Nitrogen-fixing trees for tropics/subtropics:
Leucaena leucocephala (River tamarind) is a small fast-growing mimosoid tree native to southern Mexico and northern Central America (Belize and Guatemala) and is now naturalized throughout the tropics. It is classified as invasive weed in Florida (https://plants.ifas.ufl.edu/plant-directory/leucaena-leucocephala/). It is also called lead tree.
L. leucocephala is used for a variety of purposes, such as firewood, fiber, and livestock fodder. During the 1970s and 1980s, it was promoted as a "miracle tree" for its multiple uses. It has also been described as a "conflict tree" because it is used for forage production but spreads like a weed in some places. It grows quickly and forms dense thickets that crowd out all native vegetation.
It also efficient in nitrogen fixation, at more than 500 kg/ha/year. It has a very fast growth rate: young trees reach a height of more than 20 ft in two to three years.
The young pods are edible.
Gliricidia sepium (common names: quickstick, madre xacao or madre de cacao in the Philippines and Guatemala) is a medium size leguminous tree belonging to the family Fabaceae. It is considered to be the second most important multi-purpose legume tree, surpassed only by Leucaena leucocephala.
The generic name Gliricidia means "mouse killer" in reference to the traditional use of its toxic seeds and bark as rodenticides.
G. sepium was spread from its native range throughout the tropics to shade plantation crops such as coffee. Today it is used for many other purposes including live fencing, fodder, firewood, green manure, intercropping, and rat poison. Its use expanded following the widespread defoliation of Leucaena by psyllid in the 1980s.
G. sepium is used as cut and carry forage for cattle, sheep, and goats. Its high protein content allows it to complement low-quality tropical forages. G. sepium can tolerate repeated cutting, every 2 to 4 months depending on the climate. Cutting G. sepium causes it to retain its leaves during the dry season when many forage crops have lost their leaves. In some cases it is the only source of forage feed during the dry season.
G. sepium trees are used for intercropping in part because they fix nitrogen in the soil and tolerate low soil fertility, so when they are interplanted with crops they can boost crop yields significantly, without the need of chemical fertilizers. The common name madre de cacao (literally "mother of cacao" in Spanish) used in Central America and the Philippines is in reference to its traditional use as shade trees for cocoa tree plantations.
G. Sepium tolerates being cut back to crop height, and can even be coppiced, year after year. When the trees are cut back, they enter a temporary dormant state during which their root systems do not compete for nutrients needed by the crops, so the crops can establish themselves.