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There’s nothing quite like the smell of the trees during a hike through the forest.
But the molecules responsible for that lovely scent do more than just smell good.
They’re used by trees for protection against pests and pathogens.
Researchers have found that they can even help make clouds.
Which means that trees might not be complete bystanders
when it comes to the weather around them.
Their natural emission of cloud-forming molecules
may be influencing the climate around them –
way more than we used to think possible.
And since we need to understand exactly how our climate is changing,
we’re going to need to account for weather-controlling trees.
Let’s quickly go over some organic chemistry to understand how this works.
Trees produce monoterpenes, using carbon dioxide
that they collect from the atmosphere during photosynthesis.
Monoterpenes are made up of
ten carbon atoms and sixteen hydrogen atoms,
and the exact arrangement produces slightly different compounds.
The most abundant of these is alpha-pinene.
As the name suggests,
it’s responsible for the unique smell of pine trees.
Once they’re made, monoterpenes can be stored in the leaf,
locking them up until the tree needs to use them.
Otherwise, they’re released by the tree immediately after they’re produced..
And trees emit a lot of monoterpenes —
well over 100 teragrams of carbon per year.
That’s a one followed by fourteen zeros grams.
Up in the atmosphere, monoterpenes are quick to combine
with ozone and hydroxyl molecules that are floating around up there.
They form new particles that are the beginning stages of clouds,
known as cloud condensation nuclei..
There are a few ways clouds can get started.
And research has shown that monoterpenes can be one.
And while there’s obviously a lot of variables,
the thinking goes that these clouds lead to
increased cooling as well as the potential for rain!
[ehn-aan-tee-oh-murhs] Now, a monoterpene such as alpha-pinene
can actually exist in two forms: plus and minus alpha-pinene, referred to as enantiomers.
These molecules have the same atoms in the same arrangement,
but they’re mirror images of one another.
It’s often compared to left and right hands. Same basic arrangement,
but you can’t superimpose them on each other no matter how hard you try.
And there can be differences between the way enantiomers behave chemically.
However, in this case, enantiomers of alpha-pinene
have the same reactivity with ozone and hydroxyl ions.
And because these two forms react the same in the atmosphere,
existing climate models don’t differentiate between the two.
But maybe they should.
The authors of a 2022 paper in the journal Nature
have found that these different forms of alpha-pinene are released
into the atmosphere at very different times,
especially during periods of severe drought.
These researchers looked closely at the two forms in a closed-system,
controlled environment rainforest designed to simulate the Amazon Basin.
This is literally a rainforest in a really big box,
where the chemical and climate conditions within
can be controlled and measured by researchers.
This means the researchers were able to do a few things
they wouldn’t be able to do in the natural environment.
First, they could simulate the climate conditions they were interested in studying,
which ranged from normal rainforest conditions to severe drought.
And they could also add carbon-13,
a form of carbon with an extra neutron
that is used by researchers as a way to track environmental processes,
because it doesn’t naturally occur in large quantities.
Carbon-13 can be injected into the atmosphere within the box,
where the trees will pick it up and use it
just like the more common carbon-12.
That helps researchers determine which form of alpha-pinene was
being released throughout the experiment.
If the alpha-pinene contained the carbon-13
that was added during the experiment,
that would mean it was created after they
dosed the rainforest-in-a-box with the stuff.
And what the team found during normal conditions was t
hat the trees released minus alpha-pinene in the morning.
And it was freshly made,
released almost as soon as it was created
by the trees during photosynthesis.
In contrast, plus alpha-pinene wasn’t released until later in the afternoon.
And it didn’t contain any carbon-13,
which told the researchers that this form of alpha-pinene
was being released from some kind of tree internal storage system.
So even under normal rainforest conditions,
the different forms of alpha-pinene were being emitted at different times of the day.
And as drought conditions progressed from mild to severe,
the trees switched how they released the different forms of alpha-pinene,
and also started releasing a lot more monoterpenes in general,
including other compounds like beta-pinene.
The emission of minus alpha-pinene
was delayed from morning until later in the day,
which the researchers believe was in response
to the stressful conditions created by the drought.
Instead of releasing freshly-made minus alpha-pinene,
the trees conserved energy by releasing more of it from storage,
along with the plus alpha-pinene.
And by releasing a lot more monoterpenes into the atmosphere,
it was kind of like an S.O.S. signal to the atmosphere,
signaling for clouds and rain in order to lessen the drought stress.
Not that the trees know what they’re doing, but it seems like a strong
clue that trees have evolved ways to help ease drought conditions on a local scale.
They’re not totally at the mercy of the weather –
it’s a two-way conversation,
and the trees talk with monoterpenes.
All that suggests monoterpenes are more than just a nice smell to enjoy on a hike.
Drought conditions are predicted to increase
as the climate crisis continues to rage,
so these molecules will become even more important in the future.
This new research shows
that even the different forms of monoterpenes
are an important piece of the climate jigsaw puzzle,
and it’s important to understand how their emissions will change over time,
and accurately account for them in future climate models.
In the end, those trees might be onto something.
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