The Sun: Mother of Us All!
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
With the exception of a few “iron eating” bacteria described in my Naturalist Note of May 3, 2015, all life at Tryon Creek State Natural Area (TCSNA) depends upon energy from the sun. It might have to go through several steps like plants using sunlight to create carbohydrates, insects eating the plants, birds eating the insects and bigger birds eating the smaller birds, but eventually we all depend upon the sun.
How Much Sunlight Is There?
The amount of sunlight falling on TCSNA depends upon many things. It depends, for example, upon the season of the year, the time of day and the amount of cloudiness. An important factor for the plants includes competition from other nearby plants. So how much sunlight is there in various parts of the forest?
To find out, I took measurements at TCSNA on two totally clear days, July 13 and July 30, 2016. On July 13th, I focused on an area near the Nature Center around noon, and on July 30th, I focused on the Cedar Trail in mid-afternoon. On July 13th, the full sunlight at noon in the Equestrian Parking Lot measured 104,000 lux (lux is a standard measure of light intensity). On July 30th my 2:50 PM and 3:30 PM full sunlight readings averaged out to 101,000 lux.
Out on the trail, 3 feet above the ground, there were about 1500 lux of light on July 13th, and from 260 – 610 lux on my second day in the forest.
How dark is “shade”?
I plunged into the “dark side” by measuring the light level under some plants. First I checked under a dense clump of salmonberry (Rubus spectabilis). At the soil surface there were only 60 lux of light under those plants. But the champ was a cluster of young western redcedar (Thuja plicata) which let just 50 lux of light through to the soil surface. That is less than 1/20 of 1% of full sunlight. And what was growing under that clump of western redcedar? Take a look in the picture below:
Almost nothing grew under this clump of western redcedar.
Why Don’t Plants Grow in Dim Light?
Plants use light to power the photosynthetic process to produce the carbohydrates (like sugars) they need to grow. The more light, up to a point, the more sugar. So you would think that they might grow everywhere there is any light at all. However, the flip side to photosynthesis is respiration. Respiration is the result of internal processes for cell maintenance that are vital to life. The amount of light at which the process of photosynthesis is creating the same amount of energy that the plant uses to maintain its basic functions is called the “compensation point.” At this point, the plant can stay alive, but not grow. When the light level is so low that photosynthesis falls below the respiration rate, the plant ultimately dies.
Various species of plants have different compensation points. Plants are sometimes grouped by their tolerance of shade. At TCSNA one example of an “intolerant” plant (one that can’t tolerate the shade) is our Douglas-fir (Pseudotsuga menziesii). While we have many mature Douglas-fir, the number of young ones is very small. Shade tolerant plants include both the western redcedar and western hemlock (Tsuga heterophylla).
Published standards on how many lux plants need are somewhat variable. As a general rule it appears that shade tolerant plants need at least 150 to 500 lux, while shade intolerant plants need at least 800 to 1500 lux to survive. On the high end, it appears that for all plants about 25,000 to 35,000 lux saturates the photosynthetic process.
Those Mysterious Sunflecks
Both days at the park I periodically encountered “sunflecks” on the trails beneath tall trees as shown in the picture below. They typically appear as circles or multiple overlapping circles.
The first day, the light intensity of these sunflecks typically measured between 2,500 and 9,000 lux. That is the equivalent to about 2-1/2 to 9% of full sunlight. My second day measuring sunflecks at approximately 3:00 PM, the majority were from 2,500 to 3,500 lux. So these sunflecks can provide adequate light for photosynthesis to the plants near the ground.
Sunflecks have intrigued me ever since I was delivering newspapers during a partial solar eclipse in Minneapolis, Minnesota in the early 1960s. I saw something on the side of a house that stuck with me the rest of my life. Four decades later, during the partial solar eclipse of June 10, 2002 at my home in Lake Oswego, I captured this phenomenon on film. The photo below was taken during a partial solar eclipse. This is how the sunflecks looked on the side of my house.
Amazingly, the sunflecks were crescent-shaped, not circular. Compare the sunflecks above to the diagram below of what the sun (and moon) looked like that day during that partial eclipse (based on information from the internet).
You will note that the sunflecks on the house seem to be flipped 180° from the sun’s actual shape in the sky. This is because of the sun’s image passing through a tiny hole in the crown of the tree. This is sometimes referred to as the “pinhole camera effect” as illustrated below using a tree instead of the sun. This effect was described by Aristotle in the 4th century BC.
Interestingly enough, your eye also does this, and the images on your retina at the back of your eye are all upside down.
Flipping the image of the actual eclipse by 180° we get the following image, which is virtually identical to the images on the side of the house. This is because the tiny spaces in the crown of the tree are acting as pinhole cameras.
The bottom line: the round sunflecks we see on the trail are really images of the sun.
Why aren’t all sunflecks the same?
Sunflecks have two different properties, size and brightness. Both of these properties are influenced by two factors. First, the size of the hole in the canopy, and second, the distance from the hole in the canopy to the ground. For a given size hole in the canopy, the closer to the ground, the smaller and brighter the sun fleck. In the picture below, the sunflecks are produced by the same size hole. Because both holes are letting the same amount of light through, the larger sunfleck doesn’t appear as bright as the small one.
For holes in the canopy at the same height above the ground, the bigger the hole, the bigger the sunfleck. Both sunflecks below are produced by different size holes at the same distance above the ground. However the intensity of light in the sunflecks is the same.
If you start thinking about the combination of different size holes at different distances from the ground, you can see that a vast array of different sunflecks are possible.
So in a sense, these sunflecks on the trail are constant reminders that the sun is indeed the mother of us all.