The golden leaves of Ginkgo trees are just spectacular this week.

I have a special fondness for this species: its kin date back to the Permian (>250 million years ago), so the Paleozoic lives on right here on our campus lawns. The Ginkgo is also remarkably robust and is able to live in even the most polluted cities. Ginkgo trees were among the few living creatures to survive the horrors of the atomic bombs that were dropped onto Japan. Survivors, indeed.

In addition, the species refuses to conform to our narrow notions of botanical beauty. It is dioecious (female and males are separate individuals) and female trees are currently scattering their extremely pungent seeds all over tidy lawns (the smell is butyric acid — rancid butter). The philosophical underpinning of a lawn denies the realities of biology: death and sex are nowhere in evidence on a “nice” lawn. The Ginkgo violates these standards with great flair. I can smell the trees from at least fifty meters away.

Squirrels can smell them too. When I took my Field Investigations class to see the Ginkgos, we counted ten gray squirrels under one tree, gathering the seeds. The squirrels were as fat as bear cubs. A great harvest. Humans also like the cleaned centers of the seeds, but be warned, the fleshy outer layer is highly allergenic to some people.

Ginkgos are gymnosperms, so they technically don’t make true “fruits.” The fleshy outer layer is actually just the soft husk of the seed, not formed from an ovary wall as with true fruits. (Botany, it turns out, is mostly about sexual organs, an obsession that got Linnaeus into trouble — a colleague reportedly called his focus on sexuality “loathsome harlotry.”)

Sewanee’s campus lost two of its oldest living residents last week. No fanfare ensued, although these were remarkable members of our community. They were born before 1800, one or two human generations before the founding of the University. They were likely seedlings when Tennessee first became a state.

The growth revealed by the tree rings on these oak trees showed no sign of slowing until last summer when the construction company working on the University Archives dug up their roots and routed their heavy machinery directly across most of the root zone. The trees could not recover and were cut down.

While the East Coast is getting blasted with rain and wind, here on the other side of the Appalachians, we’re dry, dry, dry.

The tuliptrees have given up on the year and are shedding their crisped leaves.

Whereas the more drought-tolerant hickories and oaks retain their summer freshness.

You can pick out the tuliptrees from a distance. Their yellowing leaves stand out against the rest of the forest.

Why do tree species differ in their ability to withstand drought? McDowell et al.’s review sums things up nicely: “Numerous hypotheses to explain mechanisms of survival and mortality have been generated via theoretical, modeling, and experimental analyses. However, a broader framework that encompasses these different hypotheses is lacking, and most hypotheses remain untested.” Despite this state of ignorance, it appears that differences in the plumbing systems of trees provide a partial answer. Oaks and hickories have water-conducting vessels that let them squeeze a living out of  dry soil whereas the tuliptrees’ system stops working in drought, cutting off the water supply to the leaves. The particularities of the width and shape of the vessels account for these differences.