Those who have traveled the American Southwest--or just about any warm, arid region of the world, have surely seen endless vistas of Mesquite. American ranchers have long considered it an invasive weed species, but I think that it is one of the most valuable gifts in our pantheon of useful native plants.
Mesquite, or the broader genus Prosopis that is found throughout the world's warm arid regions, is incredibly useful and underestimated. It is incredibly drought hardy, thriving on rainfall alone in the Sonoran Desert (and elsewhere), a location that receives around 12" or rain per year. Mesquite tap-roots have been discovered over 200' underground in copper mine tunnels--this is one hardy plant that enjoys water when it gets it, but can survive long and intense droughts. It is a leguminous tree, and is very effective at fixing nitrogen in the surrounding soil. It provides excellent, high-protein browse for livestock, its pods making excellent poultry forage. Its flowers are great attractors of polinators, and mesquite honey is renown (as is its cousin, acacia honey) for its excellence. It grows rapidly, especially considering its environment. It is probably best known today as a source of wood-chips to flavor BBQ dishes, but its wood is an excellent and very energy dense form of firewood and charcoal. With its root system established, it makes an excellent copice species--part of the problem that Western ranchers face when trying to get rid of it. It is also prized as a hardwood for structural timbers, furniture, and tool making. In fact, it is such an efficient creator of wood from marginal lands that it is being considered as a prime species for ethanol production. It is also semi-deciduous, and its leaf drop is an effective desert mulch in wind-protected areas. It has numerous traditional medicinal uses, many of which are being validated by modern medicine. It is a long-lived, zero-care tree that is a fascinating potential component for dry-land arboriculture.
Sounds like a pretty valuable tree, right? I left out the most interesting part. Its seed-pods are a traditional human food. Mesquite drops exceptionally hard seeds encased in elongated seed-pods. These pods themselves are dried and ground up into "Mesquite Flour." However, this "flour" is nothing like its wheat-based namesake. Mesquite flour is moderate sweet, has about 100 calories per ounce, and is very high in protein, fiber, and minerals. Perhaps most importantly, its carbohydrate content has a very, very low glycemic index--about 25 (mesquite flour is 80% carbohydrate, 25% being fiber, 13% protein, 3% fat). The Tohono O'odham indians of Arizona used to subsist primarily on mesquite flour and are now decimated by diabetes as they have almost completely abandoned it in favor of wheat flour. From a primitivist-diet perspective, mesquite flour does not have the same objections as grain flours do--the glycemic index is very, very low, the actual seed is not eaten, and the plant evolved an evolutionary system where the seed is extremely hard, but the pod (from which the flour is made) is very palatable to animals. Finally, one acre of hands-off mesquite production produces in the area of 2500 pounds of mesquite flour annually--that works out to over 4 million calories, or 10,000 calories per day.