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The Forest’s Fabulous Fertilizer Factory!

Red Alder: The Forest’s Fabulous Fertilizer Factory

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester


Ever wondered how a forest fertilizes itself? Miracle-Gro? Wildlife scat? What’s happening out there? Let’s start with some basics ingredients.

The trees at Tryon Creek State Natural Area use the process of photosynthesis to make sugar from three key ingredients:

  • Sunlight
  • Water
  • Carbon dioxide

This sugar is then converted into wood and other carbon-based materials that make up the vast majority of the tree.

Some more key ingredients…

While water and carbon dioxide are important raw materials for the tree, some other ingredients play key roles in the life of the tree. These are often referred to as “mineral nutrients.” These mineral nutrients make up only a tiny fraction of the tree’s great bulk. Calcium, phosphorus, iron and magnesium are a few of these nutrients. But perhaps the most critical mineral nutrient in our Pacific Northwest forests is nitrogen.

Researchers have found that adding as little as 200 lbs. of nitrogen per acre will typically boost the growth of Douglas-fir (Pseudotsuga menziesii) in forests west of the Cascade Mountains. In the photo below, you can see how much urea fertilizer (46% nitrogen) is needed per square foot of forest floor to increase the growth of Douglas-fir.


Forest fertilizer

It is easy to see why this element is so important. The key process of photosynthesis is mediated by a green pigment called chlorophyll. The structure of the chlorophyll molecule is illustrated in the diagram below.

C = carbon, O = oxygen, H = hydrogen, N = nitrogen and Mg = magnesium


Chlorophyll = “the green stuff”

Looking at the chlorophyll molecule, we can see that nitrogen atoms play a key role at the very heart of the molecule.

No nitrogen, no chlorophyll; no chlorophyll, no photosynthesis; no photosynthesis, no live trees.


But what about the forests that never get fertilized?

How do they get nitrogen? There are a couple of ways, but one of the more interesting ways Tryon Creek’s forests get nitrogen is from red alder (Alnus rubra) trees. You can recognize the alder in the forest by their smooth trunks, often with grey splotches of lichen, and some moss like the example below.

Alder bark

Alder bark


Yes, alder trees help feed the forest’s need for nitrogen. It’s not because the alders want to be nice, they are actually just as selfish as any other tree. However, as in all of our lives, sometimes there are unplanned consequences, and so it is with the alder.

But let’s start at the beginning.

All trees, and indeed all plants, need nitrogen. As luck would have it, almost 80% of the atmosphere is nitrogen. Good news, right? Well…. not so much! It turns out the nitrogen molecules in the air are composed of two nitrogen atoms, hooked together by a very strong triple bond. (This is referred to as “diatomic nitrogen”.) The plants cannot use these nitrogen molecules at all! They are useless to the plants.

A model of a diatomic nitrogen molecule is shown below.


The yellow balls represent the nitrogen atoms, and the three sticks represent the triple bond.

However, it turns out that some microorganisms CAN use the diatomic nitrogen in the air as a source of nitrogen for their growth. So the alder and one of these microorganisms, whose Latin name is Frankia alni (it’s so weird it doesn’t even have a common name) join into a partnership. The Frankia (now that we’ve been introduced, we can call it by just its first name) lives inside of the alder roots and creates little lumps called nodules. You can see a cluster of these nodules in the photo below. This one is about 4 centimeters (1-1/2 inches) long. This is bigger than most nodule clusters, but I’ve also seen bigger ones.


Cluster of nodules

Cluster of nodules


When two different species live together it is called a symbiotic relationship. The alder and Frankia have a special type of symbiotic relationship called “mutualism” which means both organisms benefit from the relationship. The alder feeds the Frankia sugar which it needs to live. In turn, the Frankia converts the diatomic nitrogen in the air into ammonia. This conversion process is called “nitrogen fixation.” The alder tree CAN use the ammonia molecules. The ammonia molecule is shown below.

The yellow ball represents the nitrogen atom, and the white ball represents hydrogen atoms.



One of the interesting things about the process of making the ammonia inside the alder nodule is that the whole process can only happen in the total absence of oxygen (this is called “anaerobic”). So the nodule has to let in the nitrogen, but prevent the oxygen from stopping the nitrogen fixation. Hmmm… Sounds tough! How does it do that? Scientists aren’t 100% sure, but there is one tantalizing little thing that might be part of the solution.

When you first cut open an alder/Frankia nodule, it looks yellow-orange, sort of like an apricot. Some freshly cut nodules are pictured below.

5 Alder nodules cut editted

freshly cut



However, after about three minutes, they change color, and look like the picture below.


colors transform

colors transform











Some of the cut nodules appear to have turned a bit reddish, and a few have fairly prominent red streaks in them. Red, red, red. Let’s see, is there anything in your body that is red? Oh, yeah! Blood is red. And the thing in your blood that makes it red is called hemoglobin. You’ve heard of hemoglobin; it’s that chemical that binds with oxygen (and then turns brighter red) and carries the oxygen around your body. Well, guess what? Scientists have discovered that both the alder, and the Frankia can manufacture hemoglobin. Not exactly like the kind humans have, but close!

An intriguing situation.

The exact role of the hemoglobin in the alder nodules is something scientists are still working on. However, when you consider that oxygen could stop the nitrogen fixation process, but oxygen is also necessary for the normal respiration of the cells, and we have a chemical (hemoglobin) that binds with oxygen….. Well, let’s call it an intriguing situation. We may not know the answer for sure, but it’s fun to think about the possibilities. Maybe some of today’s young Tryon Creek hikers will discover the answer when they get a little older.

Remember I said that alder nurtures the forest?

Well, I doubt the alder planned to nurture the other plants in the forest, but it does. Most trees have to work hard to get their nitrogen from the soil. In the autumn, these other trees suck most of the nitrogen from the leaves back into the twigs to be reused next year. Since much of the nitrogen is locked up in the (green) chlorophyll molecule, the trees let the chlorophyll break down. Then the tree can suck the nitrogen from the chlorophyll back into the twigs. However the alder has all those nodules feeding it nitrogen, and so it gets lazy, and doesn’t do a good job of having the chlorophyll break down, and sucking the nitrogen back into the twigs. So the alder leaves fall green, and they still contain lots of nitrogen. In the photo below, taken along the Old Main Trail in the autumn, the yellow & brown leaves are from black cottonwood (Populus balsamifera spp. trichocarpa). The cottonwood leaves contain relatively little nitrogen. However, the green leaves are all alder leaves, and contain an average of almost 3% nitrogen (on a dry weight basis).

Fallen leaves

Green leaves = alder leaves


As the alder leaves decay, the nitrogen they contain will be released into the soil, and available for any plant root that is nearby, or even a fungus or bacteria.

That’s how the alder nourishes the forest.


This “nurturing” function is especially important when the forest experiences a major wildfire, which significantly reduces the amount of nitrogen in the ecosystem. Then, the alder can grow on the site, contribute nitrogen to the soil and essentially prepare it for the other species of plants.


Next time you’re walking by an alder at Tryon Creek, remember to thank it for doing such a good job of nurturing the whole forest.


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