Recycling the Forest: Year 2
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Mother Nature is the world’s first, largest and best recycler! Each year in Tryon Creek State Natural Area (TCSNA), tons of organic matter like trees, dead animals and coyote feces are recycled. This recycled material is used by the forest to grow new trees, birds and mice, to name just a few. If it weren’t for recycling, the surface of the earth would be covered with unimaginable amounts of dead organic matter. In addition, it would be tough for any new organisms to grow because so many materials vital to life would be locked up in the litter.
Two years ago, I started a small study at TCSNA to determine just how fast the recycling process is in our forest. The first year’s results were published in a Naturalist Note last fall. To access this previous note, use this link, and then scroll to the bottom. This second Naturalist Note takes a look at the results after two years.
How it works…
As indicated in the previous Naturalist Note, samples of five different kinds of fresh organic matter were collected from the ground in TCSNA in the fall of 2014. Collecting them from the ground insured that they were naturally ready to start being recycled. The samples included a) red alder (Alnus rubra) leaves, b) bigleaf maple (Acer macrophyllum) leaves, c) western redcedar (Thuja plicata) branchlets, d) Douglas-fir (Pseudotsuga menziesii) branchlets, and e) scales from a Douglas-fir cone that had been chewed apart by a squirrel. Two samples of each thing were collected, for a total of 10 samples.
Each sample was placed inside a flat “bag” made of standard plastic window screen material. The bags were then taken into the forest, placed down flat on the forest floor, and fastened down by inserting a nail through the corner of each screen down into the ground.
From September 2015 through September 2016, pictures were taken on a quarterly basis. The results after two full years can be seen in the photos below. The first photo shows the array of litter bags before I disturbed them to take the photos. The silvery-gray bits seen in the photo are the exposed part of the screen bags. As you can see, there were also some plants growing up through the litter bag array.
They’ve really changed!
The rate of decay of materials from the different species varies dramatically. Specific samples are shown below. The first two sets of pictures show the two different red alder leaf bags. I’ve included both sets of red alder bags to show that there is a certain amount of bag-to-bag variation in the rate of decay. For the remainder of the species in the study, I’ve included only one of the two bags. You may observe that due to my picking up these bags periodically for photos, the position of some of the materials in the bags has shifted a bit.
While most of the leaf blades are substantially degraded, some of the veins are still intact. This is typical of many different leaf decay studies.
The following photos are of one of the bags containing bigleaf maple leaves. Again, you can see that the leaf blades themselves are pretty well decayed, but some of the veins, and petiole (the “stem” that connects the leaf blade to the branch) are still identifiable.
…Or maybe not so much!
The remaining samples are all from conifers, which retain their foliage for more than a single season. If you could feel their foliage, you could tell it was definitely tougher. This becomes evident in the photos below.
The western redcedar foliage has not only not decayed very much, but also, many of the leaf scales are still attached to the branchlets. I’m impressed!
Most of the Douglas-fir needles in the bag shown below have fallen off the twigs, but they still appear to be largely intact. Detailed scientific studies suggest that while there has probably been some substantial decay inside the needles, the outer tough surface of the needle has been somewhat resistant. One unfortunate thing with the Douglas-fir branches is that once the needles have fallen off the twig, they can easily slip through the mesh screen when they are jostled very much. I have no doubt that some of the needles that were originally on the twig have fallen out of the bag.
The final set of photos below show the tough cone scales of the Douglas-fir which appear to be only minimally changed over the course of two years on the ground. This is not surprising, given that the cones are made tough to protect the seeds of the tree. Also, the cones tend to have high resin content, and thus resist decay.
Litter as fertilizer
Litter contains vital nutrients that help trees grow. One of the advantages of relatively slow litter decay is that the litter then acts as a “slow-release fertilizer” a method of ensuring that the living plants will have ample opportunity to absorb it to assist with their growth.
But long before the nutrients trapped in the litter help the next generation of trees and shrubs grow, those nutrients help the fungus and bacteria that decay leaves grow. And just like you, the microorganisms are more attracted to a lavish banquet than to a bowl of thin soup. The more nutritious the litter, the faster it decays1. For example, red alder leaves have 4 times the nitrogen content of either western redcedar or Douglas-fir foliage. Bigleaf maple litter has more than 3 times as much phosphorus as either western redcedar or Douglas-fir. Both red alder and bigleaf maple have more than 10 times the amount of potassium as Douglas-fir litter. Viewed from the perspective of nutrient contents, it’s no surprise that the red alder and bigleaf maple foliage decays faster.
It was found in the mid-1970s that applying urea fertilizer containing 200 lbs of nitrogen per acre to Oregon’s Douglas-fir forests, significantly increased their growth rate. As noted above, red alder leaves, due to the nitrogen-fixing nodules on the alder roots, contain relatively high levels of nitrogen. The photo below shows the amounts of urea fertilizer (the typical nitrogen fertilizer for forests) and alder leaves that would be needed to provide 200 lbs/acre of nitrogen. So the decaying leaves, especially the alder leaves, are acting as a slow release fertilizer.
The box of dried alder leaves contains as much nitrogen as the small vial of white urea.
The dead plant parts like tree trunks, branches, leaves and even the unseen roots, are important parts of the whole cycle of life at TCSNA, as well as in other forests. In their role as nutrient recyclers, the fungus and bacteria that decay the forest litter play a vital role in helping maintain a healthy and vigorous forest. Recycling is an important example of how different parts of the forest work together to create the TCSNA we love so much.
1Valachovic, Y. S., B. A. Caldwell, K. Cromack Jr., and R. P. Griffiths. 2004. Leaf litter chemistry controls on decomposition of Pacific Northwest trees and woody shrubs. Can. J. For. Res. 34:2131-2146.