Pithecellobium saman (Jacq.) Benth.
Leguminosae -- Legume family
Roger G. Skolmen
Monkey-pod (Pithecellobium saman), samán in Spanish, is a fast-growing tree that has been introduced to many tropical countries throughout the world from its native habitats in Central America and northern South America. Although generally planted as a shade tree and ornamental, it has been naturalized in many countries and is greatly valued in pastures as shade for cattle. Short-boled, with a spreading crown when open grown, it forms a long, relatively straight stem when closely spaced. Its wood is highly valued in some locations for carvings and furniture (7).
The most widely used common name for the species is raintree, from the belief that the tree produces rain at night. The leaflets close up at night or when under heavy cloud cover, allowing rain to pass easily through the crown. This trait may contribute to the frequently observed fact that grass remains green under the trees in times of drought. However, the shading effect of the crown, the addition of nitrogen to the soil by decomposition of litter from this leguminous tree, and possibly, the sticky droppings of cicada insects in the trees all contribute to this phenomenon (3). The Hawaiian common name, monkey-pod, is used here because it is a logical derivation of the scientific name Pithecellobium (monkey earring in Greek). Besides monkey-pod, raintree, and saman, which is its name throughout Latin America, the tree is called mimosa in the Philippines.
Monkey-pod is native from the Yucatan Peninsula in Mexico, through Guatemala to Peru, Bolivia, and Brazil (3). It grows naturally in latitudes from 5° S. to 11° N. (13). Cultivated throughout the tropics as a shade tree, it has been found in Burma, Ceylon, India, Jamaica, Nigeria, Sabah, Trinidad, Uganda and the island of Zanzibar (12). The species is naturalized in most of these countries as well as in the Philippines and Fiji (7).
In the United States and its possessions, monkeypod grows in Hawaii, Florida, Puerto Rico, the Virgin Islands, Guam, and the Northern Marianas. It is naturalized in Hawaii, Puerto Rico, and the Virgin Islands (3,10). The tree was reportedly introduced into Hawaii in 1847, when Peter A. Brinsmade, a businessman visiting Europe, returned to Hawaii, presumably via Panama, with two seeds, both of which germinated. One of the seedlings was planted in downtown Honolulu, the other at Koloa on the island of Kauai. These seedlings are possibly the progenitors of all the monkey-pod trees now in Hawaii (1). Monkey-pod may have been introduced into Puerto Rico and Guam as early as the 16th century.
Monkey-pod grows in a broad annual rainfall range of 640 to 3810 mm (25 to 150 in). On wet sites (1270 mm [50 in] or more), its growth is often rapid. This rapid growth is at times objectionable because the tree forms a large mat of surface roots and the crown becomes top heavy, thereby overbalancing the tree (5). In Hawaii, the climate in locations where the tree is naturalized and spreading rapidly has winter maximum rainfall ranging from 1140 to 2030 mm (45 to 80 in), with a temperature range of 10° to 30° C (50° to 86° F). These climatic conditions are found between elevations of 15 to 245 in (50 to 800 ft) at several sites on three islands. Elsewhere, the tree is reported to grow at elevations of 0 to 700 in (0 to 2,300 ft) (15). It is, however, very intolerant of frost and also, if grown near the shore, of windblown saltwater spray.
Monkey-pod attains its best growth on deep alluvial soils that are well drained and neutral to slightly acid in reaction. In Hawaii, most areas to which monkey-pod is well adapted are used for cultivated crops. It has naturalized, however, on gently to steeply sloping Oxisols and Inceptisols on certain sites. On these sites it is most common in gullies where the soil is deeper and more moist than on adjacent hills and ridges. It can, however, grow well on a wide variety of soils when planted and can withstand seasonal flooding (15).
Monkey-pod is frequently found on old home sites near streams in the forests of Hawaii where it is usually associated with mango (Mangifera indica), ti (Cordyline terminalis), guava (Psidium guajava), another escaped domestic plants. Where naturalized, is associated primarily with grasses, although occasionally with such trees or shrubs as koa-haole (Leucaena leucocephala), Java-plum (Eugenia cumini), and Christmas-berry (Schinus terebinthifolius).
Flowering and Fruiting- Monkey-pod may flower at any time of the year in Hawaii, but it usually flowers from April to August, with the pea~ of flowering in May The flowers are perfect and form in umbels. The clusters, with their numerous pink stamens, 3.8 cm (1.5 in) long, look like powderpuffs in the tree crown. The flowers are insect pollinated Seed pods develop in from 6 to 8 months and fall to the ground intact, usually between December and April in Hawaii. The dark brown and relatively straight pods are usually 15 to 20 cm (6 to 8 in) long and contain from 5 to 20 seeds (3,8).
Seed Production and Dissemination- Seeds are reddish-brown beans about 13 min (0.5 in) long that drop from the pods when they open on the ground. Although the seeds are hard coated and long lived, some germinate soon after moistening by soil contact, resulting in a short period of prolific reproduction even under lawn and garden trees. Most or all of the reproduction dies or is destroyed by insects, rodents, and lawn mowing. Seeds are easily collected by gathering pods on the ground and drying them under cover until they open. Natural dissemination is by birds and rodents.
Seeds number from 4,400 to 7,000/kg (2,000 to 3,200/lb) (15). They can be stored dry at 0° to 3° C (32° to 38° F) in closed containers for lengthy periods with little loss of viability. Seeds are normally scarified; they are placed in water at 100° C (212° F), then allowed to cool overnight. Scarified seeds usually germinate 3 to 4 days after sowing.
Seedling Development- Germination is epigeal. Seedlings are usually grown from seed planted in containers. In Hawaii, polyethylene bags are now the most commonly used containers for this purpose. Monkey-pod seedlings have also been grown in seed beds and successfully planted bare-root in Hawaii, but not on a large scale. Severe drought stress usually results in high seedling mortality following bareroot planting. Nursery seedlings are of plantable size in about 4 months (15).
Seedlings grow rapidly if maintained, reaching 2 to 3 m (6 to 10 ft) within 1 year after planting. Natural seedlings, or planted seedlings that are not weeded, are strongly inhibited by competition and grow much more slowly. Seedlings and mature trees are intolerant of shade (15) and extremely susceptible to damage by overspray of herbicides used in weed control.
Vegetative Reproduction- Monkey-pod roots easily Hardwood (leafless) cuttings, ranging in size from 1 by 15 cm (0.4 by 6 in) to stems and branches of mature trees, can be rooted in moist soil on a site without use of mist or shade. In Honolulu, it is common practice to transplant huge trees by cutting away almost all the roots and all the branches. Trees grown at close spacing in the forest frequently have branch-free stems 4 to 5 in (13 to 16 ft) tall and are transplanted to parking lots and parks as "instant" full-size shade trees. Despite the ease with which it can be vegetatively propagated, monkey-pod is almost always started from seed.
Growth and Yield- One of the best known trees of this species is in Trinidad. When a little more than 100 years old, this tree had a trunk 244 cm (96 in) in diameter, was (reportedly) 44.8 in (147 ft) tall, and had a crown spread of 57 m (187 ft) (3). The large, rounded crown of open-grown trees (fig. 1) provides shade over a wide area. Huge trees such as these are extremely difficult to log, so young, smaller trees are sought after for utilization, particularly those that are forest-grown and have long boles.
Although primarily a shade tree, monkey-pod also has potential as a timber tree. After the first year of planting at close spacings in Western Samoa, monkey-pod averaged 4 cm (1.6 in) d.b.h. and 4.4 m (14 ft) tall (2). Because of its large crown, however, it requires wide spacing in plantations. A spacing of 2.4 by 2.4 m (8 by 8 ft) proved much too close in Zanzibar (12). In Hawaii, two plantings at 3 by 3 m (10 by 10 ft) failed, possibly as a result of spacing, but more likely for lack of adequate tending. Monthly weeding around planted trees greatly improved height growth in the Philippines, thus ensuring survival (6). Another planting in Hawaii that covered about 16 ha (40 acres) at 6 by 6 m (20 by 20 ft) was fairly successful and produced many trees with 7 to 10 m (24 to 32 ft), relatively straight, branch-free stems. The growth of this stand, now 85 years old, has never been measured or evaluated, however. Trees in this stand are 18 to 21 m (60 to 70 ft) tall and are about 91 to 122 cm (36 to 48 in) in diameter, and have crowns that are co-dominant in the overstory with Eucalyptus, Ficus, Persea, and other introduced trees that have invaded over the years.
Rate of growth depends on rainfall. In dry areas in Hawaii, diameter growth of open-grown trees is usually less than 13 mm (0.5 in) per year, and total height rarely exceeds 12 m (40 ft). In wet areas, diameter growth usually exceeds 2.5 cm (I in) per year. An annual growth rate of 25 to 35 m³/ha (350 to 500 ft³/acre) was reported, but a source was not cited (15). This rate may be excessive in view of the wide spacing required by this species.
Rooting Habit- Depth of rooting varies with amount of rainfall (3,5). In dry areas with less than 1270 mm (50 in) annual rainfall, monkey-pod roots deeply. In wet areas, the root system develops at or near the soil surface and can become a problem in gardens or near paved roads.
Reaction to Competition- Monkey-pod is intolerant of shade. The leaves of shaded branches remain folded during the day and contribute little photosynthate. Shaded branches die back and improve the form of trees that shade each other.
Damaging Agents- Monkey-pod on the Island of Oahu, HI, is badly defoliated each year by three caterpillars, Melipotis indomita, Ascalapha odorata, and Polydesma umbricola, with most damage attributed to M. indomita (13). The trees promptly leaf out after defoliation, so are not stressed for long.
Stressed trees, however, are sometimes attacked by the monkeypod roundheaded borer (Xystrocera globosa), which makes large galleries in the sapwood (11). In Puerto Rico, ants (Myrmelachista ramulorum) bore into branchlets, resulting in defoliation and leaf deformation (14). The defoliators can be controlled with insecticides applied to the tree trunks (13). The tree is highly susceptible to leaf damage from herbicide overspray. Leaves are also very susceptible to damage by salt-laden mist from ocean storms (called 'ehu kai in Hawaiian).
The pods contain a sweet edible pulp that supplies nutritious food for animals. Children also chew on the pods, which have a licoricelike flavor (3). Monkey-pod has long been a favorite of plant physiologists for studies of nyctinastic leaf movements (9).
Although the tree is commonly used as a shade tree in parking lots, it is undesirable for this purpose because of the sticky flowers, gum, and seed pods that fall from it during much of the year.
Monkey-pod wood has been reported as hard and heavy (12), and difficult to work (3,4). Actually, in Hawaii and elsewhere in the Pacific where it has been used much more extensively than in its native habitat, the wood is considered easy to work, particularly because low shrinkage during drying allows it to be machined while green. Articles made from green wood can be dried without serious drying degrade (10). In Hawaii, monkey-pod has been the premier craftwood used for carved and turned souvenir bowls since 1946. As labor costs increased, however, the industry spread to the Philippines and Thailand, which now supply most of the monkey-pod bowls for which Hawaii is famous.
No information on the genetics of this tree was found. It is probable that the genetic base at each location where it has been introduced is quite narrow. For example, in Hawaii, the entire population may be the progeny of only two seeds, although the ease with which seed of this species can be transported in one's pocket from the Philippines, for example, makes this unlikely.