Seventy-eight percent of the earth's atmosphere is composed of nitrogen gas (N₂) but this is not available as a nitrogenous nutrient for plants. Instead, it must be "fixed" (reduced) into ammonium (NH₄) in order to be used as a nutrient - a process that only some bacteria are able to do. Some of these nitrogen-fixing bacteria (Rhizobium and closely related bacteria) live in association with leguminous plants - the plant family that used to be called Leguminosae, but now is called Fabaceae. Some of the nitrogen fixed by these bacteria is passed to the plants - an important contribution to the fertility of deserts.

Three of the main types of tree that occur in desert regions have nitrogen-fixing bacteria in their roots - mesquites, desert ironwood and paloverdes.

Mesquites

There are two common mesquites in the deserts of southwestern USA - velvet mesquite (Prosopis velutina) and honey mesquite (Prosopis glandulosa), although these two species can hybridise where their ranges overlap. They are important nitrogen-fixing trees, with pinnate leaves which are shed in periods of drought. The leaves are an important source of food for browsing animals, and the seed pods also are eaten by many animals.

Mesquite in full leaf after a period of rains.

Mesquite inflorescences are clusters of pale yellow catkins, about 7 cm long. They are pollinated by bees, resulting in legume-like pods later in the season. (See bruchid beetles for an image of honey mesquite pods).

Paloverde

There are several types of palo verde (which means "green stems"): foothills palo verde (Cercidium microphyllum) which is usually found with saguaro on desert plains and rocky skopes of foothills and mountains; blue palo verde (C. floridum) usually found in washes, and Mexican palo verde (Parkinsonia aculeata) usually in moist valley soils. All can produce small leaves after periods of rain, but the leaves are shed in drought conditions and these plants typically photosynthesize through their stems.

Photosynthetic stems of a palo verde

Paloverde produces large numbers of yellow flowers each year, pollinated by bees

Fruits of foothill paloverde. These fruits and those of other nitrogen-fixing trees such as mesquite are often colonised by larvae of the seed-eating bruchid beetles.

Desert ironwood

Olneya tesota (Leguminosae) is a frost-sensitive tree, found only in the southernmost part of the USA, but more commonly in Mexico. Because of its frost-sensitivity, its presence is used as an indicator of sites where it is safe to grow citrus (also frost-sensitive) when desert land is cleared for irrigated agriculture. Unlike the other nitrogen-fixing trees, desert ironwood usually retains its leaves over most of the year - even in periods of drought.

Typical growth form of desert ironwood, but trees such as this are becoming increasingly rare because there is a very high demand for the hard wood, which is used for making ornaments.

Pinnate leaves of desert ironwood

Desert ironwood flowers