|Agroforestry In-service Training: A Training Aid for Asia & the Pacific Islands (Peace Corps, 1984)|
|Appendix E: Nitrogen-fixing tree resources: potentials and limitations|
The 18,000 species of legumes (Family: Leguminosae) include the vast majority of important N2-fixing trees and shrubs, many of which are in the predominantly woody subfamilies Mimosoideae (2800 spp.) and Caesalpinioideae (2800 spp.). Relatively few of the 12,000 species of Papilionoideae are arboreal, but some of these are of great economic importance. A high proportion of the tested mimosoids (92%) are able to fix N2, contrasted with the papilionoids (94%) and the caesalpinioids (34%). A few nonleguminous tree genera also fix N2, notably the temperate genus Alnus and the tropical Casuarina (Stewart, 1967; see p. 427).
Leguminous trees produce some of the outstanding luxury timber of the tropics (NAS, 1979). Notable among these are the papilionaceous genera Dalbergia (rosewood), Perocopsis (African teak), Pterocarpus (narra), and the caesalpinioid genus Intsia (ipil, Moluccan ironwood). Other important timbers include the mimosoids Acacia, Lysiloma, Parkia, and Samanea. Preferred timber species often exceed 30 m in height and are of slow-to-intermediate growth rates. With their high intrinsic value, such trees might wisely be interplanted at wide spacing (e.g., 100/ha) in plantations of fast-growing legumes, as a long-term investment.
The legume trees best known as ornamentals, offering striking displays of color when in flower, are predominantly in the Caesalpinioideae, many of which do not fix N2. The ornamental legumes include:
Caesalpinioideae: Amherstia, Barklya, Bauhinia, Brownea, Caesalpinia, Cassia, Colvillea, Delonix, Peltophorum, Saraca, and Schotia.
Mimosoideae: Calliandra, Samanea.
Papilionoideae: Butea, Erythrina, Sabinea, Sophora.
Several tree legumes provide valuable gums (Acacia spp.) and the pods of several species are excellent human foods, including:
Caesalpinioidae: Ceratonia (carob), Tamarindus (tamarind).
Mimosoideae: Inga. Parkia.
The following discussions will focus on legume trees with special significance as sources of energy or green manure. As a generalization, most fast-growing legume trees are mimosoids. Genera to be considered in the discussions of energy and green manure are listed below, together with their approximate number of species:
Caesalpinioideae: Acrocarpus (3), Cassia (600), Schizolobium (5)
Mimosoideae: Acacia (600), Albizia (100), Calliandra 9100), Desmanthus (40), Mimosa (450), Parkia (40), Pithecellobium (200), Prosopis (44), Samanea (1).
Papilionoideae: Dalbergia (250), Erythrina (100), Flemingia (35), Gliricidia (10).