Prosopsis glandulosa

 Prosopsis glandulosa (honey mesquite) (mezquite dulce)

Fabaceae

Prosopsis glandulosa var. torreyana

• Although most common in the Sonoran Desert areas of California, honey mesquite extends its range across lower elevations of the Mojave Desert as far north as Death Valley and over huge areas of desert and semi-desert in the southwestern United States and adjacent Mexico. It is a classic phreatophyte with roots that tap groundwater pools as deep as 30 feet or more below the soil surface. In semi-arid grasslands of the southwestern United States with summer rainfall, however, it behaves less like an obligate phreatophyte and readily invades desert grasslands. It must be remembered, however, that plant roots do not grow through dry soil actively seeking out groundwater. Roots will only follow moist fracture zones in soil profiles. Honey mesquite is quite variable in growth form. It may form a broad shrub 20-30 feet in diameter with multiple stems, particularly on dune habitats. Each pinnate leaf of honey mesquite is divided into two clustered sets of oblong-linear leaflets, with paired straight spines at each node. All leaves senesce and are lost in early December. 
• The plants are leafless for about three months until early March, when a cohort of new leaves are formed. Leaf production is not continuous but is constrained to a limited period of weeks and is followed by flowering and pod production in spring. Surprisingly, a second, smaller cohort of leaves and flowers is produced in August under the hottest conditions of the summer. It may seem strange that the honey mesquite should lose its leaves during the winter months -when rainfall is most likely to occur- and form new leaves in the heat of the summer even when it doesn't rain. The reason for this appears to be that the timing of leaf loss and growth arose as adaptations in regions with dry winters  and summer rainfall. 
• Honey mesquite in California still retains ancestral traits that are better suited to conditions present n the rest of its range. As a phreatophyte tapping groundwater for its growth, there is little limitations of water resources for spring and summer growth, and cool winter conditions limit potential photosynthesis. Thus, there have not been sufficient evolutionary pressures on honey mesquite in the winter rainfall regime of California to evolve away from this winter deciduous habit.
• Honey mesquite possesses nitrogen-fixing root nodules, and, with the help of symbiotic bacteria, fixes nitrogen into its root tissue. This is an important aspect of its ecological success. Being phreatophyte and using deep roots to tap groundwater allows mesquite to decouple itself from the limiting factor of water availability. Once water is no longer limiting, nitrogen availability takes over as limiting for growth. With nitrogen-fixing root nodules, however, this limiting factor is also removes-allowing for remarkable rates of productivity by mesquite trees. As mesquite roots die and decompose, they release nitrate-a form of inorganic nitrogen available for plants uptake-which greatly improves soil fertility. Nitrate, which is water soluble, normally leaches out of soils, but dry conditions allow for the buildup of soil nitrate. For this reason, many mesquite stands have been cleared for agriculture in areas where irrigation water can be supplied.
• In many respects, honey mesquite is a keystone desert species. Its broad canopy and high rate of growth provide shade for many animal species. Equally important is its high productivity and ability to fix nitrogen, which allows honey mesquite to produce huge annual crops of protein-reach leaves and pods. These calories provide an important food source for many animal species. Native Americans have long used the pods of the honey mesquite tree for food. Mesquite wood is extremely dense and has been rather overused for fence posts, building materials, and fuel. "Califoria Desert Plants, Philip W Rundel, Robert J Gustafson, Michael E Kauffmann."