Botanists know how trees turn to their gorgeous colors in the fall — as each leaf’s plumbing shuts off and the green chlorophyll fades, other compounds come to the fore. The scientists still don’t know why, exactly.

One theory is that the red pigments, at least, are forming a sun screen or signaling aphids to back off. Bright sunshine and cool nights intensify the colors, we know that, and the heavy rains of late summer spared trees and shrubs the drought stress that simply causes them to drop their leaves early, whatever their color.

But this fall seems to have been a year to remember, and I view the spectacle as some sort of cosmic payback for having to live with clammy, 80-degree nights in August. Nature’s lollipop after the dentist’s chair.

The autumn show is fleeting, but there is a moment at the end where the fading coda seems to be both at its most intense and most fragile. Such was the other day at the National Arboretum, when I reveled in the sight of all those xanthophylls, carotenoids and anthocyanins coming through the rye.

The serenity was enhanced by the lack of other folk. Given the natural beauty, one might have expected legions of long-faced poets hunched over their Moleskines and chewing on pencils. One placid soul set up an easel and started painting, but that was about it. She smiled a lot, proving once again that painters are happier than poets.

Leaf-viewing is an art and requires an active role if you are to do it right. First, there is the macro view, where you take in the pleasing structure of a tree in the landscape. Toward the end of the leaf fall, a tree reaches a sort of perfect counterpoise when the architecture shines through the thinning foliage but the tree still seems clothed.

The intense colors only accentuate this lovely moment. Reds, plums, magentas — the work of the anthocyanins — have been superb, to the point where the usual dull wine oaks have been resplendent in crimson and orange and the red maples a jaw-dropping vermilion. They are matched by the Asian maples, the acers, brought into dazzling relief against the now-leaden stars of April.

The delicate leaves in their many versions invite that other sphere of leaf enjoyment, the micro view of the leaves. By looking at leaves close up, you perceive their basic forms, the venation, the way the edges are smooth or serrated and their arrangement on a branch. The weeping threadleaf maple can be a plant of rare beauty, especially if it is pruned properly through the years, but the upright varieties deserve much more use.

They are not favored enough as ornamental trees because they don’t flower to speak of. But they are relatively small and sculptural and bestow a certain class on a garden, even the run-of-the-mill Bloodgood. Dozens of more interesting varieties are available.

It’s possible to plant a garden for fall color, for which these maples would provide a sublime staple. The list would also have to include witch hazels, fothergillas, iteas, sumacs, oakleaf hydrangeas, sourwoods and rugosa roses, along with grasses and asters. Many receding perennials can look stunning when the chlorophyll is sent packing. A massed planting of blue star(Amsonia hubrichtii) brings the ground alive with orange plumes. Solomon’s seal turns a pale yellow and then fades to the color of aged ivory.

The National Arboretum leaf watcher is ultimately drawn to the hillside gardens above the Anacostia River, to the Asian Collections. Old and maturing specimens of stewartias and acers catch the eye. The paperbark maple (Acer griseum) is valued for its coppery, peeling bark, but its crimson leaves are something to behold, too. At the end of one bed, the unusual loose-flowered hornbeam (Carpinus laxiflora) holds out arms of pendent leaves, golden tinged with pink. The tree itself is spindly, scaled for a small garden, and worth more use, surely.

Another find was a cypress that is at its northern limit in Washington. Like its relative, the bald cypress, the Chinese swamp cypress (Glyptostrobus pensilis) turns and drops its needles before winter. The foliage is a becoming pink-orange. This is a great deciduous conifer for a sheltered, wet area and something different from the dawn redwood, beautiful as that is before it drops its needles.

The season closes with another Chinese dinosaur, the ginkgo. A mature tree is hidden away at the back of the arboretum’s administration building, which is a shame because it has been fashioned into a cordon espalier as if it were a noble pear tree in some lordly garden.

When its golden leaves brighten and abruptly drop, it will reveal its interesting, stubby branch spurs. But that’s for winter viewing, a practice that takes an even keener perception. Or as Rutherford Platt wrote nearly 70 years ago, “The smallness of winter buds is greatly exaggerated.”

Put away those little screens and open your eyes.

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