To humans, a desert oasis may conjure an image of a blue pool encircled by a coronet of palm trees. But to certain mosses, an oasis takes the form of a pebble of milky quartz. The cloudy crystal dilutes the sun’s piercing ultraviolet rays and, in the dry desert heat, traps moisture beneath it, creating a microclimate perfect for a moss.
Kirsten Fisher, a biologist at California State University, Los Angeles, first spotted these miniature oases in 2014, somewhere off a highway in the western Mojave Desert in Wrightwood, Calif. She was studying the sex life of the moss Syntrichia caninervis, and progress was slow.
“They’re never having sex,” she said. “For a moss, it’s maybe once every 30 years.”
The site was studded with crystals eroded from a nearby mountain striped with pearlescent quartz. While waiting for Jenna Ekwealor, now a doctoral student at the University of California, Berkeley, to finish up a transect of S. caninervis growing in the dirt nearby, Dr. Fisher picked up one of the many glittering rocks scattered nearby and, she recalled, saw a brilliant green carpet of the moss underneath: “And I said, ‘Holy moly, there’s moss under this rock.’”
Ms. Ekwealor was sure the green moss was a fluke. It had not rained in Wrightwood for at least two weeks. But as she and Dr. Fisher flipped over more rocks, they found several patches of strangely moist moss. Dr. Fisher and Ms. Ekwealor published documentation of this relationship last week in the journal PLoS One.
“I never thought to look under the rocks,” said Brent Mishler, a biologist at Berkeley who studies Syntrichia but was not involved with the research, with an exaggerated face-palm over Zoom. (Dr. Mishler has mentored Dr. Fisher and Ms. Ekwealor.)
Bunking under a rock can be luxurious. Astrobiologists have long studied arctic and Antarctic cyanobacteria that grow under translucent rocks to trap moisture while still being able to photosynthesize. But no one has studied this survival strategy, used by organisms known as hypoliths, in anything as large as moss.
“No one’s really looked at this combination before,” said Kimberley Warren-Rhodes, a research scientist at the NASA Ames Research Center, who peer-reviewed the study.
Last September, the researchers placed sensors underneath the pebbles to measure how the microclimate changed with the seasons. Some sensors fell prey to the middens of enterprising wood rats, but those that remained found that the quartzite underbellies preserved approximately twice the humidity of the surrounding area and buffered the temperature swings by as much as 7 degrees Fahrenheit. “It’s a little quartz house,” Ms. Ekwealor said.
S. caninervis, the most common moss in the Mojave Desert, spends most of the year parched and brown — in a state of suspended animation awaiting the next rain. “It is something only a mother can love,” Dr. Mishler said. But the mosses are long-lived; a single clump could easily be a centenarian.
Although S. caninervis made up more than two-thirds of the hypolithic moss at Wrightwood, the researchers identified another species, Tortula inermis. That moss typically grows at lower, hotter elevations, but was able to thrive at the Wrightwood site, seeming to rely on the quartz for protection from the cold.
These quartzite oases, while common at Wrightwood, only emerge in what Ms. Ekwealor called a “goldilocks” situation. If the quartz is too tiny, it will be too easily windswept to let anything grow underneath. If it is too large or opaque, not enough light will shine through for photosynthesis. If it is too clear, it could become a miniature greenhouse and capture even more heat. The quartz needs to be just right: around an inch thick and milky enough to transmit up to 4 percent of incident light.
But the vastness of the desert and the abundance of pebbles means that serendipity can become commonplace, Ms. Ekwealor said: “It’s low probability, but lots of opportunity,” she said.
The study highlighted the importance of microenvironments that may be invisible to the human eye, Ms. Ekwealor added. Dr. Warren-Rhodes noted that hypolithic communities, tiny as they are, affect carbon cycling and soil conservation.
After presenting this research at several conferences, Ms. Ekwealor said she now receives sporadic texts from people identifying hypolithic mosses across the country. “I hope people start flipping rocks to see what else is out there,” she said. After a pause, she added, “And gently placing them back down again, so the moss can survive.”
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