The joys of Fern-watching?

Well, I don't know how much or how long these people actually watch ferns (not that that wouldn't be entertaining if one could only alter the flow of time; see below the fold), as opposed to simply finding, identifying, ticking on list, and moving on (which, come to think of it, is what most bird-watchers do also, anyway), but this article in a recent New Yorker certainly makes the activity sound delightful:

There is no end to the odd things that New Yorkers do on Saturday mornings. This, at least, is what drivers must have thought recently when they had to slow down to avoid a line of a dozen people flattened against the enormous embankment of the Park Avenue railroad trestle, peering with magnifying glasses and monoculars into tiny crevices in the stone. Passersby stared, asked questions, and even took photographs. Police officers stopped their patrol car and watched with suspicion, or with bewilderment—until they caught sight of the T-shirts many of us were wearing which bore slogans such as “American Fern Society” or “Ferns Are Ferntastic.”

Thus begins this lovely little report of a weekend fern-watching romp (an annual Fern Foray) undertaken by a bunch of pteridophiles and bryophiles in the middle of New York city, written by none other than Oliver Sacks! I sure hope the article remains available online. And surely any society of watchers of whatever corner of the biosphere must really have its priorities correct if it can not only pause amid the rush of Manhattan to peer at crevices in the embankment of urban railroad trestles, but also count Oliver Sacks among its ranks and hold Darwin as its icon! Isn't that enough to make you want to go looking for ferns?

But the article is much richer than merely an account of the fun they had, or lovely word-pictures of various species encountered. It actually portrays an intriguing picture of the distribution of cryptogamic (pteridophytes mostly, but also some bryophytes) diversity in midtown (or is it uptown?) Manhattan, with interesting changes in species occurrence / composition as they moved along the Park Avenue viaduct, until

...suddenly, strangely, at 110th Street the ferns stopped. From that point north, there was a startling, lifeless desolation, as if someone had decided to eradicate all signs of cryptogamic life. No one knew for sure why this was so, but we quickly crossed over to the sunny side of the trestle and began to work our way south again.

But, surely, someone must know of some explanation for this abrupt boundary? Someone has studied it, yes? Is this boundary set by geophysical/climatic/biological factors? Or sociological ones? Or is this yet another example of how often we ecologists and evolutionary biologists ignore the life that thrives amid our most extreme human enterprises to chase after more remote wildlife? This urban reconciliation ecologist wants to know - so if you are a fern-fancier or live in New York or both, and know of an explanation, or know someone who does, please let me know!

And now, if you are not averse to manipulating how time flows, or slowing down your perception of it, tell me: would you remain unmoved by this truly wonderful and amazing sight?