Listen to the yowling, ethereal sounds a Joshua tree can be made to make

The artist Scott Kildall is waving his hand over the contours of a Joshua tree, just inches from its spiky green, bayonet-like leaves.

“If I get too close to it, it will prick me and draw blood,” he says. “And it's done that before.”

In his palm, he has a microcontroller — just about the size of a credit card. It’s got a few wires sticking out, and an infrared sensor, which picks up wavelengths of light just beyond what the human eye can perceive.

“It's kind of like magic,” Kildall says. “And the magic is just revealing something that's right beyond our levels of perception.”

The magic is part of Kildall’s latest sound installation — a work he calls Infrared Reflections. He developed the piece as an artist-in-residence at Joshua Tree National Park this spring, and it transforms near-infrared light bouncing off the iconic scraggly yuccas into a shimmering mosaic of otherworldly music — essentially turning the Joshua tree into an instrument.

Kildall is neither a computer scientist nor a musician, though he does play the ukulele. This artwork relies on both disciplines, as Kildall needs to build sensors, route their data to a computer, process and smooth that data, and convert it all into something beautiful for the ear.

“With art and technology, you have to constantly think about wearing multiple hats. Are you an IT person? Are you an artist? And you have to be able to troubleshoot on the fly,” he says.

The installation relies on a fundamental interplay between the sun and the Joshua tree. When sunlight hits the plant, cells in its healthy leaves soak up lots of red and blue light, and reflect back most of the green – which is why the clusters of piercing leaves at the end of the Joshua tree’s branches appear green to the human eye. (It’s also why most plants on Earth appear green to us.)

But other types of light rain down on the Joshua tree too, including wavelengths we can’t see, like near-infrared light. The plant’s shaggy, grayish-brown bark doesn’t reflect much of that infrared light, but its healthy leaves — packed with compounds like water, carbohydrates and defense chemicals — reflect much more. And that’s exactly what Kildall sees as he passes his sensor from the yucca’s dead bark to its green, leafy buds.

“Beyond our perceptions lies a whole realm of invisible data,” he explains. “And so what I do is I find some sort of invisible phenomena such as water quality or air quality or infrared light reflection, and then map that data into sounds, so that we can hear that data.”

That technique is known as sonification, and Kildall has previously designed installations that sonify water flow in trees, or tap into the electrical signals of mushrooms. Infrared Reflections follows in that vein, but it’s much more site-specific — it’s meant to be played on the Joshua tree, which is endemic to the Mojave Desert.

With that in mind, earlier today Kildall lugged his laptop, a box of electronics and a giant speaker into a field full of Joshua trees at the Arthur B. Ripley Desert Woodland State Park, north of Los Angeles.

Gusts of wind scream through the high desert here, and Kildall eyes individual Joshua trees with the eagerness of a kid at Guitar Center, ready to pick out an axe.

“I see one Joshua tree that's about a hundred feet away that is moving a little bit in the wind and has multiple leafy sections and bark sections,” he says. “And that one really looks like it is asking to be sonified.”

After a short walk, Kildall sets down the speaker and begins fiddling with his laptop. It’s perched on a camp chair, in direct sun, and refuses to boot up — a reminder, Kildall says, of how preparing for a field installation is like planning a NASA space mission (though with non-lethal stakes.)

“You want to reduce the number of points of failure. And so with the system I have, I have backup electronics, I only have one laptop. So that's the only point of failure that I'm really worried about.”

But not to fear. The machine soon boots up, along with a local Wi-Fi network — which connects the small infrared sensor to the computer — and Kildall is ready to rock.

As he caresses the air just above the tree, the yowling of a theremin rises and falls from the speaker, lending an eerie soundtrack to the already alien-looking landscape. A few hikers walk by, but seem unphased by the guy performing reiki on a Joshua tree. The high desert is known for its eccentrics, after all.

Kildall has designed three more “instruments” to be played like this, along with the theremin sound.

His favorite combines the drone of a theremin with spiraling arpeggios.

Another sounds more like haunting electric guitars, reverberating in a cathedral.

The final sound is more pared-back and simple — resembling a kid plinking at the piano.

“In some ways, it feels a little bit more direct and accessible because we all kind of understand how to press notes randomly on the piano,” Kildall explains. “And as I move the sensor over these sort of leafy spots in the sun, those will produce a higher pitch on the piano.”

As he slides the sensor further down the branch, to the bark, the notes get lower and lower, moving deeper into the bass clef.

It’s a fascinating way to make music. The infrared-sensing technology at the heart of the work also happens to be something scientists have employed for decades.

“From our aircraft or satellites, we use it to separate the living vegetation from the dead vegetation,” says Greg Asner of Arizona State University in Hawaii. “That's very important in a grassland — is the grassland dry and ready to go up in smoke in a fire, or is it wet and green and living? And so we can translate that to fire fuel load for grasslands.”

Asner has also soared above the Amazon basin, measuring infrared signals from a plane — which allows him to tease out individual tree species, based on the fingerprints of infrared light that bounce off the trees.

“When I fly over a tropical rainforest with the infrared sensors, the maps literally are like Willy-Wonka-candy-store-style variation. They are amazing to look at,” he says.

“So there's this enormous diversity of plants when you look at them in infrared light. And unfortunately, a lot of people just see them as green because that's all we can see with our limited eyesight. There's much more variety out there. And it's a very beautiful world when you look at it in the infrared.”

Asner says he’s thrilled to see artists now experimenting with the same technology.

“Our studies for decades now have been stuck in the halls of science. And this will help translate what we have come to understand, utilize, love as scientists — it's going to translate it to a much wider audience.”

Kildall says, in some sense, that’s his goal.

“One of the things I like to do is to engage people with nature and issues of climate change and issues of ecology through means other than science articles,” he says. “Science articles are great. I read them all the time. However, they don't engage people on a more visceral storytelling level, as artwork does.”

And though this installation was conceived and developed with the Joshua tree at its center, it could work with the infrared reflections of other plants, if Kildall calibrates his code — which is a good thing, since Kildall calls San Francisco home.

“There aren't any Joshua trees in San Francisco, so I might have to go to Golden Gate Park and see what I come up with.”