Desert Plants The Ultimate Survivors
Desert shrubs, flowers and other plants have found amazing ways to survive heat, drought and poor soils
Three years into the worst drought on record, farmers in California have taken action to deal with the lack of water. Some farmers have drilled new wells deep below ground. Others are leaving fields fallow, waiting out the drought until there’s enough water again to sow their crops. Still other farmers have moved to greener, wetter locations.
When nature does not provide enough water, farmers use their brains, brawn and plenty of technology to find solutions. As clever as those solutions may seem, few are really all that new. Many desert plants rely on similar strategies to beat the drought — and have done so for thousands if not millions of years.
In the deserts of the southwestern United States and northern Mexico, native plants have come up with amazing tricks to survive, and even thrive. Incredibly, these plants routinely cope with punishingly dry conditions. Here, plants can go a year without seeing a drop of rain.
How they manage has attracted the interest of scientists. These researchers are uncovering all kinds of strategies used by desert plants to survive and reproduce. For example, the mesquite tree counts on finding better conditions elsewhere. Rather than moving — which it can’t do on its own — this plant relies on animals to eat its seeds and then scatter them with their feces. Meanwhile, the creosote bush partners with microbes in the soil. Those microbes help it survive the very real stress of living in a persistently hot and dry climate. And many wildflowers gamble with their seeds in a way that can help them outlast — and outfox — even the worst drought.
Digging deep for water
The Sonoran Desert is located in Arizona, Calif., and northern Mexico. Daytime summer temperatures often top 40° Celsius (104° Fahrenheit). The desert cools off in winter. Temperatures at night can now fall below freezing. The desert is dry most of the year, with rainy seasons in summer and winter. Yet even when the rains come, the desert doesn’t get much water. So one way these plants have adapted is to grow very deep roots. Those roots tap into sources of ground water far below the soil’s surface.
Velvet mesquite (Prosopis velutina) is a common shrub in the Sonoran desert. Its roots can plunge down more than 50 meters (164 feet). That is taller than an 11-story building. This can help slake the thirst of a full-grown mesquite, a shrub related to beans. But seedlings must find a different solution as they begin to sprout.
Before a seed can take root, it must land in a good place to grow. Since seeds cannot walk, they rely on other methods to spread out. One way is to ride the winds. Mesquite takes a different approach.
Each of these plants produces hundreds — even thousands — of seedpods. The pods look a lot like green beans but taste sugary sweet. They’re also very nutritious. Animals (including people) can eat dried mesquite pods. However, the seeds themselves, which grow inside the sweet pods, are rock hard. When animals eat the pods, the seeds’ hard coating allows many of them to escape being crushed by chewing. The hard seeds travel all the way through the gut. Eventually, they come out the other side, in poop. Since animals are often on the move, they can shed the seeds all over the desert.
After an animal scatters mesquite seeds across the desert, the seeds don’t sprout right away. Instead, they lie in wait for rains — sometimes for decades. Once enough rain does fall, the seeds will sprout. Now, they face a race against the clock. Those seeds must quickly send down deep roots before the water dries up.
Steven R. Archer studies how this works. He is an ecologist at the University of Arizona in Tucson. It’s in the heart of the Sonoran Desert. “I study ecological systems, which means the plants and the animals and the soils and the climate and how they all interact with each other,” he explains.
The Sonoran Desert doesn’t get long, sustained drenching rains, he notes. Most rain falls in short little bursts. Each might deliver just enough water to wet the top inch (2.5 centimeters) of soil. “But during certain times of the year,” Archer notes, “we get quite a few of those pulses of water.” A pulse is a short burst of rain. It might last anywhere from a few minutes to an hour.
Another common desert plant — the creosote bush — has adopted a different survival strategy. It doesn’t rely on deep roots at all. Still, the plant is a real desert survivor. The oldest creosote bush, a plant in California called the King Clone, is estimated to be 11,700 years old. It is so old that when it first germinated, humans were only just learning how to farm. It is much older than the pyramids of ancient Egypt.
Also known as Larrea tridentate, this plant is extremely common throughout large areas of the Sonoran and Mojave (moh-HAA-vee) deserts. (The Mojave lies to the north of the Sonoran, and covers portions of California, Arizona, Nevada and Utah.) The creosote bush’s small, oily leaves have a strong smell. Touching them will leave your hands sticky. Like mesquite, creosote produces seeds that can grow into new plants. But this plant also relies on a second way to keep its species going: It clones itself.
Cloning might sound like something from a Star Wars movie, but lots of plants can reproduce this way. A common example is the potato. A potato can be cut into pieces and planted. As long as each piece includes a dent called an “eye,” a new potato plant should grow. It will produce new potatoes that are genetically the same as the parent potato.
After a new creosote plant lives for about 90 years, it begins to clone itself. Unlike a potato, creosote bushes grow new branches from their crowns — the part of the plant where their roots meet the trunk. These new branches then develop their own roots. Those roots anchor the new branches 0.9 to 4.6 meters (3 to 15 feet) into the soil. Eventually, older parts of the plant die. The new growth, now anchored by its own roots, lives on.
Mesquite and creosote are both perennials. That means these shrubs live for many years. Other desert plants, including many wildflowers, are annuals. These plants live a single year. That leaves them just one chance to produce seeds before they die.
Now imagine if every single one of those seeds germinated following a rainstorm. If a dry spell followed and all the little seedlings died, the plant would have failed to reproduce. Indeed, if that happened to every plant of its kind, its species would go extinct.
Luckily for some wildflowers, that’s not what happens, observes Jennifer Gremer. She’s an ecologist with the U.S. Geological Survey. Earlier, while Gremer worked at the University of Arizona in Tucson, she studied how wildflower seeds avoid making bad “choices.” Sometimes people who place bets use the same strategy. With plants, the strategy isn’t about winning money, however. It’s about the survival of its species.
The wildflower ‘hedge’
Gremer and her team wanted to know how 12 common desert annuals hedged their bets. The experts tallied what share of the seeds germinated each year. They also counted what share of ungerminated seeds survived in the soil. (For example, some seeds end up getting eaten by animals.)
As luck would have it, another ecologist at the University of Arizona, Lawrence Venable, had been collecting data on wildflower seeds for 30 years. He and Gremer used these data for a new study.
Each year, Venable would sample desert soil and then count the seeds of each flower species in it. These represented seeds that had not yet germinated. After each rain, his team counted how many sprouted into seedlings. Venable then would watch the seedlings for the rest of the season to see if they set seeds of their own. Gremer used these data to calculate how many seeds germinated each year and, finally, how many of those ultimately went on to produce more seeds.
She suspected that if a species of desert flower is very good at surviving, most of its seeds would germinate each year. And her suspicions proved correct.
A lack of water makes it hard for plants to grow. That’s something crop farmers in California have seen only too well over the last three years of drought. In the deserts of the Southwestern United States, drought is a permanent feature of life — yet there, many plants still thrive. These plants succeed because they have evolved different ways to germinate, grow and reproduce.