by Whit Gibbons

August 9, 2015

If you like margaritas, you are beholden to a plant in Mexico known as the blue agave, the source of tequila. Blue agaves are grown as an agricultural crop in the state of Jalisco, Mexico, which means that this cool-looking plant persists with the help of another species, humans. How do other species of agaves manage to survive in the harsh environment of the southwestern deserts of Mexico and the United States without human assistance?

The persistence of desert agave is impressive, considering the soil surface temperatures of more than 150 degrees that must be tolerated in the Sonoran Desert. The roots of an agave seedling remain within three inches of the surface, a very hot spot for a seed. How does a young agave plant manage to survive summer in such an environment? For at least one species, part of the answer is known. A study by researchers Augusto C. Franco and Park S. Nobel from UCLA provided evidence for a phenomenon in which one plant (bunchgrass) positively affects the survival of another (desert agave) in a subtle but previously unsuspected manner.

Ecologists studying the phenomenon found that nearly all the young desert agave seedlings were growing in association with another plant known as desert bunchgrass. In fact, most agave seedlings occurred only in the center or on the north side of the bunchgrass plant. The shade provided by the bunchgrass gave the seedlings protection from the desert sun. Ecologists use the term "nurse plant" for a species on which another plant is dependent. Desert bunchgrass serves as the nurse plant on which many desert agave depend for their survival during early development.

The study also revealed that the soil nitrogen, a critical element for growing plants, was significantly higher around the base of a bunchgrass clump than in open soil areas. Thus agave plants benefited from an enriched soil situation. One detrimental aspect for a seedling agave associated with bunchgrass is that the amount of water available to the roots is reduced, compared to other sites on the desert floor. But the value of the shade that is essential for early survival presumably outweighs the water loss that must be endured by a young agave. Even a tiny shift in favor of long-term survival against the overwhelming odds of near certain early mortality can be crucial in a desert environment where only a limited number of perennial plant species are able to survive and thrive. In essence, without bunchgrass, there might be no agave in some areas.

Botanists have described more than 150 species of agaves, which include century plants and others from which mescal is produced. (Commercial tequila, by Mexican law, can only be made from the blue agave in selected localities; mescal can be produced anywhere from many varieties of agave.) Bunchgrass is nursing along agaves naturally in the desert without our help.

Plant ecologists have discovered many complicated biological relationships, and each time a new one is revealed it is enlightening. Hence it is gratifying to learn that the same investigators subsequently discovered that the impressive saguaro cactuses of the Sonoran Desert also benefit from bunchgrass as a nurse plant during their seedling stage. Considering the massive grandeur of these quintessential cactuses of movies, cartoons and southwestern photography, it is sobering to think that many of them owe their existence to help from another plant when they were seeds.

The desert is a relatively simple ecosystem in terms of the fauna and flora and their interactions. Imagine how complex the ecological network is in an oak-hickory forest or a tropical rain forest with its rich fauna and flora. Findings about delicate yet vital interactions among species give us a clear message that the world and its environments are far more intricate and interconnected than one might think. We need to be increasingly careful in our manipulation and management of natural systems. We ought to understand how all the pieces work together before we start making changes to the environment.

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