There are a few ways a leaf response could play out. For one thing, the leaves might reflect more or less light, or change in color—getting greener or even red, for example. “We’re definitely looking at green, but we’re also looking at all wavelengths within visible light, and then also those on either side of the visible,” says Stewart. For instance, the chlorophyll in plant leaves fluoresces, reemitting light that special optics can pick up. This signature is invisible to the human eye but not to existing technologies. It’s known as hyperspectral sensing. These devices can actually see across the electromagnetic spectrum, including visible and infrared light. Fluorescence specifically is an indicator of what’s in a plant leaf’s cell walls. Stewart and his colleagues reckon that an amino acid like phenylalanine, which is released from a dead body, could cause leaves to fluoresce in a distinct way.
Because different kinds of plants fluoresce differently, authorities are already using this technology to spot cannabis and poppy grow operations. Farmers and scientists use it for other kinds of agriculture too, because chlorophyll fluorescence is an indicator of productivity. “Hyperspectral imaging has been used to correctly classify pork and other animal fats and is also used for nutrient soil management in detecting phosphorus and other nutrients,” says Mississippi State University environmental microbiologist Heather Jordan. (She does the aforementioned work on pig carcasses but wasn’t involved in this new study.)
But to use this technology to find the remains of missing people, Jordan adds, “there would be a need to differentiate effects from human decomposition with those from other animals’ decomposition.” Humans, after all, aren’t alone among large mammals. Hogs, deer, bears—you name it, and it’s rotting out there in the wild, flooding the soil with the same nutrients a human cadaver might.
However, there could be subtle differences in how plants respond to the decomposition of different kinds of mammals. We humans have distinct microbiomes, for instance, and different fat content than, say, a bear. “I think once we find our key spectral signatures, we can then think about doing a comparative study with other large mammals,” Stewart says.
Another challenge is the variability of decay. It’s one thing to fly a plane over a forest and spot a cannabis grow operation with hyperspectral imaging, but “that’s very different than a randomly located dead person decomposing at an unknown rate,” says Mississippi State University ecologist Brandon Barton, who collaborates with Jordan. Any number of variables influence how a dead organism releases the nutrients and microbes that might affect surrounding vegetation, like how hot it is or how much moisture is out there. All of this, he says, is “a lot of background noise.”
And in Barton’s experiments with pig carcasses, the body often doesn’t stick around for long—at least not the flesh—so ultimately not all the nitrogen stored in a body will be available as a fertilizer. “The chances of all of that nitrogen entering the soil is pretty low,” he says. “Because flies are going to consume tons of it. If a coyote comes along, it will consume a bunch of it. Vultures, crows, all these organisms will be removing it.” If a plant’s leaves change only a little in response to a smaller dose of nitrogen than they would have received if a body was left undisturbed, that could make it challenging for searchers to find a signal.
Since the researchers are just beginning to collect data on how plants might respond to human cadavers, this is all still theoretical. But imaging technology will only get more sensitive over time. Even if this system can’t distinguish humans from other large mammals, if authorities know generally where to look for a missing person, they might scan a few square miles, get some hits on where plants might be changing, and investigate each spot. Some spots might turn out to be covering the remains of deer, but one might lead them to a person. And if it works, for those people’s families, this science may one day bring closure.
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