The Tree of Heaven

The ghetto palm in its natural habitat

The Tree of Heaven* is also known by its international colloquial name, the Ghetto Palm,  because it thrives in the worst conditions. It's been intentionally brought to the New World from Asia for centuries, both for its exotic ornamental qualities, and its basic use as a fast-growing shade tree that reaches up to the vast ecosystem in the sky at an exceptional rate.
*Also called ailanthus, varnish tree, and chouchun (Chinese, foul-smelling tree).

Now it's considered an invasive species. Its unstoppable juggernaut root system crumbles anything made of concrete, from sewers to foundations to highways and bridges. It also crowds out indigenous species by releasing a toxin (ailanthone) into the soil via its roots and fallen leaves. It isn't affected by herbicide; it is the herbicide.

At this time of year, in the heat of summer, it's spreading like wildfire. Not only does it produce an extraordinary amount of seeds, but it sprouts from its indomitable lateral roots, and almost 100 feet away from the source tree. You don't even have to look for it; you will know it's there by its smell. If you don't know, you will in a minute.

The Tree of Heaven smells bad. Like what? "Bad," that's what.

Can you be a little more descriptive?

Not really. We are so bad at describing bad smells. In fact, the corpus of words used in olfactory science is skewed way to the good. We have many more words for good smells than we do for bad. There's a few reasons for this, one being that we don't like to think about bad smells long enough to generate descriptors. When it comes to bad smells, "objectionable" "disagreeable" "noxious" "offensive" and "smells like shit" will suffice.

We already don't talk about smells much as it is; why waste that precious sensory indulgence on things that smell bad? We also don't like to talk about bad smells, because it can get socially complicated. You're not going to mention a bad smell while with your boss, and that's in case they are the source of it! We don't want to make them look bad. It's also too personal. It's just good social etiquette in general to not talk about bad smells, at all.

We also don't stop and take a second whiff when something smells bad. Flowers yes, the Tree of Heaven no. It will activate your disgust response, you'll reflexively twist your head back, flare your nostrils, and curl your upper lip. And you will not be going back for seconds.

Unless you're me. I've been trying to smell the Tree of Heaven for years now. It was introduced to me circa 2008 by a friend who lived on a farm; we were touring his property. "We call it the jizz tree," he said with a nervous giggle, him and his girlfriend. I don't remember smelling it, maybe it wasn't in season, who knows. I don't remember seeing it either. I don't remember anything except that there's a tree that smells like semen.

Later on I wrote a book about the language of smell, and shortly after that I came to the realization that I'm anosmic to putrescene, which means I cannot smell semen. Smellblind. People are smellblind to all kinds of things, bad things more often it seems, and roughly half of us are anosmic to something (natural gas and rotten fish will come up a lot, because they're talked about a lot, because it's a safety issue, right?).

I'll bet there's less people that know they're anosmic to putrescene. There are some people who are completely anosmic to everything, from birth, and don't realize it until they're ten years old. Kids don't even know that they have a sense of smell, and they certainly don't notice if it goes away temporarily, as in the case of those infected with the novel coronavirus of 2019 (the condition will present as them skipping meals and not being hungry, but it's likely because they can't smell).

Back to the tree. I started to notice the Tree of Heaven invading my extended neighborhood once I began taking the train for school. They grow along the train tracks really well, and they grow fast. After an extreme weather event, like drought, flood or fire, they are the first to pop back up, because they go dormant into their roots, conserve resources, and wait. They looked strange to me only because they were growing so fast. Within the first year of my taking the train, they had already grown from 5 to 20 feet. I started to notice them elsewhere, and once I started to look, there they came; these things are everywhere.

I looked them up, and found this notable characteristic -- this is the famed semen tree I've been hearing about, but never smelled. But it could be sumac, they look pretty darn close. In fact, the easiest way to identify it is to break off a leaf at the stem and smell it. If your head snaps back, it's the Tree of Semen, I mean Heaven. I went outside and grabbed a leaf, and as expected, couldn't smell anything. It could also be sumac; although it turns out I was wrong. 

Months later, I see one on the street and decide to try again, and rip off a leaf. It smells, kind of bad, maybe not, definitely not noxious, is it chocolate? Bad chocolate? What does that even mean? I must investigate further, because there's something there, although it's faint. Maybe it's just the beginning of the right season, who knows. Maybe it's still sumac.

A couple weeks later, it's getting hotter, a 12-day heat wave which is rare where I live, and I'm waiting in a parking lot for my laundry to finish (you can't wait inside, because of the virus). I'm in the back of the parking lot, against the train tracks, where there's shade, from the shade trees... There, I see it again... and now it's everywhere, I take a piece, break it off, and my goodness. My head snaps back. Terribly offensive.

I look it up again -- sumac makes red bunches of fruit, tree of heaven makes neutral colored and later in season pink to reddish-colored seed pods that kind of like maple tree "helicopter" seeds. I now confirm it is in fact the Tree of Heaven. And so is that what semen smells like? I call it pungent burnt rancid oily and dare I say, nutty, like peanuts?

Ailanthus samara seed pod, The Pennsylvania Flora Project of Morris Arboretum, 2011

How come when I look this up, in the more reputable sources such as the New Jersey Audubon Society or the Ecological Landscape Alliance, they use words like rancid peanut butter, burnt rubber, pungent, foul, and the the indispensable "smelly."

Is it because they're so "reputable" that they can't say "semen?" I think this may be half of it.

But as I go further into the informal investigation of the internet rabbit hole (this is called gray literature research, although you should probably call it reading urban dictionary entries, VICE articles and horticulturist blogs), and it starts to dawn on me. These descriptors are a mess; they're all over the place:

Rancid peanut butter, rancid peanuts, rancid cashews, cross between peanut butter and cat urine, well-used gym socks, yucky cooked meat, objectionable, disagreeable to humans, fetid-smelling, bitter, acrid, pungent, strong, and any word that refers to semen, and which can be described as a "chlorine musk," but is more directly associated with the molecule putrescene ... there's something in here about amines and ammonia also. You could also refer to it in your most prudent Victorian manner and call it simply "a man smell."
*Only the male flower smells, but both smell when their branches or stems are broken.

When I look at that list I am reminded of two things -- we are really bad at describing bad smells, and half of us are anosmic to something, and usually to bad smells.

So not only do we NOT talk about or think about the names to call bad smells, but for some of them, we can't even smell them in the first place.

When you don't talk about something, and you don't generate either a personal or a shared vocabulary for something, you will be really bad at identifying it, at discriminating it from similar things, or at categorizing it in your autobiographical database. This means you're more likely to mis-assign a name to the smell, calling plant-semen "dirty gym socks" instead. I may need some help here, because again, I'm smellblind to it, but does semen smell like dirty gym socks?

Furthermore, when you're presented with a cocktail of bad smells, as would be expected emanating from a living biochemical reactor*, you may be missing a major component of the mixture due to smellblindness to one of the molecules, and that could change your impression dramatically.
*A plant's essential oil is not the same as an isolated synthetic compound, because olfaction is a combinatorial affair, shown from about 2015 research and on.

If you combine all these factors together, you get one dirty mess of a database. There is no absolute, no discrete points. If you could manage to ask 10,000 people around the world  (or 1.5 million in the National Geographic Smell Survey) to describe 10 different bad smells, each a natural biologically-generated smell cocktail, what would that list look like?

That list would tell you something about the overall distribution of genetic diversity in the study population based on olfactory receptor genes, or it could tell you something about the cultural milieu of a sub-sample (like the Victorians!), but it won't tell you any better what the Tree of Heaven "really" smells like, or stink bugs for that matter.

When it comes to making sense of the world as an olfactory phenomenon, you're on your own. Olfactory reality is not a consensual reality. And that's unsettling, because in the Information Age, approximation seems like failure, no?

Post Script: 
Within days of my most recent experience with the potent odor of this tree, I can now smell it as a drive down the highway, from the trees on the side of the road. In my typical self-induced pseudo-hyperosmic fashion, I have become very sensitive to it.

This reminds me of an idea about regeneration of olfactory neurons and combinatorial perception, and as it relates to people recovering from the novel coronavirus of 2019. After some traumatic disturbance to your olfactory neurons, like from being attacked by a virus, you may experience changes in smell or taste. This is also called anosmia, partial anosmia, or phantosmia, the last referring to not a loss of smell but a change in the way things smell.

Phantosmia, like all phenomena in olfactory science, is not understood enough to say much from an evidenced-based point of view. I'm making a broad speculation here, not to explain, but to make someone interested enough that they will investigate further for themselves, and maybe even initiate more research into the topic.

The change in smell that comes from phantosmia is common, but its origins are often overlooked. It likely signals a change in the structure of neurons used to smell. These are the only part of your brain that pass the blood-brain barrier and rest outside your skull, in the mucous atop the epithelium skin way up in the top of your nostril canal (right where they swab that sample for your PCR test by the way, and not a coincidence). These neurons are thus both very vulnerable to damage, and able to regenerate indefinitely.

Combine this with another fact about olfaction -- it is combinatorial. That means when you smell "apple," there are a bunch of different receptors all lighting up in a pattern that means "apple." There is no Apple gene. For some there are, but for the most part, no. No single gene codes for any single cell. Olfaction is all gestalt. Take one piece out, and the entire picture gets weird as hell. Something's wrong but you can't tell what. So your brain misfires, it says "cigarette smoke" when it's really something else entirely. But after damage to your system, it is re-learning how to smell. Your nose-brain is a deep learning neural network that requires countless iterations to "learn" what a smell is. And while it's relearning after an infection, it gets confused.

In a very mild manner, and for reasons I will attribute to having been infected myself, my Tom Ford Italian Cypress lost its depth and presented as cinnamon and bubblegum, for about three days. If you've ever smelled Italian Cypress (and if not good luck it's discontinued since 2014), you would know that it does NOT smell like cinnamon and bubble gum. That's happened to me once before, and I will now assume it was because I was then also infected with a virus. But I got lucky, it wasn't bad, just weird.

And another thing -- when faced with new smells, we tend to call them bad. After repeated exposure, we can start to see them as good, but it's more likely we call them bad at first. So if your system is relearning how to smell, then lots of typical odor exposures will present as "bad" to you, simply by their being new, that is, new to your newly developing system. And all of the sudden, anything that doesn't compute properly on your new system will become "cigarette smoke," for example. Rotten meat is another good one for this, but it could be anything really (and it could also be really debilitating, just imagine.)

Eventually, the system will recalibrate and relearn how to smell, and you'll be back to normal. But that doesn't always happen. Blunt trauma can kill those neurons forever. Really bad infections too. Sometimes it doesn't come back because you're not using it, like therapy after a stroke, it only comes back if you try really hard to use it.

And some of us, well, we're just getting older. Things don't work like they used to. And not only that, changes in smell can predict all kinds of neurological diseases decades in advance (Parkinson's, Alzheimer's, etc.).

All this being said, my system has apparently, and finally, learned how to smell the Tree of Heaven.



Some good recent research on genetic variation in olfactory receptors:
Did You Smell That No I Didn't
Jan 2020, limbicsignal.com

And the most relevant among them for today:
Any two individuals differ by ∼30% of their olfactory receptor subtype genome.
Mainland JD, et al. (2014) The missense of smell: Functional variability in the human odorant receptor repertoire. Nat Neurosci 17(1):114–120.
https://www.ncbi.nlm.nih.gov/pubmed/24316890
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3990440/

The human olfactory genome contains 418 intact odorant receptor genes and their 912,912 intact odorant receptor alleles.
The 1000 Genomes Project (2008-2015), the largest public catalogue of human variation and genotype data.
https://www.internationalgenome.org

Pop Search Trivia:
"Tree of heaven smells" like peanut butter, #1 search result. (July 2020)

What is the tree that smells like dead fish?
(Callery pear trees)

What is the tree that smells like peanut butter?
(Butterfly tree or peanut butter shrub)

What are other trees that smell?

• Callery pear, Bradford pear tree - flowers emit dead rotten fish semen trimethylamine dimethylamine
• Maidenhair, Ginkgo bioloba tree - female fruit produces putrid rotten eggs vomit
• Chinese chestnut tree - male flowers emit "off-putting" smell; again I think this smells like semen and it's just not said that way because it's uncouth!
• Linden tree - smells like semen? How could it smell like semen and yet someone else says it smells like the most powerful fragrance in the plant kingdom, of honey and lemon peel?

Notes:

• The tree of heaven is a very sensitive ozone bioindicator; high levels of ozone will cause spotted and dropping leaves.
• New spouts grow fast, sometimes 15 feet per year.
• It is one of the worst invasive plant species in Europe and North America.

• Ailanthus altissima - Wikipedia
• USDA National Invasive Species Information Center - Tree-of-Heaven - Jul 2020
• The Nature Conservancy - Tree of Heaven - July 2020
• Invasive Plant Fact Sheet - Tree-of-Heaven (Nov 2011) (PDF | 213 KB)(link is external) University of Pennsylvania. The Pennsylvania Flora Project of Morris Arboretum

Best source of information on this topic:
Ecological Landscape Alliance - Tree of Heaven, An Exotic Invasive Plant Fact Sheet - May 2014

Identify and Disambiguate:
New Jersey Audubon Society - How to Correctly Distinguish Invasive Tee-of-Heaven from Native Sumac - July 2018

Post Post Script:
This gal is trying to decode the bad smell network; I made an odor descriptor-molecule network graph of her research with a regional air quality odor complaint database, interesting work, under-explored territory.

In word-searching the list from the Curren's study, "amine" turns up "fishy" and "pungent" via trimethylamine, and "pungent" via ammonia; no mentions of semen ever.

"Pungent" then brings up pentanal, 2-pentanone, formaldehyde, ammonia, trimethylamine. And  "rancid" connects to butyric acid. "Rotten" brings the expected hydrogen sulfide and dimethyl trisulfide from "rotten eggs" and "rotten vegetables."

A Case Study of Odor Nuisance in the South Coast Air Quality Management District 
Curren, J. 2012. Characterization of Odor Nuisance. UCLA.

What the Hell Does a Stink Bug Smell Like?