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Formerly called Leguminosae after the type of fruit most of its members have -- a legume (bean pod) -- leguminous trees, along with columnar cactuses (e.g., Saguaro) characterize the Sonoran Desert. In fact, north of the Mexican border, most of the common trees in the Sonoran Desert are legumes (all the trees spotlit here are legumes). Legumes occur as forbs and shrubs as well as trees and were found to comprise 8% of the 660+ plants in the Tucson Mountains (Rondeau, R.J., et al. 1996)(this link takes you to the comprehensive website). They have their origin and highest prevalence in arid tropical environments, attesting to the tropical influence we have in our Tucson flora (and fauna).

Legumes are critical to the desert ecosystem in many ways. Some of these are listed below:

  • As Food: many organisms (including humans) eat their pods and other parts.
  • As Shelter: many organisms use legumes (especially shrubs and trees) for nesting, for escaping predation, for ambushing prey, and the escaping the elements. Leguminous trees serve as nurse plants, providing protection from trampling, the hot sun, and maybe most importantly, from frost at night to other plants (e.g., the saguaro).
  • As Soil Enhancer: legumes indirectly add nitrogen, a critical nutrient often missing in desert soils, to the soil around them. They do this by providing root nodules for special, nitrogen-fixing bacteria. The bacteria take unusable (to most organisms) nitrogen from the air and convert it into usable nitrogen that then gets into the soil for other plants. Farmers will plant legumes (e.g., alfalfa, soybeans, etc.) in their fields to recharge the soil's nitrogen.

There are three subfamilies, each with different flowers.

  • Caesalpinia Subfamily (includes Blue and Foothills Paloverdes): Flower has 5 separate petals, one of which is called the banner petal because it is different in size, shape, or color from the rest of the petals. Each flower has 10 separate stamens. All members of this family are woody. The two other subfamilies are descended from this basic pattern.legume
  • Mimosa Subfamily (includes Catclaw and Whitethorn Acacias, Velvet Mesquite, and Fairy Duster): The petals are fused and so tiny as not to be easily seen. But the stamens are long. Many flowers cluster tightly to form a ball (powder-puff) or cylinder (catkin). As in Caesalpinia, all species are woody.
  • Papilionoid (Pea) Subfamily (includes Desert Ironwood): Flowers have 3 upper petals (banner and wings) and 2 lower petals fused along the bottom (keel). Nine of the 10 stamen filaments are fused and the 10th is separate and they (and the stigma) are housed in the keel. In short, these are the typical sweet-pea flowers.