefer Douglas-fir (Pseudotsuga menziesii) over pines (Pinus spp.) or true-firs (Abies spp.), probably because of the bark. Older Douglas-fir have a greater tendency to develop nicer protective bark crevices than most other species.
Winter Homes: Hibernacula
Bats’ third home is their winter home where they hibernate during the insects’ “off-season.” A Portland area survey showed a sharp decline in the number of active bats between August 6th and September 23rd. This was probably due to declining insect numbers and cooler weather, a time when bats might start thinking about hibernation. Bats oftentimes hibernate in caves, or for some species of bats, barns, sheds or attics. These places are called “hibernacula.”
Regrettably, the location of the overwintering hibernacula for the bats inhabiting TCSNA in the summertime are unknown. The best general information we have is from a study* of wintertime collections at museums, and a survey of 650 caves plus 70 buildings that occurred in Oregon and Washington from 1982 through 1989. Combining the results for the 1980s study and some museum collection records, the closest overwintering location for little brown bats was in Washington County near Sherwood. For long-legged myotis (Myotis volans), the closest overwintering individuals were located in Gladstone. The closest winter home of big brown bats (Eptesicus fuscus) was near Corvallis. The closest wintering locations of the California myotis (Myotis californicus) were near Gladstone and Vernonia.
It is important to note that this nearly decades-long study only found a total of only 174 overwintering bats. With the exception of the long-legged myotis, all the overwintering bats were found either singly or in groups of less than ten. This is in sharp contrast to some other areas of the country where overwintering bats occur in groups of hundreds to million in a single cave. However, it does demonstrate that at least some of our bats spend the winter in this area.
While long winter hibernations are the rule, California myotis and silver-haired bats (Lasionycteris noctivagans) (both species reported at TCSNA) have been observed feeding between November and February near Olympia, Washington.
Picking out a roost
Several reports indicate that the absence of suitable day roosts is an important factor limiting the bat population. Long-legged myotis, a species reported at TCSNA, was the subject of a study in the Oregon Cascades east of Springfield. Sixteen radio-tagged female bats were tracked for 8 days. In this period, the bats used a total of 41 different day roosts scattered over 12 square miles of forest. Of these, 1 roost was in a rock crevice, 4 were in green trees, and 36 were on, or in, snags (a dead, or nearly dead, standing tree).
This study also found that different bats from a common night roost frequently used different day roosts. An individual bat might use the same day roost repeatedly, or switch day roosts every couple of days.
Two of the key factors in selecting roosts seemed to be the presence of bark on the snag, and a snag being taller than the trees immediately around it. The tall snags towering above the rest of the forest are in fall sun all day, and thus are probably warmer places for the bats to roost.
Different species of our bats here at TCSNA have different companionship preferences. While the little brown bat might roost in colonies up to several thousand, silver-haired bats are solitary roosters, except when a female is with her young.
What do bats eat?
The bats at TCSNA all eat insects. What kind depends upon the bat, of course, but bat food includes flies, midges, leaf-hoppers, gnats, moths, caddisflies, beetles, mayflies, wasps and mosquitos. Adult bats can eat up to 1000 to 1200 insects per hour. That’s about 1/3 to ½ of the bat’s body weight. This is the equivalent to an insect every 3 seconds! Yikes! That’s a lot of chasing, but bats sometimes fly into swarms of insects which probably helps a lot. Interestingly, bats can catch insects in their tail or wings, not just their mouths.
When I first read this, I thought that adding that much body weight each night would make the bat too heavy to fly. Never fear, the bats have solved this problem. Bats can complete the processing of “food to feces” in 35-54 minutes!
Life cycle of bats
Males inseminate the females in the fall. However, the semen is held within the female, and the eggs are not fertilized until spring. The babies are born in June or July. Baby bats learn to fly within 3 weeks. Little brown bats live an average of 6 to 7 years, but the record is 31 years. Other species live an average of up to 20 years.
Bats are amazing creatures that play a vital role at TCSNA. As our forest naturally ages, there should be a greater number of snags providing more roosts. This could mean the bat population grows. Hopefully, these fascinating little critters will be with us for many generations to come.
*Perkins, J. Mark, John M. Barss and Joshua Peterson. 1990. Winter Records of Bats in Oregon and Washington. Northwestern Naturalist 71:59-62.