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By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
All photos by the author unless noted.
The forest at Tryon Creek State Natural Area (TCSNA) is literally rooted in the soil. What happens below the surface of the ground is vitally important to the forest, but something which is generally hidden from us. Rarely, Mother Nature provides us with a glimpse into her underground world, and it can be very enlightening.
The Root System Revealed
In late September of 2013, a little more than 5 years ago, a tree growing by the side of the road leading up to the “Horse Lot” at TCSNA blew over during a rainy, stormy period. Seizing the opportunity, I started studying this event, with the idea of creating a series of Natural Notes possibly stretching over many years.
When the tree blew over, the root system was revealed. Photos taken within 2 or 3 days of the blowdown are shown below.
After the tree fell over, for at least a couple of days afterwards, there was a pool of water covering the clay layer exposed when the tree had fallen over. It turns out that the soil under this tree, just like most of the soil at TCSNA, has only about a 2 to 3 foot layer of soil suitable for growing plants, underlain by a thick layer of clay. This clay is very resistant to water flow and root growth. For safety reasons, the Park Staff filled in this hole shortly after the tree fell over. The photo below gives an indication of the size of the root wad. The red arrows show moderate- to large-sized roots growing horizontally, not downwards, because they hit the clay layer.
— Photo by Anonymous Park Visitor
I measured the thickness of the root wad within a week of when the tree fell over. I did this by pounding a thick metal rod through the root wad, and measuring how much of the rod stuck out of the soil. A picture of this method is shown below. I pushed the rod through the root at 4 different locations, both 2 and 3 feet on either side of the trunk of the downed tree. On average, the thickness of the root wad was 24-1/2 inches.
As you can see, there are no roots growing straight down out of the root mass. The clay layer beneath this tree was not hospitable.
The Aging Root Wad
Five years after the tree blew down, I returned to the site, and measured the thickness of the root wad again, in the same way I had measured it when the tree first fell down. In the 5 years since it fell over, the soil on the root wad was 4.9 inches thinner than it was when it first fell down. Based on other trees that have fallen over in the forest, like this one along the Maple Ridge Trail shown below, I anticipate that sooner or later, all the soil will be washed off the skeletal root system.
An Underground Dam
These root systems all raise questions about the depth of the “plant friendly” soil, which in much of the park, seems to be pretty shallow. In many cases of fallen trees, the soil which is exposed is substantially clay. Clay of course is resistant to water flow. Just how resistant? I collected a sample of clay from the root system of a tree that had recently fallen at TCSNA.
At home, I drilled holes in the bottom of a plastic cup, as shown below. Water flowed easily through the holes as you can see in the picture below.
For my test, I put about 1/3 of an inch of clay into the pot, and gently pressed it down into the pot. Then I filled the pot with water. There was some tiny amount of water that flowed through the holes but not much. I let the pot sit with water in it for a couple of days. Then I once again filled the pot with water and let it sit inside of a plastic tray. I sat the pot on two pencil stubs to keep the bottom of the pot up off the tray, so water could easily run out of the holes. This is illustrated below. I left out the plastic tray in order that you could see the rest of the set-up more easily.
In the course of 20 hours, not a single drop of water leaked out of the cup. The clay used in this demonstration is clay that is found underground throughout much of TCSNA.
The nearly impenetrable layer of clay found at TCSNA means that the forest we love is dependent on approximately the top two feet of soil. To put this in perspective, when leading hikes for students at the Park, I will oftentimes ask them this question: What would they think if I went to Washington Square and dumped 2 feet of dirt on top of the asphalt parking lot, and declared that I was going to start growing a forest there? Almost always the kids will say something like “You are crazy!” But in fact, that is essentially the situation we have here at the Park.
The photo below shows me with a cardboard box the same height (24.5”) as the depth of soil supporting the trees at TCSNA. This is the depth of soil I would pour onto the parking lot in order to create a forest at Washington Square Mall like that at TCSNA.
The thin layer of soil supporting the forest at TCSNA is one reason that the trees need to shelter each other if they are going to resist being blown down by the wind. They really do constitute a “Forest Community.”
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The forest at Tryon Creek State Natural Area (TCSNA) contains marvelous plants that we can enjoy at different seasons for different reasons. They range from the beautiful trillium (Trillium ovatum) blossoms in the early spring to the bright red leaves of the vine maple (Acer circinatum) in the fall. But these individual displays of beauty are transitory, as are the plants themselves. This is summed up in the Latin title of this note which means: “thus passes the glory of the world.”1
I started a phenology study in mid-2013. This involved, for the most part, identifying and tagging specific individual plants and monitoring their developmental stages each year. These stages were things like when I could first see the veins on the new leaves, and the first time I found open flowers on the plant. Now, just five years later, I am surprised at how many of those individual plants I was following have died in that short timeframe.
Plants are Persistent
As we realize, plants are persistent. In the photo below, you see the result of a very old injury to the trunk of a Douglas-fir (Pseudotsuga menziesii) growing at TCSNA. Long ago, the upright shoot of this tree was damaged or killed, and several side branches competed to take over the role of “leader.” The branches marked with red arrows lost the race to become leader and are now dead. The branch indicated by the blue arrow won, and became the leader so successfully, that it looks almost like it was always the leader. Several branches that were lower down the tree when the top was lost are marked with green arrows, and they remained horizontal.
Another example of a persistent plant is this mature black cottonwood (Populus trichocarpa) located alongside the Old Main Trail. Normally, mature black cottonwoods don’t have little branches popping out along the main trunk. However in this case, the reason can be seen in the wet dark seepage at the base of the tree. This tree appears to be infected with some microorganism (Fungus? Bacteria?) which is excreting a smelly fluid out of a crack in the tree. When Ranger Deb and I bored into the tree, the heartwood was definitely wet and smelly, evidence that it was decaying. In these cases, the tree doesn’t do such a good job of controlling the sprouting of the buds on the tree trunk.
Finally this Douglas-fir near Old Main Trail, which still has many green needles, sports numerous fungal fruiting bodies which indicate it is heavily decayed.
Plants are persistent, but…
Sometimes the trees have problems from which they never recover. The red alder (Alnus rubra) pictured below probably just aged out. Estimates of what constitutes “old age” for an alder varies from 60 years to a maximum of 100 years. Red alder is a species that likes full sun light and most frequently gets started on disturbed sites. So no surprise that we would find one this size dead.
A little more surprising is the dead western redcedar (Thuja plicata) pictured below.
This species is very shade tolerant, and under normal circumstances commonly lives several hundred years. So why is this relatively young tree dead? My best guess is based on the fact that this was found on the uphill side of the trail. Trails often serve as unintentional “dams” to the normal flow of underground water (great example: Old Main Trail near the Nature Center). A couple of years ago we had an extraordinarily rain-soaked winter season and I hypothesize that this cedar got “drowned out.” Yes, cedar frequently grows in wet-ish areas, but there is a limit to everything.
Individuals from several shrub species have recently died as well. This red elderberry (Sambucus racemosa) located just off the Old Main Trail (pictured below) is the plant that began my awareness of this topic and thus this whole article. This plant died before the recent winter with heavy rains. It was the first plant which was part of my multi-year phenology study that died. Additional walks around the park revealed many other dead elderberries. Again, it appears to be a fairly short-lived plant.
Perhaps the most dramatic die-off I’ve witnessed occurred near the upper section of the Red Fox Trail. Last year I noticed that many of the Indian plums (Oemleria cerasiformis) seemed to turn yellow and lose their leaves a little earlier than normal. This year, a relatively large number of them never leafed out. I laid out a 1/20 acre plot (a circle with a radius of 26.3 feet) and counted all of the Indian plum stems. I also measured their diameters at ground level. To the best of my ability, if I was able to determine that multiple stems were part of a single plant, I only measured the largest stem. Important confession: I chose an area with a very high density of dead stems. The results are summarized below:
In some cases these plants were quite large, both in height and diameter. I laid one of the stems on the sidewalk near the top of the Red Fox Trail to make it easy to see.
I selected a few of the larger Indian plums, and counted the annual rings at the base of the stem. They were between 15 and 19 years old.
As a final example, I’ve also noticed this year a number of dead salmonberry (Rubus spectabilis) in the forest. I don’t think this is the result of some climatic fluke or disease, because there are also a very large number of healthy salmonberries in every area where I’ve see a dead one. One example of a dead salmonberry is pictured below:
Below is a cross-section of stem from a dead salmonberry. Note the relatively large whitish pith in the center.
And let’s not forget the animals
Sometimes animals play important roles in the life of plants. A couple of years ago, beavers decided that a lot of the young cedar trees near Obie’s Bridge were ready to eat, and went in for the harvest. The results were evident by the number of chewed off stumps, like the one seen below.
“This too shall pass”2
The forest we see today is not the forest we will see tomorrow. Barring huge environment shifts, the major trend that we should expect is that much of our uplands forest will evolve to a predominantly redcedar-hemlock forest type. Douglas-firs will be relegated to a tiny role. Red alders may persist in some of the bottomlands near the creek. This of course will bring some shifts in the animals that inhabit our forest as well. It will be different, but still just as fascinating as it is today!
1Documentation on the web indicates this phrase was used as early as 1409 during the installation of the Pope.
2According to Wikipedia, this is an ancient Persian expression that worked its way into the English language sometime in the 1800s.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
In the forest, like much of life, timing is everything! It’s why most animals have their young in the spring or early summer when food is abundant. It’s why most plants don’t bloom in December, when there’s a good chance that their flowers would be killed by a subsequent frost. The study of the timing of different biological events is called “phenology.”
What’s a Phenology Study?
A phenology study involves identifying when different organisms enter different stages of their life, or behave in particular ways. My phenology study focuses largely on plants. Plant phenology frequently involves studying the behavior of selected individuals over the course of several years. Some “events” in a plant’s life that can easily be tracked are, for example, when the veins on the new leaves are first visible (aka, bud break), or the first time you can see the sex organs inside of a flower.
I started the phenology study at Tryon Creek State Natural Area (TCSNA) early in 2013. For this study I tracked ten different species growing along 4 different trails at the park; Red Fox, Old Main, Cedar/West Horse Loop (referenced here as “Cedar”) and Middle Creek/Big Fir (referenced here as “Middle Creek”). For perennial species with above ground parts, I tagged the plants and followed them each year. For annual plants, or those species arising from underground organs, I identified a given patch of ground and studied plants at that location. As time went by, I started including observations on a few other species like a delightful patch of bleeding hearts (Dicentra formosa), and some spittlebug nymphs (suborder: Auchenorrhyncha).
I made observations on a weekly basis, with a few exceptions caused by vacations, and extreme weather conditions. For a couple of reasons, mostly related to “learning curve” issues, I starting collecting useable data in 2013 part way through the growing season.
One of the challenges in conducting a phenology study is the issue of when you should report the results. In this case, the differences I observed between 2016 and 2017 are dramatic enough that it is time to provide you with a report. This probably won’t be my last phenology report, “God willing and the creek don’t rise” (to use an old expression).
The Drivers of Plant Development
Plant development is driven by several factors, key among them being day-length, temperature and moisture availability. When it comes to spring budburst in our area of the world, temperatures probably are the primary driver.
The temperature plays two important, and quite different, roles in bud break. The plant needs to hold off on bud break until the threat of a killing frost is past. Thus most perennial plants in our area have a “chilling requirement.” This means that the buds have to experience a certain amount of chilling before they can start growing. Secondly, the buds have a “forcing requirement” which is a certain amount of warm temperatures to get the buds growing after the chilling requirement has been met. As anyone who has ever walked through the forest in the spring knows, these requirements vary dramatically between different species of plants. If the plants receive less than the normal amount of chilling curing the winter, they will need a greater amount of warm “forcing” in the spring. Although it is clear from the diagram that there is at least some minimal amount of chilling needed to ensure that the buds will eventually open.
The diagram below shows the generalized nature of the relationship of chilling and forcing for both Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). While the curves have a basically similar shape, it is apparent that with low levels of chilling, the hemlock will break bud first, but with large amounts of chilling, the Douglas-fir will break bud first.
Chilling and forcing requirements of Douglas-fir and western hemlock1
In reviewing these bud break results, please be aware that there is very little agreement on the exact temperature that separates the “chilling” and “forcing” functions. I believe most scientists would think 50 degrees is little bit too high, but to be blunt, this is the base I used because it is the best database to I have available. As you can see in the chart below, there have been dramatic differences between years in the number of growing degree days in the first three months of the year, primarily that 2017 has a much cooler spring.
*Data from the Aurora Airport, approximately 15 miles from TCSNA.
So What Happened?
Presenting even a summary of all the data I collected would be a sure cure for insomnia, so I’ve picked out a couple of examples from the study which are fairly typical of the general trends. The first example is to look at the behavior of the Pacific waterleaf (Hydrophyllum tenuipes). This is a plant that has underground roots, stems and buds.
The graph below shows the results for the years 2015 through 2017. On average, the appearance of the first leaves in spring 2017 was delayed an average of 2.5 weeks from the first appearance of leaves in the prior two years. (And yes, the absence of data for the week of Feb 11, 2015 is unfortunate!) The date of first flowering in 2017 was on average 3.0 weeks later than first flowering in 2016. The primary lesson here is that both budburst and flowering in 2017 was much delayed compared to the two prior years. Interestingly, the average date of when the last leaves died was nearly identical in 2016 (33.0 weeks) and 2017 (33.75 weeks).
Similarly, the leafing out of the vine maple (Acer circinatum) was also delayed about 3 weeks in 2017, as seen in the chart below. For the vine maple, so few of the plants I followed produced flowers on a regular basis that the data is probably not worth presenting, although what data there is follows the same general pattern as the Indian plum above.
At first glance, these lines all look fairly similar. However, looking at Week 20, for example, the number of growing degree days in 2016 is about double the number for that same date in 2017. This is a huge difference!
The final set of plant data that I will include here is for snowberry (Symphoricarpos albus), a reasonably common, but not abundant shrub at TCSNA. Here again, both the budburst and flowering of these plants is three to five weeks later in 2017 compared to 2016, as seen in the graphs below:
And it’s not just plants
Below is a chart of the sightings for two years of spittlebugs. In the beginning of 2016, if there were no spittlebugs seen, I just left the space on my datasheet blank. Midway through that year, I recognized the folly of that approach, and started making a clear record showing that no spittlebugs were seen. My notes on the March 2016 spittlebug indicate that it was just one individual bug, and a small one at that. Sometimes Mother Nature will show off one specimen of something way out of season, but that probably doesn’t really mean the season has started. No matter what you think about the March 2016 outlier, it is abundantly clear that the spittle bug, like many of its botanical associates, was late in 2017 by at least three weeks.
The Connection to Global Warming
As documented here in the differences between the various years, the organisms in the forest are sensitive to environmental temperatures. This will be very important as we consider global warming, and what we should do about it. I love the fact that Mother Nature provided us with a great example of the fact that the trend of global warming is not a straight linear process, but will have some hiccups like 2017. By the way, the early readings from this year (2018) show that some plants are starting growth way earlier than they did in 2017. But that’s grist for another note at least a year away. A fascinating aspect to this, which we may be many years away from experiencing, is that almost all woody plants require a certain amount of chilling before they break bud. If global warming ever gets to the point where the winter temperatures are not adequate to chill the buds, already completed research tells us bud break will be significantly delayed. Then we could have real problems. But for now, enjoy the forest that we have!
1Harrington, Connie and Peter Gould. 2016. Rise and Shine: How Do Northwest Trees Know
When Winter Is Over? Science Findings, Issue 183. USDA Pacific Northwest Research Station.