Deserts might have a reputation as being barren and devoid of life, but the truth is that many deserts--especially those in the southwestern United States--are ripe with life. The flora and fauna there might be a little non-traditional, but that's because they've had to adapt to the harsh conditions of constant, bright sunlight, high temperatures and little water. Fortunately, there are a number of different ways to do this, and a number of reasons plants can thrive in the desert.
Oftentimes, desert plants will take root along streams, in areas with a high water table, and along areas that will flood during rains. These plants can grow root systems that reach as far as 50 meters to the water table. Others, such as cacti, have shallow, wide root systems that are capable of absorbing as much water as possible in as short a time as possible. Saguaro--among the largest of the cacti species--have root systems that rarely extend deeper than 4 inches.
Most non-desert plants lose moisture through tiny openings on their leaves, called stomata. Desert plants have these features, however, they have adapted them to be better suited to desert life. Many have stomata that are sunken into the leaves, called stomata crypts, that help reduce the amount of moisture lost to the air.
When desert plants do come across water, they have to be able to take advantage of it. Therefore, most desert plants are succulents, which means they have thick, fleshy areas that are used for storing water, often in the leaves, stems or roots of the plant. Cacti are the most obvious example of these, with their thick stems that are perfectly adapted for absorbing and storing water. Some succulents store their water underground in their roots; this also helps the plants reduce the surface area available to lose moisture through by eliminating the need for leaves.
Some desert plants, such as yuccas and some bromeliads, use a version of photosynthesis that is very water-efficient. Called crassulacean acid metabolism, these plants process water and nutrients at night. When water becomes extremely scarce, these plants can shut down their photosynthesis processes almost entirely. This slows their metabolism and allows them to go into a holding pattern in the long, dry spells between rains.
Avoiding the Dry Spell
Some plants deal with the shortage of water in an entirely different way--they avoid it all together. Annual plants will grow and germinate only in a small window of time; usually this is in the fall, and during the rainy season. During the dry seasons, there is little left of these plants but their seeds. In order for these seeds to germinate in their narrow window of time, they must receive enough rainfall in a short amount of time before they will leave their dormant state to grow.
The drier the desert, the greater the proportion of annual flowers there are. In some areas, up to 90 percent of flora are annuals that are dormant for the most brutal part of the year.
Hiding from the Sun
Sunlight is needed for all plants, but too much of it can be just as damaging as none at all. Desert plants can often be found fighting each other for shade; in fact, saguaros spend their first years growing in the shadow of more mature plants before they are strong enough to stand in the sun alone. Shade is a valuable commodity in the desert, whether it's found from other, larger plants, overhangs or crevices.
Many desert plants have small, narrow leaves that reduce the surface area that is exposed to the sun. Many can orient their leaves so the least amount of surface area is exposed, and still others are coated with a salty excretion that helps to reflect the sunlight.
Saguaros are one of the most recognizable of all desert plants, even though they only grow in select areas of the Sonoran Desert--so specific that they are protected by law. Saguaros can live to be over 200 years old; they aren't even considered adult until reaching 125 years. They are very slow-growing plants, but by the time they reach adulthood they can weigh more than 6 tons and reach a height of more than 50 feet.
Saguaros provide shelter for many desert birds and reptiles who make their homes in and around the behemoth. They are also a water source, and will be eaten by some desert animals when water is extremely scarce.
Even though a saguaro can produce more than 40 million seeds in its lifetime, not many reach adulthood. This is because of the tough weather conditions including drought and freezing conditions, animals eating the seeds, and the different uses humans have found for the seeds.
Animals know that succulents store water in their thick stems and leaves, and some can also smell water from miles away. In order to survive, desert plants had to adapt to combat this menace as well.
The spines of the cacti are among the most obvious of these defenses. Others include bad smells and a bitter taste. Many plants are poisonous to the herbivores they share their desert with, and animals are nothing if not fast learners.