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Sugar, red and Japanese Maples: You can drive up and down America’s east coast to enjoy their fiery pyrotechnic shows each fall. Along the way, you may want to hop out of the car, take a deep breath and hope you catch a whiff of the katsura tree’s sweet scent.“I can barely smell it, but as people walk through my garden, they shriek, Cotton candy!” said Ken Druse, a gardener and author of “The Scentual Garden,” a book about how humans smell and process scented plants.

Autumn seems to belong to pumpkin spice, and odors are often overlooked when it comes to fall foliage. We rave about how leaves die colorful deaths and rarely discuss how their scent changes with old age. But right about now in Mr. Druse’s garden and elsewhere around the country, the leaves of katsura — which can be found all over New York City and in many other parts of the United States — are just beginning to turn. The katsura is also called the caramel tree. In Germany, they call it “kuchenbaum,” or “cake tree.” And if you’re able to pick up its scent, you’ll see why.

Known scientifically as Cercidiphyllum japonicum, the katsura tree is native to Japan and China. Fossil records suggest it once grew widely in North America and Europe before going extinct in the cold Pleistocene epoch. It returned to the United States around 1865, when an American diplomat in Japan shipped seeds home to his brother in Manhattan, who operated a family nursery.

He planted the seeds on the Upper East Side, but they also made their way to Queens. Today in that borough’s Kissena Park, within a 14-acre plot known as Historic Grove, a stand of katsura trees still stands, one of the park’s most aromatic attractions.

The earthy scent typical of autumn is the work of fungi and bacteria that decomposes plant matter in the soil. But a different chemical reaction in katsura leaves conjures fall spice, caramel and burned sugar. As the tree’s heart-shaped leaves ignite, changing from plum purple or green to buttered-rum yellow, they abandon the grassy, hay-like smell of leftover chlorophyll and adopt a scent more appropriate for a bakery.

“We were wondering, every autumn, about the pleasant Madeira cake-like smell” emanating from a tree growing nearby, Ralf Berger, a flavor researcher in Germany recalled in an email. With a lab full of instruments for flavor analyses, “we decided to take a look.”

In the 1990s, they collected and analyzed leaves throughout the year, starting just two weeks after the leaves appeared through when they turned brown. The chemical compound they found was maltol, which is used in flavor enhancers, perfume and incense. Breads, warmed butter, uncured smoked pork, chicory, milk, cocoa, coffee and roasted malt and barley all emit the chemical, Mr. Druse said.

In young katsura leaves, Dr. Berger found that maltol concentrations were lower in spring and summer. But as leaves began fading in the fall, the compound partnered with a sugary molecule and the scent revealed itself. The chemistry was most fragrant in brown leaves and lasted about a week.

Source: It’s One of Autumn’s Best Scents, but Not Everyone Smells It