Desert mosses use quartz rocks as umbrellas

Living under a translucent rock can be quite comfortable, if you are a moss in the Mojave Desert.

A graduate student from the University of California, Berkeley, discovered that some mosses in the California desert seek protection from relentless sun and heat by taking refuge under translucent quartz pebbles, essentially using rocks as umbrellas.

The soil under these rocks retains more moisture than the exposed desert soil, Jenna Ekwealor said, while enough light seeps through the milky quartz to allow the small mosses to remain green with chlorophyll. Mosses actually prefer dim light, making these conditions ideal for growth. In contrast, mosses near full sun are dry and black.

A species of desert moss appears capable of surviving at high elevations only under milky quartz, which occurs on outcrops scattered throughout the desert. Mosses do not grow under granite stones in the area because sunlight cannot get to the ground.

“We were there (in the Mojave) studying population biology and reproductive biology of mosses, and picking up these cold quartz rocks, like, oh look at this beautiful rock,” said Ekwealor, a doctoral student in integrative biology who works at the University and Herbal Jepson at UC Berkeley. While all the other mosses were dry and dormant, “he saw that there was moss growing under the quartz and it was bright green. That was the first clue that something was different, that they responded to the environment differently.”

Working with their former master’s thesis advisor, Kirsten Fisher of California State University, Los Angeles, they placed temperature and humidity sensors under milky quartz pebbles to record the microclimate from September last year to February 2020.

“The rock acts as a buffer for extremes of weather,” said Ekwealor. “The desert is high up, it is very hot in the summer and very cold in the winter. And the days can be hot and the nights cold. The rock keeps the mosses below cooler during the hottest parts of the year and warmer during the cold periods. The increase in relative humidity was positive, something important for these plants that dry when the relative humidity is too low. “

Astrobiologists have long studied the cyanobacteria that live under translucent desert rocks, a possible model for the extreme life types that might exist on other planets, but this is the first green plant to take advantage of these natural havens.

“In the desert, for all organisms, it is like life or death all the time,” he said. “So every time you can find a little boost, a little profit, it makes a big difference.”

Ekwealor and Fisher reported their discovery this week in the newspaper. MORE ONE.

Fresh moss

As one of the first land plants to evolve more than 300 million years ago, mosses are well adapted to extreme environments, capable of surviving repeated drying and freezing. Some produce sunscreens to protect against intense ultraviolet light. Some may remain essentially dead, with zero metabolic activity, for a decade, and then revive in seconds when wet. They are found in the Arctic and Antarctic, as well as in the warmer and drier deserts.

Syntrichia caninervis, one of the moss species that often lives under milky quartz, generally grows in soils exposed to biocrusts, an association with lichens and cyanobacteria. These crusts are a feature of many deserts, protecting the soil from erosion and providing nutrients for other plants.

As part of the biological desert crust, Syntrichia are discrete: about 1 millimeter in diameter and 5 millimeters long. They grow only when wet, and since the Mojave can receive only five “precipitation events” per year, half of which could be snow, which is not conducive to growth, they grow slowly. Ekwealor estimates that Syntrichia grows only a couple of millimeters per year.

The rest of the time, Syntrichia mosses on exposed soil remain dormant, turning dark brown-black, possibly as sunscreen, something Ekwealor is trying to discover for his Ph.D. thesis.

He found that, in contrast to exposed mosses, mosses that crawl under a rock grow 60% faster.

“The mosses that normally live at that height are small and brown on the ground surface, and under the rocks they are tall and green,” he said.

At its lowest point, the average relative humidity under the rocks was approximately twice that of the exposed soil: 63% versus 33%. The daily fluctuation of temperature in the “hypolytic microenvironment” decreased by approximately 4 degrees Celsius (7 degrees Fahrenheit) from the soil surface.

A second species of moss, Tortula inermis, commonly grows in full sun at lower altitudes, but at the study site elevation of 1,900 meters (6,200 feet), it appears to grow only under milky quartz.

“Rocks offer mosses two great benefits: a boost in their normal habitat, or they can live in a habitat you wouldn’t normally live in,” said Ekwealor.

Although mosses prefer low light and wet conditions, they, like all plants, require some light or turn white and die. According to his measurements, between 4% and 0.4% of the incident light is transmitted through milky quartz rocks, depending on the size of the rock. At their study site, in a place called Sheep Creek Wash, the quartz was 25 to 14 millimeters thick, or an inch to two-fifths of an inch.

The researchers suspect that quartz not only reduces light and overall heat, but also provides protection against harmful UV rays. Mosses that grow under quartz have less pigmented sunscreen than those that grow in exposed areas.

Ekwealor, self-styled “moss evangelist,” is fascinated by the small world of moss and occasionally leads local moss walks for the bryophyte chapter of the California Native Plant Society.

“For these little plants, something like this giant quartz rock above you that blocks all the sun and keeps you moist is like a huge habitat,” he said. “You may be in the middle of the desert, but a small rock makes you feel like you are in a spring. We have to remember to see the world from the perspective of a very small plant.”