You may be thinking, “Ginkgos don’t seem like a candidate for “rare or endangered.” The certainly are common, as one of the most popular street trees worldwide. There are an estimated 74 billion trees in the United States alone! Their popularity is surprising to some people - not everyone wants ginkgo trees, also known as maidenhair trees, on their streets. Some neighborhoods don’t like to plant them because their seeds smell, according to some, like dirty socks. City planners try to avoid this problem by only planting male tress, which cannot produce seeds. However, ginkgo trees are able to change sex in some circumstances, so that strategy doesn’t always work.

 

Given the problems with ginkgo, you may wonder why so many city planners in western cities lined our streets with them The answer lies in the history of the ginkgo tree – they are tough! While the Chinese have long known about, and cultivated, ginkgo trees, they first earned a reputation in Europe for being tough during the London Smog of 1952. This smog was caused by the toxic, acidic smoke pouring from London’s many coal-burning factories and a particular set of weather events. This thick stew of air pollution killed most of London’s street trees, along with at least six thousand people. Although the London Smog was a terrible tragedy, a surprising discovery was made. Workers at a local botanical garden noticed that while other trees were withering away, their ginkgo trees were thriving. After sharing this information with London officials, a plan quickly took unfolded to replace the dead trees with ginkgo. From London, other cities in Europe adopted the ginkgo as a street tree.

 

Why were ginkgos so resistant to the smog? Well, it turns out that gingkos are resistant to many environmental stresses that would kill other trees.  For instance, months after the United States dropped a nuclear bomb on the Japanese city of Hiroshima, new leaves grew from the roots of their destroyed ginkgo trees. Ginkgos have a long and distinguished history of survival; fossilized ginkgo leaves have been found in rocks 270 million years old, making them older than dinosaurs by almost forty million years. This means that over the span of ginkgo’s existence, three separate mass extinctions have failed to drive them extinct – perhaps it is no wonder that they do fine in our polluted cities.

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Although there are many individual ginkgo trees to be found around the world, they not doing as well as it might seem. Ginkgo trees have survived hard times over hundreds of millions of years, but their future is at risk. Although the ginkgo’s native wild range once spanned the world, it is uncertain at this point if it is still present as a wild plant anywhere. Its likely range is a small area of eastern China. However, those trees are very similar genetically, and so may actually be a remnant population of a cultivated stand. Genetic diversity is a measure of how variable the genetic codes of the members of a population are.  To understand this, consider humans.

 

Humans, a genetically diverse species. are sometimes born with resistances to certain diseases or environmental conditions. For example, some people are resistant to malaria, while others are resistant to ultraviolet radiation. When a genetically diverse population is exposed to a problem, like a disease or environmental change, the chance that some members of the group are resistant to that problem is related to the likelihood of that population going extinct. If all of the individuals are the same, genetically, they are similarly susceptible to the same problems.

 

It is under this definition of rarity, low genetic diversity, that ginkgos are in a difficult position. Because of their smelly seeds, most ginkgo trees are male, and are cloned from other identical males. Clones all have the same genetic code, and thus there is very little genetic diversity in their population. So, while ginkgos may be tough, if some environmental change or disease starts to negatively impact one population of ginkgo, other populations are likely susceptible, too.