The introduction of landscaping into densely populated urban settings has always been a thorny issue, pun fully intended. Steeped in emissions and incarcerated by impervious surfaces, plant life across urban environments typically only thrives against the odds. Certain flora that can flourish in a suburban front yard are scarce in downtown settings, for obvious reasons: hydrangea macrophylla, for example, require considerable amounts of water. The heavily paved environment in a city setting results in considerable stormwater runoff, creating extremes of either scarcity or standing water rather than a good and even percolation, making these otherwise hearty plants rather high maintenance and unsuitable for urbanism. Other plants need prolonged sunlight over multiple hours of the day, which urban environments, with their pronounced penumbral protrusions, prevent. If a five-story building casts a shadow over a patch of grass for all but four hours each day, it’s difficult for some sunlight-dependent plant life to prosper.
But then there’s a genus of plants that seems to defy all of these limitations: the gingko. One of the most anomalous tree species in existence, it resides its own botanical division (phylum); I’m sure I’m not the only one who remembers those primary school textbooks that showed it standing alone among vascular plants, taxonomically unrelated to all other trees because, with the exception of the surviving gingko biloba, all other species died millennia ago, and the lone man standing has been slow to evolve. This one species’ resiliency may help explain why it enjoys the reputation as being a particularly hearty tree, good amidst the grime and concrete of city life, and in turn why a tree of Chinese origin is visible throughout the rest of the northern hemisphere, like this urban setting in the historic Old Town neighborhood of Alexandria, Virginia.
This photo, taken in mid September, suggest that the tree is approaching its peak autumnal colors and beginning to shed its leaves, which is unusually early for northern Virginia within the Washington DC metropolitan area. The humid subtropical climate, in which an urban heat island and the sea-level, marsh-like conditions amplify the heat, results in an uncharacteristically late green season. Many trees in northern Virginia don’t reach peak colors until early November, while if one travels 80 miles outside of the dense metro (even to the south) the trees will have largely shed their leaves at that point. When mature, gingko trees in particular have a much-loved brilliant yellow hue during the peak of fall. By the time of this article, the trees from the above photo are probably approaching that golden glow. (I featured some gingkoes at peak fall color in an article from downtown Indianapolis many years ago.) But most of the other trees in this part of Alexandria haven’t begun to change, as is evidenced by a simple pivot.
Another gingko on the right, seemingly farther advanced into doffing its leaves than the first two. But notice the other trees, both to its left and in the background, on the other side of the street. They’re gingkoes too. Male ones. Atypical of almost all flowering plans, gingkoes achieve outlier status once more for being dioecious—the sex characteristics of trees are unique to that tree, rather than hermaphroditic. And here we encounter the juxtaposition of the male and female gingkoes, both sharing that same distinctively fan-shaped leaf.
How do we know the one on the right is female? Easy—it produces and deposits its distinctive rosy seeds.
There they are: spherical and often in pairs. Incidentally, the female gingko is typically much less desirable in urban settings because of these seeds, encased in a fruit-like cellulose material that butyric acid, a chemical with a smell many compare to vomit. And the early fall is the peak time for the female gingko’s seeds to fall and rot on the ground—the one time of year that weakens their desirability in comparison to the male trees. This sex discrimination has become so pronounced that male cultivars enjoy considerably greater marketability from tree nurseries, and they still yield that much-loved autumn gold (incidentally, the name for a male gingko clone). One minor disadvantage to male trees is that they produce pollen, making them considerably more allergenic than the females But apparently more people have problems with the stench than the sniffles.
I’m not sure entirely why the deciduous cycles of male and female gingko trees would differ so greatly. Frankly, I’m not well versed in the botanical nomenclature that would make for an easy research project, and in that regard, I’m lazy. Additionally, I have a sneaking suspicion that the characteristics seen here in Alexandria, where the female gingko gets a tremendous head start on fall coloring over the male, is geographically limited. Those climatic or ambient variables may not be visible even a few miles away. But it’s still surprising how marked the difference is between the two sexes of gingko, reminding me (of all things) of caribou.
Yes, the caribou, the undomesticated version of the reindeer, is actually among the least sexually dimorphic in the cervid family. While we typically distinguish most deer through the male’s antlers, among caribou both sexes grow the cartilaginous protuberances. But they shed them annually at different times: males at the end of the autumnal mating/rutting season, while females shed their antlers at the end of the birthing/calving season (spring). These biological cycles no doubt incentivize a sort of extended pairing, so that males can protect females when they’re birthing their young, and females may actual fend off more predators during the winter, when males are bald. (At the same time, a typical male during rutting will likely impregnate multiple females, so the coupling isn’t exactly akin to swans, who usually remain monogamous for life.) Not entirely sure why the gingko made me think of caribou, since the dioecious seasonal differences are less pronounced, but there can be no doubt this achieves some evolutionary advantage, which in part explains why a historically obscure tree that has at times faced threatening levels of population decline is now a standard figure in urban America. Just don’t step on that stinky fruit.