But as well as an inner journey, the trees offer us an invitation outward, into the community of life beyond our species. It is no accident that so many of the midwinter traditions revolve around trees and their products.
Not only Christmas trees, garlands and wreaths, but also dried oranges, warm apple cider, pomegranates, cinnamon (tree bark), cloves (flower buds of a tree) and bonfires. In the darkest time of the year, we turn to trees for rituals of connection and renewal.
These celebrations offer a chance to remember that we humans belong in a world full of other species. We follow our noses, literally, back into the forest through the smells of the season. The golden, resinous smells of spruce, fir and pine come from chemicals called terpenes. Trees make dozens of terpene varieties, and combinations of these give each species its own aromatic profile. For the trees, these chemicals are both defensive and communicative. Terpenes deter and, if eaten in large doses, poison many of the insects that try to nibble on leaves or bore into trunks.
The chemicals double as airborne messengers, carrying from tree to tree news about insect pests, soil conditions and the health of leaves. When we smell trees, we’re eavesdropping on botanical conversations.
Tree aromas also connect the forest to the sky. Yearly, vegetation worldwide sends more than a trillion kilogrammes of aromatic molecules into the air. This great heavenward exhalation primes the sky for rain. Each aromatic molecule remains in the air for only a few hours before it degrades or is reabsorbed by living beings, but during this time it can become the seed around which raindrops first gather.
The sky is partly made of forest. Next time it rains, know that many of the falling raindrops coalesced on tree smells.
Indoor trees bring the sensory richness and ecology of trees home, literally. This experience might also inspire us to seek experience of trees outside, in places less circumscribed by ritual.
Can you find some holly growing in your neighbourhood? What of mistletoe growing on oak branches? The fruits of both these species are important food for birds, and so keep your eyes and ears open for wildlife as you seek out the special trees of the season.
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Do native evergreens grow where you live? Or do deciduous trees outside the door remind us that our Christmas boughs are largely species from elsewhere? Nordmann fir comes from around the Black Sea, Fraser fir from the Appalachian mountains of the United States and Norway spruce from northern Europe.
If you were to pick one tree indigenous to your neighbourhood to symbolise and celebrate the season, what would it be? Part of the pleasure of smelling trees is that sensory exploration ignites curiosity. We go to the world with eager senses and come back buzzing with questions about the more-than-human world.
In a time of ecological crisis, we are in dire need of sensory connection to the living world. Traditions around the darkness of the solstice remind us that we find hope in the vitality of other species, especially trees.
The aromas of Christmas trees are not just portals to the past, but suggest paths to a better future. By taking delight in the sensory richness of trees and other beings, we can reconnect to the living earth. There is joy in this reconnection and in the possibility that by paying attention, we might be better neighbours and kin to the other species on this planet of marvels.
David George Haskell is a professor of biology at Sewanee, Tennessee. His book, Thirteen Ways to Smell a Tree is out now (Hachette, £12.99)
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