Tuesday, January 27, 2009

Plant Survival Strategies in the Desert

It's been dry out here for a long stretch, and whenever that happens I can't help but marvel at the interesting ways plants have evolved to live in such a dry climate. 

Prickly pear-Opuntia sp.
Xerophytes are plants that have changed their physical characteristics to withstand long periods of dryness. One classic example is a cactus. Instead of having leaves, which have a big surface area and thus lose a lot of moisture to evapotranspiration, a cactus has spines. The stem (cactus pad) is green and contains chlorophyll, acting as leaves do in other plants. However, this stem is often covered with a waxy coating which prevents water from leaving. In fact, many cacti can store water within their stems. They also have shallow, spreading root systems to absorb any available moisture from a brief rainstorm. 

Gray rabbitbrush-Chrysothamnus nauseosus
Another adaptation of plants that live in the desert is very small leaves, so there is less surface area for water to escape. Some plants' leaves have the ability to close their pores (stomata) to keep water in them under the intense sunlight, and some have fewer stomata than similar species found in wetter conditions. The plants may be very hairy to reflect sunlight and reduce the amount of air movement next to the leaves, which will wick away the moisture.

Stonecrop-Sedum lanceolata
Succulent plants, like stonecrop, can store water in the vacuoles, which are specialized tissues in their cells. All cacti are also succulents, and barrel cacti are famous for being able to contain large amounts of water. Many desert plants can also store water in their roots.

Joshua tree-Yucca brevifolia
The metabolism of desert plants is often very slow to reduce energy requirements. A Joshua tree only grows about two feet tall in ten years. A Saguaro cactus takes 30 years to grow just a few feet tall.

If you've looked out over a scene of desert plants, it may have seemed sparse. The plants do not live close together on purpose--there are so few nutrients and so little moisture that it takes quite a bit of space for each plant to obtain them. Spreading root systems help keep others away. In addition, some plants, like the creosote bush, have roots that are poisonous to other plants so that it's not crowded and has plenty of room to get the moisture and nutrients it needs.

Fourwing  saltbush-Atriplex canescens
Most plants need photosynthesis to live. Common photosynthesis uses what's called the C3 cycle, in which the plant fixes carbon into a three carbon compound (phosphoglyceric acid) in order to make carbohydrates. But that's not the only way photosynthesis can occur. Some desert plants like those in the goosefoot family (like the fourwing saltbush above) and bunchgrasses use the C4 cycle, which creates a four carbon compound (aspartate or malate acid). This process is more efficient in maximizing energy.

There is yet another method of photosynthesis, called Crassulacean Acid Metabolism (CAM). It is used by succulents and is even more efficient. These plants open their stomates at night to absorb carbon dioxide, and then during the day use it for photosynthesis. With the stomates open only at night, they use only one-tenth the amount of water as some plants.

Greasewood-Sarcobatus vermiculatus
Other desert plants have developed long root systems to tap into underground water. These phreatophytes are good indicators of a relatively shallow aquifer system--something important for desert travelers to learn to identify. Mesquite trees can have roots that go down 80 feet, while greasewood roots may grow 50 feet to reach water. Many of these phreatophytes, like the creosote bush, will also have some roots close to the surface to capture rainfall. 

Ocotillo-Fouquieria splendens (BLM photo)
Another adaptation to desert dryness is to go dormant when not enough water is available. The ocotillo is a classic example. After rain, it will grow leaves and flower. As the climate dries, the leaves will fall off, and the ocotillo will go dormant again. It may repeat this cycle five times in one year! Greasewood also will lose leaves if its deep root loses contact with the aquifer. It may appear that the plant is dead, but once enough water is present, the leaves will come back (unless it has been dry too long!).

Indian Paintbrush-Castilleja sp.
Annuals like Indian paintbrush will only bloom when their seeds have had the right moisture and temperature. Sometimes the seeds will lie dormant for years and years until the right conditions are met. Then the plant will produce spectacular flowers. In a few days or weeks, the seeds are scattered and await the next set of climate conditions that will allow them to show their beauty again. The drier the conditions, the more annual plants that live there. It's estimated that half the plants in the Sonoran Desert are annuals, while even drier climates may consist of 90% annuals.

Tufted evening primrose-Oenothera caespitosa
Pollination can be a challenge for some desert plants, because not many animals are active in the extreme heat of the day. One adaptation is to flower at night, when more potential pollinators are active during the cooler conditions. 

Another adaptation of desert plants is to go to the water. Plants around springs and streams are extremely different than those just a few meters away, where conditions are much drier. Changes to that water source will reduce the diversity of the plant communities.

As you can see, plants have developed a large number of ways to survive in the desert. Some have developed unique ways to store and gather water (succulents). Others have learned to tolerate drought conditions by going dormant. And some plants avoid drought by staying in the seed stage until the conditions are just right for a short burst of life. Because of these adaptations, desert plants are often easy to recognize. They have to deal with extreme conditions, and in doing so, they have become interesting plants with recognizable characteristics. Although the desert may seem a harsh and inhospitable place, indigenous plants have learned to live--and thrive--with the dry conditions.


english said...

great post! as always, thanks for the time and research you put into these reports from the desert.

I Am Woody said...

I have always loved ocotillo - it just looks so vicious!

Anonymous said...

I also love ocotillo...although I never thought of it as vicious....I still have a dried piece of it from one of the cave trips we did in New Mexico that long ago summer....

}}i{{ Alex said...

Thank you :)

farmerjanis said...

i was just taking out the trash (late dusk/early after dark) and was dive bombed by several very large moths? working over an ocotillo. when i stopped ducking and looked at the plant, there were at least 20 of these insects either flying around or working the ocotillo blooms. i did a quick search to see if there were any known night pollinators of ocotillo but came up empty handed. got any ideas? thanks.

Unknown said...

Thanks -- I learned a bunch!

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