Sharp elbows in the desert

How do plants do it...

...some stand-off-ish plants have figured out how to keep their distance from each other. They have evolved clever adaptations - sharp elbows - to keep their neighbors away. This behavior is important for survival, especially in the desert where plants compete for water.  

I have noticed this phenomenon especially when I've been on a plane flying low over the desert. Where there are plants, they are uniformly distributed but not touching each other. They don't form clumps. They seem to know how far apart to grow. How is that?

Why is it these desert plants don't form clumps? They weren't planted this way on purpose (Palm Springs, CA).

It seems so obvious to me now that I recognize what is going on, but it took me a few years of hiking in the desert to be mentally conscious of how widely separated, but yet uniformly distributed in the landscape many plants seem to be. How is it that plants are able to keep what appears to be a relatively uniform distance from each other? And, even stranger, plants of the same species are separated, but at the same time often surrounded by plants of a different species.

Yellow blossoms and fuzzy fruit of the creosote bush, Larrea tridentata

Chemical elbows

The creosote bush (Larrea tridentata) is ubiquitous in desert southwest of the US and northern Mexico. It has been one of the most widely studied plants for its ability to disperse inhibitory chemical messages to its neighbors. These chemicals inhibit the germination of seeds and plant growth of any competing plant in the immediate surroundings. The gives the creosote plant a survival advantage.

The ability of plants to produce chemicals to inhibit competing plants has been called allelopathy (the word literally means reciprocal perception). The study of this phenomenon has increased in recent decades due to the possibility of using natural methods of weed suppression, rather than broadly non-specific toxic chemicals in agriculture. These natural chemicals have been called allelochemicals.

Other plants common to the desert also have this ability. The allelochemicals that each plant produces may be different, but the results are the similar: competitors are inhibited from growing too near. Notably, sage brush and brittlebush also have this remarkable ability to spread out in a uniform distribution.

Uniformly distributed creosote bushes, along a switch-back trail rising in the distance

The palms are happy to clump together along the stream, but the common fish-hook barrel cacti and brittle bushes prefer to keep their distance from each other, with the assistance of allelochemicals (Palm Springs, CA)

More than just sharp elbows

In addition to allelochemicals, there are other ways plants have adapted to keep competitors away. For example, the plant might provide unique shelter for a particular bird or animal that eats all of the seeds which might fall into the zone surrounding the plant.

Another adaptation is the widely distributed fine roots which surround the plants, effectively vacuum up all of the available moisture in the soil, preventing any competitor's seed from germinating.

Yellow blossoms of the brittlebush, Encelia farinosa 

Brittlebush blossoms, with a bee

Uniform distribution of brittlebushes in full bloom (Palm Springs, CA)

Evenly distributed mixture of plants, mainly creosote bushes and saguaro cacti (Saguaro National Park, Tucson AZ)

While some cacti have the ability to produce allelochemicals, others do not. For example, the saguaro cacti reportedly do not use allelochemicals to inhibit the growth of nearby cacti. Rather, they appear to compete by widely dispersed shallow roots which efficiently absorb water, starving other plants from growing too near.

I'm always surprised by the complexity of the environment that surrounds us. If we to continue to ask questions - and scratch a little bit to find the answers - what we find is often amazing and beautiful!

References:

Cheng F and Cheng Z (2015) Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy. Front. Plant Sci. 6:1020. doi: 10.3389/fpls.2015.01020

Brahmsteadt, Taryn R. 2024. Encelia farinosa, white brittlebush. In: Fire Effects Information Systems, (online). U.S. Department of Agriculture, forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (producer).

Mata-Balderas, J., et. Al. Structure and Diversity of Larrea tridentata. Microphyllus Desert Scrub in Northeast Mexico.Interciencia, June 2018, vol.43, Number 6.

Hierro, J.L. The Ecological Importance of Allelopathy. Annual Review of Ecology, Evolution, and Systematics. 2021.52:25-45.