Walking in New York this season, I stumble into a time warp. The smell of fir from sidewalk Christmas tree vendors plunges me into memories: sparkling red and silver lights on childhood holiday boughs, the judder of a saw in my hand as I built spruce bookshelves, and the songs of wrens and hermit thrushes along a forested trail in Canada. This portal into the past is opened by odor alone, by the fir needles perfuming whole city blocks.

No other sense is as direct as smell. No other sense is as ancient. Smell bypasses the neural processing centers that mediate all other senses. The aroma of fir trees flies me directly into specific wordless memories: childhood holidays, hand-sawn woodwork and my feet tramping through wet forests. When I breathe in, signals run along nerves wired straight to the parts of my brain governing emotional memory and associative learning. My conscious brain catches up later, adding a patina of language.

This inward journey into human remembrance is mirrored by an outward connection to the lives of trees and forests. The chemical conversation between cells and air- and waterborne aromatic molecules unites all living beings, from animal noses to plant root tips to the cell surfaces of bacteria. The scents of holiday trees evoke primal experience.

Other trees and their products join fir’s resinous spice in the seasonal bouquet. Roasting chestnuts loose sweet, earthy scents on street corners. Hanukkah olive oil perfumes the kitchen — sharp and green from early-season harvests, rounded gold from later pressings. The spiced wine of pomegranate mingles with the tannic nose-bite of olives and tree nuts on Persian Yalda tables marking the winter solstice. In the Southwest, piñon pine wood smoke is the winter incense of the desert mountains.