In the Sonoran Desert of Arizona and Northwest Mexico, many long-time residents claim that with the onset of the summer’s monsoonal rains, a feeling of elation and relief comes as fragrances fill the air in a way that makes it seem as though “the desert smells like rain.”
For decades, geologists, botanists, atmospheric scientists, and ecologists have debated the causes and triggers of this euphoric sensation. Some scientists have focused on fragrances emitted by cryptogamic or biological soil crusts during rains, while other have focused on the terpentine-like smell of the creosote bush known in Sonoran Spanish as hediondilla, ‘the little stinker.” But now two scientists from the University of Arizona have teamed up with an herbalist-author and owner of an herb nursery (the Desert Canyon Farm) in Southern Colorado to propose a novel, but more comprehensive answer:
The Sonoran Desert flora is one of the richest in the world in plants that emit fragrant volatile oils, and many of those fragrances confer stress-reducing health benefits to humans, wildlife, and the plants themselves. What’s more, the biogenic volatile organic compounds (BVOCs) that evolved to protect plants from damaging solar radiation, heat waves, drought stress and herbivores may also have protective value for humans as climate change turns the Earth into “Planet Desert.”
Initially, desert scientists focused their attention on an earthy fragrance called petrichor that is emitted from the biological soil crusts by a compound called geosmin. Geosmin underlies the earthy taste of beetroots, with notes like eucalyptus, cinnamon, and cloves and can be detected by the human nose at concentrations as low as 400 parts per trillion. It is secreted from dead microbes in the soil crusts of many different kinds of landscapes but is now known to be emitted only sporadically in Sonoran Desert soils after summer rains.
Ecologists who studied the North American deserts then tried to explain this phenomenon through a “single cause” focus on one of the most common plants in the Mohave, Sonoran and Chihuahuan desert: Larrea tridentata, known in English as the creosote-bush. Curiously, it emits more than 35 distinct terpenes and other BVOCs, some of which (like trans-caryophyllene) are generated by an endophytic fungus growing “hidden” within the plant’s tissues. With the onset of monsoons, the high density of shrubs forming “creosote flats” emit terpentine-like fragrances (like isoprene) as potent as any botanical emissions into the atmosphere. Nevertheless, this dominant plant is by no means the only major emitter of BVOCs that give Sonoran Desert habitats their renowned fragrances.
The new research from the Desert Laboratory on Tumamoc Hill has found more than 60 species of 178 native plants in the ancient ironwood-giant cactus forests of the Sonoran Desert which emit fragrant biogenic volatile organic compounds (BVOCs) immediately before, during and after rainstorms. storms. From these desert species, more than 115 volatile oils have been identified, as high a number as is known from any biogeographic region in the world. In particular, the researchers Gary Nabhan, Eric Dougherty and Tammi Hartung identified more than 60 potent fragrances emitted from the foliage and flowers of desert plants during the monsoonal rainy season of the iconic “Sonoran Desert summer.”
The authors hypothesize that the a “suite” of 15 particular BVOCs emitted from this diversity of desert plants during the monsoons may function synergistically to generate tangible health benefits. Just 5 of these fragrances confer most of the health benefits now amply documented half-way around the world along the “forest bathing” (Shinrin-Roku) trails used by millions of Japanese and Korean dwellers to reduce the stresses of their urban lifestyles.
Many of these BVOCs can be readily absorbed by the human body through inhalation, so that they register within the brain in as little time as 22 seconds. It then takes less than 90 more seconds more for them to be released into the bloodstream. Within a half hours’ time, they may be found present in every cell of the body and reach all the body’s organs. It takes two and a half hours or less for most of therapeutical aerosol inhalation of volatile oils to be metabolized in ways that may potentially affect human health in a more lasting manner.
The fragrant BVOCs from desert plants may in many ways contribute to improving sleep patterns, stabilizing emotional hormones, enhancing digestion, heightening mental clarity, and reducing depression or anxiety. Their accumulation in the atmosphere immediately above desert vegetation can reduce exposure to damaging solar radiation in ways that protect the desert plants themselves, the wildlife which use them as food and shelter, and the humans who dwell among them. As climate change accelerates, regular exposure to these BVOC health benefits may become more important to prevent or mitigate diseases of oxidative stress and other climate maladies in a hotter, drier world.
The lead author—Dr. Gary Paul Nabhan of the University of Arizona Southwest Center– has recently been co-designing “desert smells like rain gardens” in public spaces like the Sonoran Desert Inn and Conference Center in Ajo, Arizona; the base of Tumamoc Hill at the University of Arizona Desert Laboratory in downtown Tucson; and the Seri Indian (Comcaac) fishing village of Punta Chueca, Sonora Mexico. These public gardens will not only produce nutritious foods, but offer residents, out-of-town guests, and hikers a powerful opportunity to sense how the desert smells like rain. Nabhan’s classic natural history book by the same title was first published 40 years ago this spring and will be re-released in a 40th anniversary edition with a new introduction this year by the University of Arizona Press.
Nabhan GP, Daugherty E, Hartung T. Health Benefits of the Diverse Volatile Oils in Native Plants of Ancient Ironwood-Giant Cactus Forests of the Sonoran Desert: An Adaptation to Climate Change? International Journal of Environmental Research and Public Health. 2022; 19(6):3250. https://doi.org/10.3390/ijerph19063250
Nabhan GP, Daugherty E, Hartung T. Why Does the Desert Smell Like Rain?: “Bathing” in the Diverse Fragrances of Plants and Soil Microbes in Ancient Ironwood-Giant Cactus Forests of the Sonoran Desert. Desert Plants, 20 2 (February); 37(2):18-36.