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 groundwater 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 for 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 seeds all over the desert. Getting eaten helps the mesquite in a second way, too. The hard coating on its seeds also makes it difficult for water to get into them. And that is needed for a seed to sprout. But when some animal eats a pod, digestive juices in its gut now break down the seeds’ coat. When those seeds finally get excreted in the animal’s feces, they will, at last, be ready to grow. Of course, to grow well, each mesquite seed still needs to land in a good spot. Mesquite usually grows best near streams or arroyos. Arroyos are dry creeks that fill with water for a short while after the rains. If an animal goes to the stream to have a drink and then does its business nearby, the mesquite seed is in luck. The animal’s feces also provide each seed with a little package of fertilizer for when it does start to grow.
Taking root
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. The 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. Archer and his team wanted to see how two plant species respond to these pulses. The experts worked with velvet mesquite and a related shrub, cat’s claw acacia (Acacia greggii). In tests, the scientists doused seeds with varying amounts of water. They delivered it in varying numbers of pulses. Later, they measured how fast the seeds sprouted and grew roots. A storm that drops 2 centimeters (0.8 inches) of rain provides more than enough water for seeds of a mesquite or acacia shrub to germinate. That much rain can keep the top 2.5 centimeters of soil wet for 20 days. That period is crucial. Each seedling “has to get a root deep enough the first few weeks after it germinates to survive the long dry period that will inevitably come,” explains Archer. In the Sonoran Desert, in fact, one-quarter of all perennials — plants that live for many years — die in the first 20 days after they germinate. Inside a greenhouse, the scientists planted seeds of velvet mesquite and cat’s claw acacia. They then soaked them with between 5.5 and 10 centimeters (2.2 and 3.9 inches) of water over 16 or 17 days. At the end of the experiment, the scientists measured the plants’ growth. Mesquite seeds germinated quickly. They sprouted after 4.3 days, on average. Acacia seeds, by contrast, took 7.3 days. The mesquite also grew deeper roots. For the plants that got the most water, the mesquite roots grew to an average depth of 34.8 centimeters (13.7 inches), compared to just 29.5 centimeters for the acacia. In both species, the roots grew longer with each additional 1 centimeter of water the plants received. The acacia grew more above ground; the mesquite put most of its energy into growing a deep root as fast as possible. Growing a deep root very fast helps ensure a mesquite’s survival. One study looked at a different type, honey mesquite (P. glandulosa). Most young plants of this species that survived their first two weeks after germination went on to survive for at least two years. That study was published on January 27, 2014, in PLOS ONE.
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