People love the scent of a balsam fir tree on a hot summer day or when they chop it down to become their annual Christmas tree. If people concentrate, they can probably pick up additional scents when hiking in the forest. Compared with many forest animals, the human sense of smell is extremely limited. The forest air is awash with thousands of air borne chemicals emitted by plants, fungi, microorganisms, and animals sweeping through the air. Most are well beyond the ability of human noses to detect.

Other animals that need to survive here have much better scent detection abilities. They put people to shame. Animals also emit scents. Look no further than skunks. But most animal scents are much more subtle, when animals drop their urine, they are sending a message to competing intruders. The strongest odors often fade quickly, while longer-lasting scent molecules continue marking territories for animals with more sensitive noses.

What about plants? Flowers use fragrances to attract pollinating insects. Some are even strong enough for people to smell. Most are not. Scent is an important way for plants to communicate. When worms and caterpillars begin chewing on its leaves, some plants release airborne chemical signals that attract predators or parasites of the insects feeding on them. Many fruits advertise their ripeness through scent, helping attract birds and mammals that disperse their seeds.

Decaying plant matter is another source for forest fragrances. As microorganisms break down dead plant matter, they release carbon dioxide along with countless other compounds that create the rich, earthy aroma many people associate with a healthy forest.

Rain changes the forest’s chemistry in the air. Some airborne compounds are removed, while others are released from leaves, bark, soil, and decaying wood. After a shower, the forest often smells entirely different. People often notice the scent of a forest most clearly after rain. Part of that familiar aroma is called petrichor, but it represents only a tiny fraction of the chemicals swirling through the air.

The forest welcomes this amazing confluence of millions of aroma molecules that originate from all these sources. Pitifully, people cannot appreciate the vast majority of them. If humans did have better olfactory sensors, they would discover another dimension of the forest that is currently hidden from them. Minute by minute, every breeze stirs up a new mixture in the air, as each moment adds its unique facet to the true scent of the forest.