If you’re lucky enough to live in a place where nature puts on a show in the fall, your nose might know it’s fall as soon as you step outside.
“This is what fall is all about. The leaves are falling from the trees and the bacteria and fungi in the soil are actively digesting them,” said Theresa Crimmins, director of the US National Phenology Network. “And in the process, various [gases] are being released, and that’s a lot of the smells.”
So, if fall is in full swing and yet your local foliage isn’t showing up, the holdover could be related to the warming climate.
In the fall, scents can be clearer and more prominent because of changing atmospheric conditions, Crimmins said. (Phenology is the study of cyclical events in nature, such as when leaves change color or animals migrate, as well as how they are influenced by forces such as climate.) The heat and humidity of summer air it catches all kinds of smells, he said. creating a “measure” for our noses.
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But as the days get cooler and crisper, there are fewer volatile organic compounds in the air and we can better distinguish those released by dying and decaying vegetation, leaving that sweet smell front and center.
Temperature also plays a crucial role in telling trees and other vegetation that it’s fall, Crimmins said, in addition to cues from humidity and day length, which govern how much sunlight they receive. Leaves change color in the first place because chlorophyll, the pigment they use to make food as well as what makes them green, begins to break down. This process reveals the golds and oranges and reds that were previously invisible to us, Crimmins explained. The trees then reabsorb nutrients from the leaves before they fall to stock up on resources in anticipation of the following spring.
Decaying leaves that have fallen to the ground give off their own earthy smells. Photo by Getty Images
But not everyone is guaranteed a spectacular annual show. Generally speaking, autumn has started to appear later in the year compared to previous decades, mainly due to warmer average temperatures driven by climate change. And when trees endure drought, perhaps during a long, hot summer, they sometimes drop their leaves early in the year because “the cost of leaves outweighs the benefit,” said Benjamin Wong Blonder, an ecologist at the University of California Berkeley College of Natural Resources.
Sometimes they don’t bother with their usual fiery displays due to the stress of previous seasons. After a particularly dry year, much of the East Coast is not expected to have particularly vibrant foliage.
Many tree species are not adapted to the climatic extremes we are already seeing. As the effects of climate change are expected to worsen in the coming years, this trend is likely to persist: autumn with unusual timing, leaf fall and no guarantee of colorful production.
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That’s bad news for trees because the timing of when leaves first emerge and when they fall is crucial to their long-term health, Blonder added. He also noted that his team’s research suggests that leaf fall patterns may be influenced by climate stresses that happened even a few years earlier.
“Imagine you’re this plant, and once you start growing, you’re stuck there. And you might be stuck there for hundreds or thousands of years, depending on the species. And every year, the climate might be a little different,” Blonder said. “It can be hot and dry one year, or very wet the next. So being able to deal with all that variation without being able to move anywhere is a real challenge.”
To get a sense of when various seasonal events are happening, including when the leaves change color, Crimmins’ organization invites nature enthusiasts turned citizen scientists to report what they’re observing. They can use this data to better understand the phenomenon of phenology, while for you, it’s just another way to use your senses for all the fall feels.
