Leucaena leucocephala is a small fast-growing mimosoid tree which. It is a legume, and therefore, nitrogen fixing. Leucaena leucocephala is very 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.
It is native to southern Mexico and northern Central America (Belize and Guatemala) and is now naturalized throughout the tropics. Common names include white leadtree, jumbay, river tamarind, subabul, and white popinac.
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.
From Geoff Lawton: "With the cooler weather of winter arriving at the Greening the Desert Project in Jordan, it’s time to chop-and-drop. The dense canopy that has grown over the summer needs to be opened up to allow in both the sun and rain. The canopy will change radically, and the thinning will enable trees beneath it to grow. The dropped material will add lots of organic mulch and nitrogen to the soil, feeding and protecting the soil life. This is how we design the way a forest grows and falls.
For those not aware, placing nitrogen-fixing legume trees between fruit trees will provide natural fertilization on the root level, as well as through the chop-and-drop of the mulching material. There should also be ground covers, often more nitrogen-fixing plants or herbs to distract/attract insects or forage crops for grazing animals, which provides even more natural fertilization as they munch along. In essence, we are assembling support plants and other elements to continuously benefit our productive trees without us having to regularly interact with the system.
Leucaena is the main chop-and-drop tree we’re using here, and it produces a very nitrogen-rich mulch as well as small branches that can be cured into firewood. The trees are pollarded to about head height, removing all foliage to be piled around the base of the trees. Then, the leucaena can regrow several meters throughout a year (Other trees, such as Jerusalem thorn and hibiscus, are performing similar functions but to a lesser extent). This cycle produces a tremendous amount of carbon and organic matter to form quality humus on the forest floor every year.
In the case of a leucaena, within a month of pollarding, lots of sprouts will have emerged along the tree’s trunk and from around the cuts. These tender shoots should be snapped off by hand on a weekly basis, fostering the growth of just three or four vertical branches over the next year. This will keep the trees from crowding the understory, as well as constantly feed the soil with new material. Done on a weekly basis, the pruning is possible by hand, but left longer, cutting back the branches will require a saw. Over time, the damaged areas from where branches have been pruned will develop scar tissue, which prevents new growth in that spot.
The chop-and-drop system feeds the soil and helps to develop fungal systems and mycelium webs to structure the soil nicely. This is how we can use the fast functioning of weedy, legume species to build new forests by design. From there, we adjust to the site’s evolution, adding and amending irrigation here and there. The pollarded trees will regrow over the winter such that they provide shade in the summer when needed, and the healthy cycle repeats."