Republishing an old blog post of mine from 2021 that’s very relevant in late May and early June.
Those Fluffy White Seeds that cover Vancouver every late spring? Unlocking the secrets behind them could be a very powerful tool in fighting climate change.
Every late May the sky of Vancouver is full of swirling and dancing cotton seeds. They fly in through open windows, up noses, and cover the ground like snow. The source of these mischievous, poofy snowflakes is the towering Black Cottonwood (Populus balsamifera ssp. trichocarpa). It’s the tallest deciduous tree species in BC and maybe in Canada. Its long, thick trunk can hold its own against the mighty Douglas-Fir. However, unlike our giant conifers, at the top the canopy opens to a lush green and silver crown that seems to flash like a starry night sky in the wind.
And it doesn’t make any sense how it can get this big.
Or at least it didn’t. Until recently Science could only speculate how the Black Cottonwood could grow to such a massive size. Broadleaf trees, that lose their leaves every fall, require a lot of nutrients to grow and produce these leaves every summer. This is unlike evergreen conifers that use nutrients more efficiently by creating longer lasting leaves/needles. Typically, the tallest broadleafs are found in nutrient rich grassland or forest soils, but the Black Cottonwood somehow is able to grow to its huge size on disturbed or flooded soil where nutrients are lacking.
cottonwood_treeAs you pass by an empty city lot or dirt-covered back alley, take a closer look and you are sure to see some cottonwoods springing up. Their windblown seeds allow them to travel and invade soils from long distances away. How do they do it? How can they grow so big in such empty places?
The main nutrient trees need that is often lacking is Nitrogen, the key element in protein. It’s needed by all life for growth, movement, and DNA. In trees it is especially important because it is used for chlorophyll, the site of photosynthesis. No nitrogen, no leaves, no growth.
If you can’t remember your high school biology (or if you failed your UBC Bio Exam…), nitrogen is very abundant in the atmosphere, but it is in a form that can’t be used by plants or most life on earth. In fact, the agricultural revolution, and the reason you can find such a diversity of food at your local grocery store, is all thanks to the discovery of how to fix nitrogen in a usable form via a mechanical process. This “mechanically-fixed” nitrogen is how farmers manage to grow a new crop every year without exhausting the nitrogen in the soil. It’s also now where most of the nitrogen in your body ultimately comes from. It can be traced back to a nitrogen fixing plant somewhere.
There’s a big “but” here though. A lot of nitrogen gets fixed by this mechanic process but some brilliant plants have figured out how to acquire it on their own. Well. Not on their own. They’ve made a deal, a symbiosis, with unique nitrogen-fixing bacteria. They provide a home (nodules on their roots) and food (sugars from photosynthesis) for the bacteria and the bacteria will supply them with usable nitrogen. Only a relatively small number of plant species know how to do this though: The legumes (bean plants like soybeans) and another very recognizable BC tree: the Red Alder.
Red Alder – easily recognized by its smooth bark and being the only broadleaf tree with woody cones – is the Black Cottonwoods main competitor when it comes to invading barren land. They are both BC’s main “pioneer” tree species. Alder doesn’t grow near as big or live as long as Cottonwood, but it is a feisty individual nonetheless and will give Cottonwood a run for its money. In the long run though, Cottonwood will outgrow and outlive it.
This leads us back to our problem. If Alder has a great survival trick by making this deal with nitrogen-fixing bacteria on their roots. How the heck can Cottonwood out-compete it?
The answer is a recent scientific finding that just adds to an ongoing scientific revolution in rethinking what we know about plants. Bio textbooks become ancient so quick these days! Doctor Sharon Doty and her team have recently discovered that Black Cottonwood (and likely other plants!) also engage in microorganism relationships to fix nitrogen. Unlike alders and legumes though, they do it on their stem, near buds and nodes where branches attach, and without clear “swellings” for “homes” like root nodules.
So why is this revolutionary? How does this address my clickbaity, over-used title of fighting climate change? Well Doty’s research leads to the possibility that we don’t need to machine-fix nitrogen in order to make it usable to plants. That we could instead, by learning more about how this cottonwood-bacteria relationship works, provide plants with stem-nitrogen-fixing microorganisms and allow them to fix their own nitrogen. This would be a huge deal, because the mechanical-nitrogen fixing process is a major source of fossil fuel usage (2% of worlds total). Also, a large amount of machine-fixed nitrogen that gets added to soils is leached into our water system and is a huge source of water pollution. Cottonwoods might be the key to allowing plants to find their own sources of nitrogen and use it more efficiently.
This isn’t the only Cottonwood survival strategy we could make use of to fight climate change though. Another common thing you will notice when you observe a Cottonwood tree is – especially at this time of the year – how many tiny branches full of leaves it drops. If you look up at the tree, you’ll often notice a number of dead branches without leaves still on the tree. Why doesn’t the tree drop those dead branches? Why drop the ones with leaves that could still photosynthesize?
Here we get at the heart of the defining feature of the Cottonwood. Those tiny branches it drops will grow into trees!
Cottonwoods on river banks will drop branches that will flow down the river, wash up on some empty soil, and put out a long thin root. If it can find a water source deep in the soil, it’ll start growing and become a tree. An exact clone of the parent tree. Straight out of a SciFi story, Cottonwoods drop tiny clones of themselves that can soon take over an empty area of land.
I’ve seen first-hand! Took a tiny cottonwood branch and put it in some soil in a tall pot on my window. Dug it up a couple weeks later to find a long, thin tubed root that had grown down the bottom of the pot (twice the length of the branch) in the search for a water source.
cottonwood_leafThis makes you ask some philosophical questions about how we count age. The oldest cottonwoods in the world have been found in Alberta, aged via tree-ring dating to 400 years. However, some of these likely came from fallen branch clones from other trees, which may have been branch clones from other trees, and so on. So how old are these cottonwoods really? This is an ongoing debate in science. Is the oldest tree in the world the 5,000-year-old Bristlecone Pines in the southwestern U.S. that have been dated from their stems? Or is it the 9,000-year-old Norway Spruce in Norway that has had multiple stems grow and break over its lifetime, whose age has been dated from its roots? Or is the 14,000-year-old clonal forest of a single Aspen, with multiple stems and roots in the Midwest US, whose oldest parts have long since faded away to make room for new stems and new roots?
Regardless! Cottonwood’s ability to reproduce like this has made it a favorite of plant biology labs everywhere. It’s the perfect tree for experimenting on because it by far is the easiest and quickest to grow. A “model organism”. In fact, Black Cottonwood has the distinguished honor of being the first tree to have its genome fully mapped!
How will this help fight climate change? Well, already experiments with Black Cottonwood have shown that we can crossbreed it with other species of cottonwood to produce hybrids that grow quicker than their parents. This allows us a much faster, more efficient way to “farm” trees by growing them in fields instead of needing to go out and cut down trees in undisturbed forests. Hopefully more research with Cottonwoods can help us make a stronger case for leaving BC’s old growth forests – and the huge amount of carbon and diversity they retain- alone.
Another fascinating but counterintuitive use that Cottonwood has been modified for in labs is growing it to have less wood. Wait. They are modifying trees to grow with less wood? Oh yeah! The main use for Cottonwood wood right now is pulp and paper, and to make pulp and paper you have to remove lignin from the wood. Lignin is the strong, resistant molecule that gives wood its hardness. What we want from wood for paper creation is the softer molecule in wood, the cellulose. Removing Lignin from wood is, like machine-fixed Nitrogen, an intensive process that produces a lot of greenhouse gases. Growing Cottonwoods with less lignin means we have more efficient way to produce paper that causes a lot less pollution.
The exploration and discoveries that we will get from Black Cottonwood are just getting started, now that we have an incredibly efficient gene-editing tool of CRISPR. All sorts of genetic experiments can now be done to better understand how trees grow and how we can use/protect them better. So, the next time you see some white fluff float by, look up to the sky for the tallest broadleaf tree you can find, that’s your Black Cottonwood. Go grab some of the tiny branches below it and plant them somewhere. Let that tree live forever.