This is a creosote bush, Larrea tridentata, that I saw on my first trip to Joshua Tree National Park with my wetlands class. We had gone to see the desert riparian habitat, but stopped off at this creosote patch, as I had never seen them before and was interested in seeing them up close.
They grow in discrete patches... originally it was thought that the compounds they produce kept other seeds from germinating. Now it is known that it is not chemicals, but their superior water gathering skills. There is no water around them to allow for a seed to germinate, they suck it all up!
The compounds that they produce inhibits digestion of the leaves and keeps it from being grazed by too many creatures, although there are some insects which have evolved to deal with the toxins (the creosote katydid and grasshopper). The leaves themselves are able to retain water, as they are thick and waxy. The creosote bush has been known to survive for 2 years without water.
There are three distinct populations of creosote bushes, and while they all share the same scientific name, they are genetically very different. The Chihuahuan desert population (western Texas and also in South America) has 26 chromosomes (2n), and is the founding population for the other two populations. The Sonoran desert population (Arizona) has 52 chromosomes (4n), and the Mojave desert population (California) has 78 chromosomes (6n). The Mojave population is the newest population, only coming into the area a little less than 11-12,000 years ago.