Side-Benefits of Birches’ and Beans’ Friendships

Frankia bacteria with vesicles

Although it’s common knowledge that there is a beneficial relationship between members of the legume family and Rhizobium bacteria, less people are aware that there is a similar relationship between the members of the birch and alder family and a bacteria called Frankia. What’s noteworthy is that in both mutualisms, legumes and birches gain more than just ammonium, which the bacteria form by converting nitrogen from the air with their special enzyme nitrogenase. (For those of you who are also salivating to find out what plants do with ammonium, they use it to give glutamate an extra amino group as it becomes glutamine. The latter yields an amino to α-ketoglutarate which regenerates glutamate as it transfers it to variable intermediates. Those finally become a variety of essential amino acids. )

Before revealing the other benefits, let’s look at the Frankia nodules that form around the roots of Alnus glutinosa, the common alder tree. They are the orange clumps you see throughout the picture below.

A mutualism between alder roots and Frankia nodules

When alder or birch trees are still young, it’s been shown that the Frankia infection lowers their ability to produce tannins and other compounds, making them more appetizing to herbivores. But the drawback is short-lived. The synthesis of deterrents accelerates as the treelings quickly mature, thanks to the extra nitrogen from Frankia, and this allows the trees to survive.

a Mexican Bean beetle

Lima beans, which form nodules with Rhizobium, use the nitrogen-bonanza to make their leaves richer in cyanogenic glycosides that are poisonous to Mexican Bean beetles. The more nodules they have, the more protective compounds they produce. Researchers confirmed that hypothesis by chemically degrading the poisons with enzymes in closed Thunberg vessels and then using spectrophotometry to measure the amount of hydrogen cyanide  HCN released.

Having more nodules also improves bean plants’ ability to make volatile organic compounds when they first get attacked by the beetles, which drives them away.  Meanwhile, the more “infected” lima beans become with Rhizobium, the less extrafloral nectar they produce.  This makes them less attractive to ants, who otherwise farm aphids at the expense of bean plants.


  • Ballhorn, James and al. Colonization by nitrogen‑fxing Frankia bacteria causes
    short‑term increases in herbivore susceptibility in red alder
    (Alnus rubra) seedlings. Oecologia. 2017
  • Thamer, Sylvia and al. Dual benefit from a below-ground symbiosis: nitrogen fixing rhizobia promote growth and defense against a specialist herbivore in a cyanogenic plant. Plant and Soil.  April 2011.
  • James Mauseth. Botany, An Introduction to Plant Biology. Jones and Bartlett. 2008

Why I Keep Trying to Grow Poplars

P1160987Pictured is my fourth attempt at growing a poplar from seed. The seed, which was within a capsule-like fruit, landed in my garden after it drifted in the wind, courtesy of an attachment to a cottony puff. Most people, if they notice them at all, think they are an annoyance that have to be skimmed off  swimming pools. They are in instead another species’ way of perpetuating itself. And they are more ephemeral than most flowers. In fact, prior to germination, the soil must have been moist because poplar seeds are only viable for a few days.

The third attempt failed. After germinating in the same manner and location, I transplanted a one-foot poplar to the green strip between the sidewalk and the street, in the spot where a city tree had died. But after 5 years of neglect, officials decided to finally plant a new one where my poplar stood. I moved it to my yard, but my neighbour’s dog chewed a ring around its  bark, removing the phloem, which led to the starvation of its roots.

The second attempt was a prequel of the first third one’s history, with the same cause of death—girdling, it’s called—but with a different perpetrator. A city worker with a noisy, fossil fuel-powered weed wacker inadvertently killed it. It’s why I surrounded the vulnerable trunk of Poplar Number 4 with a mesh, in case the neighbour’s dog returns with the same intention, which it already has, or in case the cord of the weed wacker somehow slips and covers three times its intended radius.

Attempt number one happened decades ago when I was still a teenager living at my parents. I had been watching its quick growth when one day I found it in the garden with its roots pointing to the sky. I revived it temporarily, only to be told by my father that it was  a useless tree. It’s not, actually. Aside from its beauty and ability to remove carbon dioxide from the air and convert water to oxygen, the wood can be used for plywood and matches. But that was never my reason for caring about poplars.

Between the ages of 5 and 10, there were hundreds of poplars in what we considered to be our backyards, given that there was no fence separating our properties from the woods. In those woods we built cabins and fires, ate berries, climbed trees. My grandmother even taught us how to make bows and arrows from the soft, easily peeled wood of poplars. Then one spring, a bulldozer, in a matter of hours, wiped out the natural playground of our childhood. Three of us screamed at the operator. When he told us to get lost, I picked up a small rock and threw it at him with Rusty Staub-like accuracy. Luckily, it did not hit him in the head but in the back. He tried to chase us, but we ran away like rabbits. The exhilaration from the escape was short-lived, but the urge to spread poplars all over the city has never gone away.


Life’s Essential Elements

Life’s Essential Elements is available at Amazon at

Just prior to publishing the book, I created a new cover to please my daughter. Authors are told not to expect their family members to read their book. My parents don’t read English, so that’s a given, but I do hope that my daughter will have a go at it. In January, she will start her PhD in chemistry, and while she will have no choice but to get bogged down in detail, it might be a refreshing break for her to get a bird’s overview of the elements’ role in life.

Remember all of my profits (not Amazon’s unfortunately) will go to a Montreal charity (CDH) for autistic children.

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