By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The plants growing at Tryon Creek State Natural Area (TCSNA) require a supply of nutrients to stay healthy and keep growing. An important part of supplying those nutrients is the decay of dead organic matter like leaves and branches. This decay process releases the chemicals in the dead leaves and branches so those nutrients can be reused by plants that are still growing.
To study this process, I collected leaves, branches or cone scales of several species of trees found at TCSNA in September 2014. I only collected fresh materials which had recently fallen to the ground. Within two days I placed these in wire mesh (screen) “envelopes” and fastened the envelopes to the ground with nails. During the first part of the study I took photographs of the envelopes on a monthly or quarterly basis, but starting in September of 2016 I switched to once-a-year monitoring. The results after one and two years have already been published in earlier Naturalist Notes.
The one-, two- and three-year end results are presented here. While I had two envelopes for each of the materials, I only selected the most photogenic envelope for inclusion in this report.
Red Alder (Alnus rubra) Leaves
Red alder is a common tree at TCSNA, and is renowned for dropping its high nitrogen leaves onto the ground while they are still green. The results from this study are seen in the photos below.
One university research project studying the decay of dead red alder leaves indicated that 93% of the nitrogen in the alder leaves was released into the soil via the decay process the first year. The same study found that 91% of the calcium and 97% of the potassium (both key nutrients) were also released during the first year of leaf decay1. This relatively rapid decay of the red alder leaves was attributed to the fact that they generally contain higher levels of nutrients than most trees, thus nourishing the decay organisms.
Western Redcedar (Thuja plicata) Branchlets
In sharp contrast to the red alder leaves, the western redcedar foliage contains lower levels of many nutrients, thus making it a somewhat less attractive food source for microorganisms. In addition, redcedar foliage and twigs contain many terpenes. Terpenes not only give the tree it’s distinctive “cedar smell,” but terpenes also have anti-fungal properties. The major terpene in western redcedar (α-thujone ) is shown below.
No wonder that the cedar is decaying a little more slowly than some of the other samples.
Bigleaf Maple (Acer macrophyllum) Leaves
With no special chemical defenses against fungi, and a fairly high nutrient content, the maple leaves are another fast decaying material.
Douglas-fir (Pseudotsuga menziesii) cone scales
These Douglas-fir cone scales were already lying on the ground at the time of collection. The cones they came from had probably been chewed apart by a squirrel looking for the nutritious seeds.
Superficially, the cone scales after three years look almost exactly like they did the day I put them in the bag. Obviously the scales are made up of a very hard material, not too much different than the wood in the main stem of the tree. Mother Nature made the cone scales very durable to protect the precious young seeds, and as a result, they are being recycled very slowly.
Again, the Douglas-fir twigs and foliage contain some terpenes similar to the terpenes found in the redcedar, and thus, they decay more slowly than the alder and maple leaves. Notwithstanding the resistance to decay, all the needles have fallen off the twigs.
The End Result
The nutrients within each of the samples used in this study will be released and then absorbed by living plants and fungus, helping the forest to grow the next generation of life. This study shows the enormous differences in the decay times of different kinds of tree litter. The softer materials, like the alder and maple leaves also have the highest nutrient content, and lowest concentration of anti-fungal chemicals. But, as the old saying goes, “All in good time!” All these nutrients will once again join living organisms, ensuring the continuation of TCSNA’s forest!
1Radwan, M. A., Constance A. Harrington and J. M. Kraft. 1984. Litterfall and nutrient returns in red alder stands in western Washington. Plant and Soil 79(3):343-351.
Mother Nature: Master Recycler
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Recycling has become a “big deal” for humans in the years since I was born! As a kid, I remember burning newspapers in the burn-barrel by our garden, and throwing a massive number of tin cans in the garbage which doubtless wound up in some landfill. Now, the Oregon Department of Environmental Quality (DEQ) proudly reports that in 2013, Oregonians recycled 53.9% of our post-consumer waste. Nice try, but we don’t even come close to Mother Nature’s record of recycling: 100%!
Here at Tryon Creek State Natural Area (TCSNA) the forest constantly recycles biomass like leaves, logs and dead animals. Sometimes it’s fast, and sometimes it’s slow, but it’s always thorough!
We need look no further than some of the TCSNA’s old logs and stumps to see that sometimes recycling takes quite a while. This rotting log is approximately 18” in diameter. It is on the side of Old Main Trail and hasn’t changed much in the last 5 years, and I don’t expect it to completely disintegrate any time soon. In fact, research foresters report that fallen Douglas-fir (Pseudotsuga menziesii) logs sometimes take almost 200 years to completely decay!
But other stuff “recycles” faster, doesn’t it?
Absolutely! In order to see how fast things are recycling (“decaying”) at TCSNA, I set up a small study. Scientists who want to study recycling in the forest often use things called “litter bags.” (The term “litter” here refers to the fallen leaves, twigs and branches on the ground, not to candy wrappers and used Kleenex!) I collected the plant material for this study off the ground, so this material was ready to start decaying.
I cut square pieces of window screen to make my bags. I placed the plant material on half of each piece of screen, and then folded the other half over the top, and stapled it shut. I fastened each bag to the ground using four big nails, one in each corner. I set up the bags in an area of TCSNA where they wouldn’t be disturbed. I put out some bags on September 11 and others on September 13, 2014.
In each bag, I put one of five things; Alder (Alnus rubra) leaves, bigleaf maple (Acer macrophyllum) leaves, western redcedar (Thuja plicata) twigs with their green scaly leaves, Douglas-fir twigs with needles, and finally, the scales from a Douglas-fir cone. I had two bags of each type of material. Then I fastened the bags to the ground.
So what happened?
This is what it looked like on September 13, 2014, after the full study was installed. You can see some of the green leaves inside the bags.
I took pictures of every bag each month. When I took pictures I brushed off the top of the bag, loosened two of the nails holding it to the ground, and slipped a piece of white plastic underneath the bag to provide contrast to the material inside the bag. I refastened the bags and replaced the litter following each photo. Below are some highlights.
By October 3, 2014 some natural forest litter had fallen on the bags. This is totally realistic. There were times when the litter bags were almost completely covered with natural litter from the trees.
Having the litter inside bags did create a certain amount of unrealism. This point was made dramatically during my March 2015 visit to the litter bags, when the little critter pictured below was crawling over them. To the extent that snails might accelerate litter decomposition, my study was only an approximation of reality.
So let’s see the decay process!
The decay rates for the samples in my litter bags varied a lot between species, and sometimes between particular leaves of the same species.
The alder leaves decayed dramatically over the course of a year. After 79 days, the leaves had lost their color, but had only just started to disintegrate. By the end of March, the leaf in the upper half of the photos was pretty much reduced to the mid-rib (the tough “vein” going from the base of the leaf right through to the tip) and the lateral veins. In contrast, the leaf in the lower right hand corner still had a lot of the leaf blade left.
Once again, the maple leaves were significantly decayed after the first year, but the petiole (the stalk that attaches the leaf to the branch) being more “woody” than the leaf blade is still largely intact.
Western redcedar is loaded with hydrocarbon molecules that impart decay resistance. The most amazing thing was that in November 2015, after more than two months on the ground, most of the redcedar branch was still green! (Confession time: The other redcedar branch had turned completely brown at this point.) After over a year on the forest floor, this branch, and its needles, was still largely intact.
Unlike the alder and maple samples, this bag includes the woody twig in addition to the foliage. The Douglas-fir twig rapidly shed all its needles, producing an un-photogenic combination of a bare twig, and clumps of dead needles. The slight movement of the bags in preparation for taking photos is what caused the needles to gather in clumps. The needles, though brown and scattered, are individually maintaining their structural integrity. As with the western redcedar discussed earlier, the presence of hydrocarbon molecules in the needles and stem are helping resist decay.
Douglas-fir cone scales
The Douglas-fir cone scales are tough and woody. To tell the story of their decay in the first year, we only need two photos. In the 12+ months in the litter bag, there was no perceptible change in the Douglas-fir cone scales, except they are now slightly darker! Again, shifting the bag for photos results in shifting the scales around within the bag.
The Cycle: Life > Death > Life
As organic matter decays, important chemicals like nitrogen and phosphorus are slowly released to soil for growing plants. The partially decayed organic matter in the soil dramatically increases its moisture holding capacity, and water infiltration rates, among other things. Better than most of us, Mother Nature knows that the rotting leaves and stems of today are the key to the towering trees of tomorrow! Without recycling, there would be no forest as we know it.