A forest smells the way it does because of hundreds of volatile organic compounds released by trees, soil microbes, decaying leaves, and even insects, all blending into what your nose registers as a single green, resinous, earthy atmosphere. The dominant molecules are terpenes, a vast family of hydrocarbons that trees produce in staggering quantities, but the full picture includes everything from the musty smell of wet earth to the sharp sweetness of sap and the faintly medicinal bite of pine needles. What makes forest scent fascinating is that almost none of it is accidental: the chemicals you smell are doing real biological work, from protecting leaves against heat damage to calling in predatory insects to eat the caterpillars chewing on a branch.
Terpenes Are the Backbone of the Forest Smell
If you have ever walked into a pine or spruce forest and noticed a sharp, resinous tang in the air, you were breathing in terpenes. These are a class of hydrocarbon molecules built from repeating five-carbon units, and trees pump them out through their needles, bark, and even their roots. The two you would encounter most in a coniferous forest are alpha-pinene and beta-pinene, which together give pine forests their signature scent. Research in a maritime pine forest found that every part of the tree, from ground-level roots to branches, releases pinene day and night, with the strongest emissions from the ground and stem during moist conditions. On hot summer days, the branches also release compounds like ocimene and linalool, which have a more floral, herbaceous character.
Terpenes come in different sizes, and the size matters for what you smell. Monoterpenes, made of ten carbon atoms, are small and volatile enough to evaporate easily into the air, producing the sharp pine-and-citrus notes that carry over long distances. Sesquiterpenes, with fifteen carbons, are heavier and tend to linger closer to the forest floor, contributing a woodier, earthier undertone. The even larger diterpenes are barely volatile at all and mostly stay locked in tree resin. When you lean against a pine tree and your hands smell like sap for the rest of the day, that is largely diterpenes.
Tropical forests have a different terpene profile. In a Southeast Asian rainforest, measurements showed that isoprene, the simplest terpene (just five carbons), accounted for about 80 percent of the reactive carbon emitted by the canopy, with monoterpenes contributing roughly 18 percent.1Atmospheric Chemistry and Physics. Fluxes and concentrations of volatile organic compounds from a South-East Asian tropical rainforest Isoprene has a faint rubbery smell on its own, but it is so reactive that it quickly transforms into other compounds in the air. Plants that produce isoprene gain a real advantage: they handle heat stress and ozone damage better than plants that do not, which is one reason tropical broadleaf trees are such prolific emitters.2PubMed Central. Isoprene emission from plants: why and how So the smell of a tropical rainforest is not just different from a boreal pine forest because of different tree species; the chemical strategy the trees use to cope with their environment is fundamentally different, and you can smell the difference.
The Smell of Rain on Earth
One of the most universally recognized forest scents has nothing to do with trees at all. That rich, earthy smell you notice after rain falls on dry ground comes primarily from a molecule called geosmin, produced by soil bacteria. Humans can detect geosmin at astonishingly low concentrations, on the order of parts per trillion. Researchers have proposed that geosmin acts as a kind of warning chemical: the bacteria that produce it are often toxic or unpalatable, and the scent may have evolved to discourage soil-dwelling organisms from eating them.3PubMed Central. The Ubiquitous Soil Terpene Geosmin Acts as a Warning Chemical The word for the smell of rain on dry earth, petrichor, was coined in the 1960s, and geosmin is its primary ingredient.
What is interesting is that geosmin is itself a terpene, just one made by microbes rather than trees. It is heavily conserved across many groups of soil organisms, including actinobacteria, myxobacteria, and certain fungi. That conservation across such distantly related organisms suggests the molecule has been performing a useful function for a very long time. So when you walk through a forest after a rainstorm and inhale that deeply satisfying earthy smell, you are detecting a microbial defense system that predates the trees themselves.
Trees Talk Through Scent
Perhaps the most surprising thing about the chemicals you smell in a forest is that many of them function as a communication network. When an insect starts chewing on a leaf, the damaged plant does not just sit there and take it. It releases a burst of volatile compounds into the air, and neighboring plants pick up on those signals and prepare their own defenses before the insects even reach them.
This is not speculative. In experiments with wild cotton, plants that received airborne volatiles from insect-damaged neighbors produced more extrafloral nectar and attracted more ants, which then attacked caterpillars on the leaves at higher rates.4PubMed Central. Tri-trophic consequences of plant-to-plant volatile signalling and its contingency on plant relatedness in wild cotton The plant essentially hired bodyguards by releasing a chemical signal. In a separate study, even the eggs laid by butterflies on a plant triggered volatile signals that warned nearby plants. Caterpillars that later fed on those pre-warned plants grew significantly less than caterpillars feeding on plants that had received no warning.5PubMed Central. Oviposition-induced plant volatiles prime defences against impending herbivores in neighbouring non-damaged plants
This phenomenon even works across species. Tomato plants grown alongside basil showed a faster, stronger wound response when damaged, because the basil’s volatiles had primed their defense pathways. Caterpillars fed on the basil-primed tomato leaves grew less than those fed on control leaves.6PubMed Central. Companion basil plants prime the tomato wound response through volatile signaling in a mixed planting system This is one of the mechanisms behind companion planting in agriculture, a practice gardeners have used for centuries without fully understanding why it works. The volatiles that give basil its distinctive smell are not just pleasant to us; they are chemical primers that put nearby plants on alert.
So when you walk through a forest and notice the scent intensifying, especially on a warm afternoon when insect activity is high, part of what you are smelling is an active conversation between plants about the threats they face.
How Forest Scent Changes With the Seasons
A forest does not smell the same in July as it does in October. The volatile cocktail shifts dramatically over the course of a year, driven by temperature, light levels, and the biological needs of the organisms producing the chemicals.
In a pine forest, pinene emissions can vary by one to two orders of magnitude from day to day, depending on conditions. Moisture drives stronger emissions from the ground and stems, while hot, sunny days push out light-dependent compounds like ocimene and linalool from the branches.7PubMed. Compartment specific chiral pinene emissions identified in a Maritime pine forest This means a humid forest morning and a dry forest afternoon literally smell different, even on the same day, in the same stand of trees.
Autumn brings its own distinct olfactory shift. As leaves begin to change color in deciduous forests, the volatile emissions change in a specific sequence. Research on silver birch found that compounds associated with insect defense, like ocimene and linalool, disappear once the green leaves are gone. Methyl salicylate, which serves double duty attracting certain insects while repelling others, persists into the early stages of color change but then drops off. As the leaves actually fall and begin to decompose, a compound called cis-3-hexenol dominates. It is the same molecule responsible for the “green” smell of freshly cut grass, and in autumn it signals cellular disintegration as the leaf tissue breaks down.8PubMed. Leaf volatile emissions of Betula pendula during autumn coloration and leaf fall That bittersweet, slightly sharp smell of a forest in late October is, chemically speaking, the smell of controlled cellular death.
Winter forests are not scentless, but they are quieter. Coniferous forests retain their terpene emissions year-round since the needles stay on, though at much lower rates when temperatures drop. Deciduous forests lose most of their canopy emissions entirely, leaving just the soil, bark, and fungal volatiles. This is why a winter walk in a deciduous forest smells predominantly of wet earth and wood, while the same walk in a spruce forest still carries that faint resinous bite.
Forest Volatiles Shape the Atmosphere
The scale at which forests emit volatile compounds is enormous. Globally, vegetation releases an estimated several hundred million tons of volatile organic compounds into the atmosphere each year, far outstripping human industrial emissions. These molecules do not just drift around smelling nice. Once airborne, they undergo chemical reactions with ozone, hydroxyl radicals, and nitrogen oxides, transforming into heavier, stickier molecules that clump together into tiny particles called secondary organic aerosol.
This aerosol formation matters for climate in a direct and surprising way. In a boreal forest in Finland, researchers found that higher loads of biogenic organic aerosol led to increased cloud liquid water content, likely because the aerosol particles serve as seeds around which cloud droplets form.9Geoscientific Model Development. On the formation of biogenic secondary organic aerosol in chemical transport models: an evaluation of the WRF-CHIMERE (v2020r2) model with a focus over the Finnish boreal forest More forest emissions, in other words, can mean more cloud cover. And clouds reflect sunlight, cooling the surface below them. This creates a feedback loop in which forests, by smelling the way they do, actively influence regional weather and possibly help buffer global warming to a degree.
Research in a hemi-boreal forest found that sesquiterpene oxidation products were sometimes the dominant fraction of the aerosol particles, even though their gas-phase concentrations were much lower than those of monoterpene products. Sesquiterpenes partition into particles so efficiently that their contribution to aerosol formation has been systematically underestimated.10Atmospheric Chemistry and Physics. The importance of sesquiterpene oxidation products for secondary organic aerosol formation in a spring-time hemi-boreal forest In a subtropical forest, monoterpene oxidation products like pinonic acid and pinonaldehyde were found in both the gas and particle phases, with roughly 42 percent ending up in the condensed (particle) phase.11Atmosphere. Contribution of Terpenes to Ozone Formation and Secondary Organic Aerosols in a Subtropical Forest Impacted by Urban Pollution The heavier, woodier-smelling compounds you barely notice at nose level are, ironically, the ones doing some of the most important atmospheric work.
Why Walking in a Forest Feels Good
The Japanese practice of shinrin-yoku, or forest bathing, has generated real scientific interest, particularly around the idea that the volatile compounds in forest air have measurable effects on the human immune system. The specific compounds in question are often called phytoncides, a term that broadly covers the antimicrobial volatiles that trees and other plants release.
Laboratory studies have found that phytoncides significantly increase the activity of natural killer cells, a type of immune cell that targets virus-infected and cancerous cells. In one study, exposure to wood essential oils boosted NK cell activity in a dose-dependent manner and increased the expression of several key cytotoxic molecules within those cells.12PubMed. Phytoncides (wood essential oils) induce human natural killer cell activity A systematic review and meta-analysis that pulled together the available evidence found a statistically significant increase in NK cell activation following phytoncide exposure.13Pharmacological Research – Natural Products. Phytoncides and immunity from forest to facility: A systematic review and meta-analysis
That said, it is worth being careful about what this actually means for a casual forest walker. Most of the immunological studies use concentrated essential oil exposures in controlled settings, not ambient forest air. The concentrations of phytoncides you encounter on a trail are far lower than what researchers use in a laboratory diffuser. It is entirely plausible that spending time in a forest improves immune markers, but disentangling the effects of the volatiles themselves from the effects of exercise, stress reduction, sunlight exposure, and simply being away from traffic and screens is a challenge researchers have not fully resolved. Forest air smells wonderful and may well be good for you, but the mechanism is muddier than the headlines suggest.
City Trees and the Ozone Problem
The same volatile compounds that seed clouds and protect leaves create a complication in urban environments. When tree-emitted terpenes mix with nitrogen oxides from car exhaust and industrial sources, the photochemical reaction can produce ground-level ozone, a pollutant that irritates lungs and damages crops. This has led to an awkward tension in urban forestry: planting trees is good for shade, carbon capture, and mental health, but planting the wrong species in a high-pollution area can make air quality worse.
A review of the role of biogenic volatile emissions in cities found that adding tree-sourced volatiles to an urban atmosphere already loaded with nitrogen oxides can shift the chemical balance in ways that accelerate ozone formation.14Environmental Pollution. Role of Biogenic Volatile Organic Compounds (BVOC) emitted by urban trees on ozone concentration in cities: A review Species like oaks and poplars, which are heavy isoprene emitters, are the biggest concern. Low-emission species, such as certain maples, elms, and many fruit trees, are better choices in polluted urban corridors. This is not a reason to avoid urban trees altogether. It is a reason to be thoughtful about which trees go where.
The irony is that in a rural or remote forest, these same chemical reactions produce aerosol particles that seed clouds and cool the planet. The chemistry is identical; the difference is context. In clean air with low nitrogen oxide concentrations, terpene oxidation produces particles and does little harm. In dirty urban air, the same terpenes combine with pollution to generate ozone. The forest smells the same either way, but the consequences depend on what else is in the air.
How Climate Change Could Alter What Forests Smell Like
Rising temperatures, elevated ozone, and changing nitrogen deposition are all expected to alter the volatile emissions of forests, though the direction is not always straightforward. An open-field experiment with Scots pine seedlings found that insect herbivory massively increased terpene emissions, as expected, but warming actually reduced the herbivory-driven spike in monoterpene emissions by about 77 percent. The interaction was not simple across all compound classes, though: the combination of warming and elevated ozone boosted sesquiterpene emissions from herbivore-damaged shoots by sixteen-fold.15Agricultural and Forest Meteorology. Herbivore-induced BVOC emissions of Scots pine under warming, elevated ozone and increased nitrogen availability in an open-field exposure
What this means in practical terms is that a warmer world will not simply produce more of the same forest smell. The balance between different terpene classes is likely to shift. Sesquiterpenes, those heavier, woodier compounds, could become relatively more prominent under combined warming and ozone stress, while the sharp, piney monoterpenes could become less dominant in certain scenarios. Given that sesquiterpenes are particularly efficient at forming aerosol particles, as noted earlier, changes in their emission rates could have outsized effects on cloud formation and regional climate feedback. The scent of forests is not just a passive product of the environment; it is a moving target that responds to and, in turn, influences the atmosphere.
How Plants Evolve Their Scent
Plant volatile profiles are not fixed over evolutionary time. When the selective pressure to attract pollinators disappears, the scent changes. Research on the genus Capsella, a group of small flowering plants, found that species that evolved to self-fertilize independently lost many of the same fragrance compounds. The similarities were so consistent that researchers could classify plants into mating types based solely on their scent profiles. The compounds most reliably lost in self-fertilizing species were the same ones linked to attracting pollinators, including beta-ocimene. Meanwhile, sulfurous compounds with an unpleasant smell, which repel many generalist insects and can act as neurotoxins, increased in concentration.16Current Biology. Convergence and molecular evolution of floral fragrance after independent transitions to self-fertilization
This tells us something about the forest scent experience that is easy to overlook: the volatile blend around you is not just a byproduct of metabolism. It has been shaped by millions of years of coevolution between plants, their pollinators, their herbivores, and the microbes in their soil. When a forest “smells like spring,” part of what you are detecting is the result of evolutionary arms races and alliances playing out in real time. The flowers are advertising for pollinators. The leaves are warning each other about pests. The soil bacteria are telling predators to stay away. It is all layered together into a single olfactory impression that your brain files under “forest,” even though the individual signals have wildly different origins and purposes.
Why Some People Smell More Than Others
Not everyone walking the same trail at the same time will experience the same scent. Human olfactory sensitivity varies enormously between individuals, and the thresholds for detecting specific terpenes differ by compound. Research on non-canonical terpenes, modified versions of common ones, found that the detection thresholds for different variants spanned a wide range, with some detectable at concentrations as low as a couple of nanograms per liter of air and others requiring much higher concentrations to register.17PubMed Central. Odor Characteristics of Novel Non-Canonical Terpenes Genetic variation in olfactory receptors means that some individuals are more sensitive to certain terpene classes than others. Age, medications, and even recent viral infections can dampen or alter smell perception.
Temperature and humidity also change what reaches your nose, independent of what the trees are actually emitting. Warm, humid air carries volatiles more effectively and keeps them in the gas phase longer. A cool, dry morning may have just as many emissions at the source but less of the scent at nose height, because the molecules are condensing onto surfaces or not evaporating as readily from the leaf. This is part of why a forest after a warm rain smells so overwhelmingly rich: the geosmin is being kicked up from the soil by raindrops, the trees are releasing moisture-driven terpenes from their bark and roots, and the humid air is carrying all of it to your nose with unusual efficiency. The forest did not suddenly start smelling; conditions just got ideal for you to notice it.