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.
— Read more in It’s One of Autumn’s Best Scents, but Not Everyone Smells It: Foliage is lovely, but so is the odor of the katsura, also known as the “caramel tree” at The New York Times. Hat tip to Quarry and Kathryn!