We are immersed in a world of odorants, and we have a sophisticated olfactory system to process them. But compared to research on the benefits of seeing nature, however, we don’t know nearly as much about how olfactory cues—such as the scents of a pine forest—affect us.
We know that spending time in nature is good for us, proven by hundreds—perhaps thousands—of scientific studies. But most of what we know about nature and its benefits have come from research focused on how seeing nature affects us. Now, however, an international team of scientists is hoping to shed light on how what we smell in natural settings impacts our health and well-being.
Plants emit odors for a variety of reasons, such as to attract pollinators, to communicate with each other, to repel harmful herbivores and insects, or to respond to changing environmental conditions. But in this time of great biodiversity loss, is the emission of these scents waning? The answer will probably surprise you: it turns out that species-rich forests release less of these gases into the atmosphere than monocultures.
The smells in nature are now even being harnessed to help restore degraded ecosystems. Reestablishing grasses, trees and other vegetation is an essential part of such plans, but a new survey of more than 2,000 restoration projects from nearly every type of ecosystem on Earth finds that most of them fail to recognize and control one of the new plants’ chief threats: hungry herbivores. Plant aromas are coming to the rescue.
In 1984, one of the first studies looking at the relationship between hospital rooms and patient outcomes found that those who had access to a window with a view of nature had shorter stays after surgery than those who did not have a natural view.
How the smells of nature benefit our health
Research has shown that contact with nature can influence thoughts, lift moods, reduce stress and improve physical health. Even brief exposures to nature can be beneficial. One well-known study found that hospital patients recovered faster if their rooms included window views of natural settings. Knowing more about how nature affects our bodies could not only help our well-being but could also improve how we care for the land and design cities, homes and parks.
Yet studies on the benefits of contact with nature have typically focused on sight. Less attention has been given to what we smell. And that’s something that a group of researchers wants to change.
In a paper published in May 2024 in the journal Science Advances, researchers from the University of Washington and colleagues from around the world—who are experts in atmospheric science, ecology, olfaction, psychology, public health and other fields—outline ways to expand research into how odors and scents from natural settings impact our health and well-being.
Plants exude volatile organic compounds (VOCs) that can persist in the air for days. VOCs can attract pollinators or repel herbivores. Some researchers are now studying the impact of plant VOCs on people.
At its core, the human sense of smell, or olfaction, is a complex chemical detection system in constant operation. The nose is packed with hundreds of olfactory receptors, which are sophisticated chemical sensors. Together, they can distinguish more than 1 trillion scents, and that information gets delivered directly to the nervous system for our minds to interpret—consciously or otherwise.
The natural world releases a steady stream of chemical compounds to keep our olfactory systems busy. Plants, especially, exude volatile organic compounds (VOCs) that can persist in the air for hours or days. VOCs perform many functions for plants, such as attracting pollinators, channeling communications or repelling herbivores. Whole forests, then, release a complex mix of these VOCs.
But what’s the impact of exposing people to plant VOCs? The paper’s authors propose a framework for how to investigate the intimate links between nature, olfaction and human well-being. Some chemical compounds, including a subset of those from the invisible realm of plant VOCs, may be acting on us without our conscious knowledge. In these cases, olfactory receptors in the nose could be initiating a “subthreshold” response to molecules that people are largely unaware of.
Nature leaves its signature smells in the air. Some scents have a “universal” interpretation, such as the pleasantness of a sweet-smelling flower.
Other olfactory cues are picked up consciously, but scientists still don’t fully understand all their impacts on our health. Some scents, for example, may have “universal” interpretations to humans—such as a sweet-smelling flower that always smells pleasant. Other scents could be closely tied to specific memories or have associations and interpretations that vary by culture and personal experience. Previous research has shown that the cultural significance of smells, including those from nature, can be passed down to each new generation in a particular community.
Just as the smells of nature may be exerting an influence on us, human activity is most likely altering nature’s olfactory footprint—both by pollution, which can modify or destroy odorants in the air, and by reducing habitats that release beneficial scents.
The scientists hope that as research illuminates more of these links, it will help policymakers make more informed decisions about our impacts on the natural world and the volatile organic compounds that come from it. We live within the chemical contexts that nature creates, they conclude, and further understanding can contribute to our health and advance efforts to protect the natural world.
How do the concentrations of aerosols in the air change as plants are stressed by drought? An interdisciplinary scientific team investigated this question.
How plant scents are linked to biodiversity
The VOCs of plants not only act as chemical signals, but they also play a role in regulating air quality, atmospheric chemistry and climate. This is because these VOCs form biogenic secondary organic aerosols (BSOAs) in the air, or particles in the atmosphere. These aerosols, in turn, affect air quality, cloud formation and the climate.
So, do emissions and concentrations of aerosols in the air change as biodiversity declines or plants are stressed by drought? An interdisciplinary team led by scientists from Germany’s Leipzig University investigated this question.
The scientists collected data at the MyDiv tree diversity experimental site. The site, located near Bad Lauchstadt in Saxony-Anhalt, covers about five acres and has 80 plots with 10 tree species growing in either monocultures or mixtures of different species. For this study, air samples from 10 of the 36-by-36-foot plots were collected, which grow four tree species (common ash, wild cherry, rowan and sycamore) in different combinations.
Previously, it was thought that species-rich forests and grasslands released more gases into the atmosphere than species-poor ones. However, it’s now been shown that the opposite is true.
The results of the study, which were published in the journal Communications Earth and Environment in November 2023, show that the amount of VOCs decreases with increasing biodiversity, in most cases. Therefore, it’s estimated that global VOC emissions from vegetation will increase by around a third because of climate change and higher temperatures.
Previously, it was thought that species-rich forests and grasslands released more gases into the atmosphere than species-poor ones. It was reasoned that since species-rich systems produce more biomass because they utilize resources—such as light, nutrients and water—more efficiently, that would mean more leaf surface area from which gases could be emitted. These new results, however, suggest that plants in species-rich forests and grasslands release fewer gases because they are under less stress. Compared to monocultures, they face fewer herbivores and less heat and drought.
This is just a hypothesis for now. The researchers note that much more research is needed to better understand how plant scents are linked to the atmosphere and biodiversity.
Herbivores, such as moose, can cause substantial damage to crops and at-risk plants. Traditional methods to deter foraging are often expensive, ineffective or lethal.
How odors thwart hungry herbivores
Reestablishing plantings of grasses, trees and other vegetation is essential for restoring degraded ecosystems. But a survey of almost 2,600 restoration projects showed that while most took steps to exclude competing plant species, only 10% attempted to control or temporarily exclude herbivores, even though they are one of the new plants’ chief threats.
Plant-browsing damage caused by mammalian herbivore populations, such as deer, elephants and wallabies, is a growing global concern. Such damage is one of the greatest limiting factors in areas of post-fire recovery and revegetation, destroying more than half the seedlings in these areas. Globally, it also threatens endangered plants and causes billions of dollars of damage in agriculture and forestry.
By not protecting plants in their early stages, conservationists are missing out on a great opportunity to significantly speed restoration, improve outcomes and lower costs, said an international team of researchers affiliated with 20 institutions and universities who conducted the survey. Their analysis, published in November 2023 in the journal Science, shows that introducing predators to keep herbivore populations in check or installing barriers to keep them at bay until plantings became more established and less vulnerable can increase plant regrowth on average by 89%—a gain equal to or greater than that realized by excluding competing plant species.
The decline of large predators that normally keep herbivore populations in check is likely a contributor to high grazing pressures. Using predators, such as wolves, to control herbivores at restoration sites could help boost plant diversity and remedy struggling ecosystems in less time and at lower costs.
Herbivores’ effects were particularly pronounced in regions with higher temperatures and lower precipitation; all of which, say the scientists, leads to the conclusion that if we want more plants, especially in this time of climate change, we have to let more predators in or restore their populations.
The decline of large predators, such as lions, sharks and wolves, that normally keep herbivore populations in check, is likely an important indirect cause of high grazing pressures. Conventional restoration is slowing our losses, but it’s not expanding vegetation in many places; and climate change could make that even more difficult. Using predators to keep herbivores in check at restored sites, say the survey’s creators, is a relatively untapped approach that could help boost plant diversity and restore ecosystems that are vital to environmental and human health, in less time and at lower costs.
Once a planting is established, the scientists, conclude, the herbivores will be essential, too. The plants just need a small break from being eaten to get restarted. Once they establish, herbivores are key to maintaining plant ecosystem diversity and function.
Ku-ring-gai Chase National Park located north of Sydney in New South Wales, Australia, encompasses bushlands, coastlines, rain forests and a river. The park has significant Aboriginal sites, including ancient, red-ochre rock engravings and paintings.
But what if introducing or reintroducing predators isn’t feasible? Some researchers at the University of Sydney in Australia have an answer. They’ve just shown that it’s possible to shield plants from the hungry maws of herbivorous mammals by fooling them with the smell of a variety they typically avoid.
Since many herbivores use plant odor as their primary sense to forage, the Australian researchers created artificial scents that mimicked the smell of plant species they naturally avoid. In an experiment conducted in Ku-ring-gai Chase National Park in Sydney, the swamp wallaby was used as a model herbivore. The researchers selected an unpalatable shrub in the citrus family, Boronia pinnata, and a palatable canopy species, Eucalyptus punctata, to test the concept.
The study compared using a Boronia pinnata solution and the real plant and found both were equally successful at protecting eucalyptus seedlings from being eaten by the wallabies. Findings from the study, published in the journal Nature Ecology and Evolution in February 2024, show tree seedlings planted next to the decoy smell solution were 20 times less likely to be eaten by the animals. That’s equivalent to the seedlings being surrounded by actual plants that are unpalatable to the herbivore. This gently “nudged” problematic herbivores away from areas the scientists didn’t want them to be, providing a new strategy that could be used to help protect valued plants around the world, either in conservation work or in protecting agricultural crops.
Biologists have discovered that “fake news” works for swamp wallabies. In an experiment in Australia, smell solutions that produce undesired aromas tricked the herbivores into leaving some plants alone.
Using the decoy smell solutions offers many advantages over employing real plants as a repellent. Real plants compete for resources and water, which can outweigh their protective qualities. In addition, this approach should be transferable to any invertebrate or mammalian herbivore that relies primarily on plant odors to forage. Current solutions to herbivore-related problems often involve costly and environmentally impactful measures, such as fencing or lethal control. Animals tend to habituate to unnatural cues—such as using chili oil or motor oil—so, such deterrent effects are only temporary. By contrast, by mimicking the smell of plants herbivores naturally encounter and avoid in day-to-day foraging, the animals are less likely to habituate to these smells.
This technique was also tested successfully with African elephants, as you can see by watching the short video, below.
How scents make long-lasting memories
On our nature excursions, we often focus on the amazing sights: a herd of wildebeest on a vast, African plain; a lone polar bear on a snowy tundra; or thousands of butterflies lounging in the sun on oyamel fir trees on a Mexican mountaintop.
We often focus on the sights of nature, but the smells of nature can be just as memorable, such as the scent of rain in the remote Scottish Highlands.
But the smells of nature can be just as stunning: the clear, clean air in an Antarctica breeze; the fresh rain on a Scottish Highlands moorland; or the refreshing scents of cedar, fir and spruce trees in a Pacific Northwest rain forest.
In fact, your sense of smell may be a better memory trigger than your sense of sight. Because of your brain’s anatomy, memory and scent are closely linked. As an odor enters the nose, it’s first processed by the olfactory bulb, which starts inside the nose and runs along the bottom of the brain. The olfactory bulb is part of the limbic system, or the emotional center of the brain. Within this emotional center, lives the amygdala—which plays a role in emotional memories—and the hippocampus, which is critical to developing memories. When scents pass through this area of the brain, emotions, memories and smells become intertwined.
It’s often said that when we travel, we should “take only photographs and leave only footprints.” Perhaps we should allow ourselves to carry home some memories of the scents we encounter on our journeys, too.
Here’s to finding your true places and natural habitats,
Candy
