A Weed No More: The Red Alder Has Nitrogen-Fixing Superpowers


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Alder leaf. Image courtesy of mzacha at Stock Exchange.

It used to be called a “weed tree” by foresters. Now, the superpowers of the Red Alder (Alnus rubra) are coming to light. Like a pushy neighbor, it moves into areas where it isn’t wanted. Some might call it the forests of straggly red alders pushing up to seek the sun weedy, however, the alder brings gifts, and plenty of them, in the form of nitrogen-fixing bacteria.

Why is Nitrogen Fixation Important?

The majority of the atmosphere is composed of nitrogen. Yet in the soil, plant growth is restricted by the availability of nitrogen. What if there were something to move nitrogen from the air into the soil? There is, in the form of nitrogen-fixing filamentous bacteria (actinomycetes) like Frankia, the bacteria that allow alders to become a key primary forest species.

The Red Alder Can Grow in Harsh Environments

Visit any clearcut, landslide, or recently flooded area in the Pacific Northwest, and you’ll find alder. It even grows in the most challenging of circumstances. Go to the middle of a city and look at a vacant lot. More than likely, there are alder trees growing in that lot. They might look like weeds, with the way that they spread across the landscape. However, these little alders are preparing the soil for new and less-hardy plants.

Gardeners know that growing legumes is a great way to move nitrogen into the soil. Think of the Red Alder as the legume of the forest. It’s not really a legume, but it does the same job, taking nitrogen from the air and moving it into the soil; getting the land ready for other trees. In the Pacific Northwest, this means that big, beautiful trees like the Western Redcedar, Western Hemlock, and Douglas-fir can move in.

Nitrogen Fixation is Key to the Alder’s Success

How does the red alder transfer nitrogen from the air to the soil? If you were to dig up a red alder tree, you would find small bumps, called nodules, on its roots. On a seedling, these nodules can be up to an inch wide, while on larger trees, they can be over 3 inches wide. Even when the seedlings are only a few months old, they already have dozens of nodules. As the alder tree grows, tiny bacteria enter the root hairs and stimulate the growth of the nodules.

Having made a home for themselves, the symbiotic Frankia bacteria get to work. The alder tree provides the bacteria with sugars and minerals, and the bacteria use these substances to create enzymes called nitrogenases. Nitrogenases can convert airborne nitrogen into ammonium, which then converts into amino acids.

Think that legumes are amazing soil-enrichers? A stand of red alder that covers a hectare of land can place from 200 to nearly 500 lbs of nitrogen into the soil. When the leaves fall, they also add mulch and more nitrogen as well as providing food for worms and other decomposition agents.

Red Alders are a Pacific Northwest Pioneer Species

How long do the red alder’s soil-enriching powers last? Compared to the thousand-year-old trees that commonly grow in old growth Pacific Northwest forests, the red alder is a short-lived plant. The tree lives between 60 to 100 years, long enough to create excellent soil for generations of shrubs, and long enough to create shade to shelter the conifers that require it for their early growth.

Where can you find this fascinating tree? Visit a site with disturbed soil anywhere from southern California to southern Alaska, and you’re likely to meet the champion of the nitrogen-fixers, Alnus rubra.

Del Tredici, Peter. A Nitrogen Fixation: The Story of the Frankia Symbiosis. Accessed May 16, 2011.

Harrington, Constance A. Alnus Rubra USDA Forest Service. Accessed May 16, 2011.

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