Ah, spring. The sun is out, the streets are humming, the days are getting longer, and the air smells like… like… um… say, can anyone else smell that? It’s not just me, right? Right?? It’s not just me. All over America, spring is getting smellier every year, and the culprit is the Bradfo rd pear, a tree that gained popularity in the mid-20th century for its ornamental properties. But it has since proven to be kind of a nightmare. As well as its distinctive smell (described as smelling like rotting fish, sweat, or semen), it has a nasty habit of shedding branches during storms. Its hardiness makes it awfully difficult to get rid of, to the extent that it’s now considered invasive in most of North America. Oh, and you can’t eat its fruit, either, because it’s full of cyanide. The tree isn’t just a problem here in the US, either. In my home country of Australia, we have a folk song called “Give Me a Home Among the Gum Trees,” which has been repurposed in an entirely predictable manner to celebrate the apotheosis of the Bradford pear. So if you’re wondering where these trees come from, why they’re everywhere, why they stink, AND what on earth they have to do with 9/11… read on. Ah yes, spring has arrived…and it smells terrible. Image: Popular Science Why does it smell like that? We should start with the thing that makes most people aware of the Bradford pear: its smell. People tend to step coyly around the tree’s unique odor: it’s been described variously as “funky”;  “fishy”, and “ammonia-like.” But we’re all adults here, so let’s be honest: Bradford pears smell uncannily like semen. So why is this? Like most scents emitted by plants, the Bradford pear’s perfume exists to attract pollinators. Unfortunately, the tree’s pollinators are flies, rather than bees, so rather than a heady sweet scent, the Bradford pear pumps out smells that attract flies. These scents of death and decay get their odors from nitrogen-based compounds called amines. Amines are a vast and diverse family of organic compounds, and their common feature is that they’re all derivatives of ammonia, which is the simplest simplest compound of nitrogen and hydrogen. The diagram below shows ammonia’s structure: the blue nitrogen atom is bonded to three grey hydrogen atoms, forming a pyramidal structure.  In amines, at least one of those hydrogen atoms is replaced by a carbon atom. In the simplest case—methylamine, a name that’ll be familiar to Breaking Bad fans—that one carbon atom simply has three hydrogen atoms bonded to it, as shown in the diagram below. Note that the blue nitrogen atom now has only two hydrogen atoms bonded to it, with its third bond going to the black carbon atom. Methylamine is a simple molecule, but more complex amines may include one or more carbon chains, which may branch off further, connect to other groups, and so on. It’s no surprise, then, that there are a lot of different amines, and they turn up everywhere–from psychotropic substances like ketamine and amphetamine to dyes and fertilizers. But for all their diversity, they have several things in common. They are generally bases; they’re volatile, with low boiling and melting points; and they all kinda stink. Amines are also ubiquitous in living organisms, where they form long chains called amino acids. When living things die, those amino acids slowly break down into smaller amine molecules with cheery names like putrescine and cadaverine. It’s these molecules that release a smell that creatures like flies and beetles associate with death. This is all very well, but what does it have to do with the smell of semen? Well, semen also contains amines. It does so because of their alkaline properties, which helps counteract the acidic environment of the vagina and keep sperm alive long enough to swim for glory. And it seems that the mix of amines is very similar to that released by… Bradford pears. Why are there so many of these damn trees? There’s at least one other glaring question that remains unanswered here: if these trees’ scent falls somewhere between swingers’ party and graveyard, why on Earth are there so many of them? Well, say what you like about Bradford pears, but they are pretty. They were originally bred in the early 20th century after a fungal disease called fire blight tore through pear crops across America. The Department of Agriculture went searching for a species resistant to fire blight, and found what they were looking for in the form of the Callery pear, a species native to Asia. That tree was attractive to look at, resistant to disease, and able to thrive in a variety of environments. Unfortunately, it also had nasty thorns all over its branches, and while it brushed off fire blight, it fared less well with several North American insects. Scientists started trying to breed new varieties without these shortcomings, and the result was the Bradford pear, a thorn-less and insect-resistant tree that arrived in the 1950s. Initially, people loved them, and they quickly gained popularity as ornamental street trees, planted on sidewalks from coast to coast and everywhere in between. The New York Times declared famously in 1964 that “few trees possess every desired attribute, but the Bradford ornamental pear comes unusually close to the ideal.” One of these attributes was supposed to be that the tree was sterile. (No, the irony of this is not lost on us.) And it’s true, Bradford pears can’t pollinate other Bradford pears. They can, however, cross-pollinate with other species of pear, and they did so gleefully once they were planted nationwide, resulting in weird hybrid species sprouting up.  It’s unclear whether the original Bradford pear smelt so distinctive, or whether it’s these rogue cultivars that really stink. Early reports about the tree, such as the New York Times’s weird botanical hagiography, don’t mention the smell at all. Although, it’s unclear whether this is because of some sense of propriety, or because early Bradford pears didn’t smell quite as awful as modern versions. Either way, since those early trees were first planted on suburban streets, the Bradford pear has become North America’s answer to the cane toad: a non-native species introduced to solve one problem and ended up being a significantly bigger problem itself. And just like the cane toad, the Bradford pear doesn’t seem to be going anywhere. The hardiness that made it so attractive to scientists in the 20th century has proven a nightmare for scientists in the 21st: The tree can survive in a wide variety of environments, and while it’s relatively short-lived, it also spreads quickly—thanks, no doubt, to its attractiveness to the local fly population. The ultimate proof of its indestructibility came, believe it or not, on 9/11. As well as leveling the World Trade Center, the attacks on that terrible day destroyed street infrastructure throughout Lower Manhattan. But in the days following the attacks, first responders noticed that one tree remained standing at Ground Zero: a Callerypear, the species that was originally brought to America for its ability to resist fire blight. It was badly damaged but still alive. A callery pear tree became known as the “Survivor Tree” after enduring the September 11, 2001 terror attacks at the World Trade Center. Image: Getty Images Toshi Sasaki As per the 9/11 Memorial Website: “The tree was removed from the rubble and placed in the care of the New York City Department of Parks Recreation. After its recovery and rehabilitation, the tree was returned to the Memorial in 2010.” It still stands there today. Botanists today tend to joke that there’s really only one way to prune a Bradford pear: with a single cut, at the base. But as an avatar of resilience—imperfections and all—well, you could do worse. The post Why spring smells like semen and rotting fish appeared first on Popular Science. ...read more read less