This is not a tree farm.


I mean, it is, in the sense that these trees were planted by human hands with the eventual intention to put them to economic use, but that’s not why they’re actually /here/.


These 36,000 poplar trees are located just up the road from SciShow’s studio in Missoula, Montana, and their job is eating poop.

I see them when I'm flying home from anywhere, I'm like, "there's the big weird poplar plantation." People ask about them, and I get to tell them about it. 

More specifically, these trees are preventing excess nitrogen and phosphorus from the local wastewater treatment plant from going into the Clark Fork River. 


Poplar trees are one of a few types of plants that have absolute superpowers when it comes to cleaning up human messes. Regular ol’ poo is just the beginning – there’s almost nothing these trees can’t do when it comes to pulling stuff out of the soil.


But as great as this sounds-- using trees to solve problems-- nothing’s perfect, and these poplars might turn out to be a /crappy/ solution.


[intro]


The City of Missoula planted 90,000 hybrid poplar trees in 2014 in an effort to reduce wastewater pollution being dumped into our river, although that number is down to about 36,000 today. 


In town, we call this the Pooplar Grove, which is very funny, but might be a misnomer. See, they’re not there to remove human waste, at least not directly. The problem these pooplars are solving is an overdose of nitrogen and phosphorous. 


Although of course these nutrients are essential for life, it’s possible to have too much of a good thing. Too much can lead to algae blooms and other imbalances in the river’s health. 


And the pooplar grove actually /is/ a pretty great way to help out the river. According to the city, the grove has been able to keep almost 5000 kilograms of nitrogen and almost 300 kilograms of phosphorus out of our river every year.


Plus, there was another benefit to growing these particular trees.


The city’s plan was to grow a grove of poplar trees to help treat the wastewater, and then find a way to sell the lumber when they were mature. At the time, they were anticipating a pretty big market for poplar wood, and even had a mill in Oregon lined up to process it. 


But the trees would need 12 to 15 years to grow. So let’s leave them to it and see what else poplars can do.

The technical term for these trees’ superpowers is phytoremediation. It can work in a bunch of different ways, and poplars are good at nearly every single one of ‘em. 

One is called rhizofiltration, and it involves the plant slurping up contaminants directly from water into its roots. 


This is how poplars soak up nitrogen and phosphorus, and they don’t just do it in Missoula.


For example, a 2018 study looked at the potential of planting poplar and willow trees along riverbanks in Canada to keep phosphorus concentrations down. 


And they identified a particular hybrid variety of poplar was best at that task overall, due to its big root system and the overall amount of phosphorus it was able to store. 


Incidentally, you’ll find nearly all of this research refers to /hybrid/ poplar trees.


This is because of a really weird phenomenon called heterosis, or hybrid vigor. Basically, when you cross a plant with another compatible plant, it gets… better. Bigger, more productive, better at cleaning up pollution. You name it. 


It’s really common in plants, but the thing is…  like, nobody knows /why/. Like, it has to have something to do with how their genes are regulated when you mix ‘em up, but beyond that scientists are completely lost. 


What they do know is that when you cross one poplar species with another, you often get a hybrid whose superpowers are even… super-er. 


Which is good, because we’re just getting started in terms of what poplars can do. I mean, nitrogen and phosphorus, that’s just fertilizer. Show me a plant that /doesn’t/ love those things. But what about cleaning up heavy metals?


Poplars are some of the best plants out there for removing toxic heavy metals from the soil. 


This particular flavor of remediation is called phytoextraction, and it’s pretty simple. The plant just pulls contaminants out of the soil and tucks them away. 

It can do that because plant cells usually have a large storage space inside them called a vacuole.


Like, nice cells. Ooh, they have pockets!


If the plant encounters something that’s stressing it out, like, say, toxic heavy metals, it can throw them in those vacuoles where they’ll do less damage. 


A 2024 study at a mining site in southern China looked at the ability of different hybrid poplar varieties to clean up cadmium, mercury, arsenic, and lead. 


They found that the poplars were especially hungry for cadmium, which, lovely for them. They also found that hybrid vigor had a huge impact, especially when the cross was between different species of poplar, as opposed to within the same species. 


The trees weren’t as great at picking up the other metals, but the researchers found that could be improved by breeding and choosing the right hybrid. Which could be useful if you’re trying to clean up multiple heavy metals at the same time. 


But it doesn’t end with metals. These trees can also solve our toxic chemical woes.

Trichloroethylene, or TCE, is a solvent that was used as a cleaning agent before we figured out it was bad news bears.

It’s not soluble in water, and it tends to seep into the soil and just… stay there, slowly releasing contamination. 

Enter poplar trees once again, this time with a combination of phytodegradation and phytovolatilization.

This script's so fun.. there's so much words to say!


Degradation refers to a contaminant being broken down, while volatilization means the plant is taking up the contaminant with its roots and releasing it into the atmosphere through its leaves. Which is only better if you’re fine with the thing being in the atmosphere, and at least with TCE, it seems to break down pretty fast once it’s in the air. 


Poplars have been shown to carry out a combination of both when it comes to TCE, although degradation seems to predominate. They appear to break it all the way down into CO2 and chloride ions, which is way better than what it started as. 

But before we get into the future of pooplar trees, we're going to pay the bills with a quick ad break.
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So I’m not seeing any reason why we shouldn’t slap these bad boys down all over. At least in their native range. Poop trees for everyone.


Well… we chatted with an official from the city, and unfortunately, there’s a catch. Or rather – about 36,000 of them. 


See, the trees are all fully mature, and the city’s previous plan to harvest the trees and have them milled in Oregon isn’t working out, on account of the fact that the mill has closed. 


And there don’t seem to be any other takers. Poplar wood has big growth rings and is tricky to mill, so apparently not many people want it.


If you’re wondering why they don’t just leave the trees there and let them keep doing their thing, that’s not really an option either – poplars have short lifespans and some of our Missoula pooplars are already starting to die off. 


Basically, the problem with growing a bunch of trees is that when you’re done, now you have a bunch of trees. The city kind of needs to figure out what to do pretty quick.


So they’re thinking about whether to use a different plant next time that will be a little easier to get rid of – like alfalfa, which could be sold to use as animal feed.


Whatever they decide to do, the trees did great! They are superstars and I am proud of them! I’m not mad at the city for trying to keep the river clean, because I love a clean river.

You cannot get into the Clark Fork without getting a little bit of river water in your beer. That's just a thing that happens. And I want it to be ... not hurting me. 


The reality is, cleaning up human messes is hard and no plan is ever perfect. But I’m still on team trees.