Prosopis pallida (Humb. & Bonpl. ex Willd.) H.B.K.
Leguminosae -- Legume family
Roger G. Skolmen
Kiawe (Prosopis pallida), also known as algarroba or bayahonda, is one of the 44 species of Prosopis recognized. The genus has a confused taxonomy. Burkart's revision, used here, assigns the designation P. pallida to the species introduced into Puerto Rico and elsewhere in the Caribbean formerly called P. juliflora. Prosopis glandulosa, mesquite, also formerly included in P. juliflora and four to six other species grow as shrubs or trees over an extensive area of the Southwestern United States and Northern Mexico (2,7,10).
More than 60 700 ha (150,000 acres) of dry kiawe forests in Hawaii are descended from a single tree planted in 1828 at the corner of a church in Honolulu. That year, Father Bachelot, the first Catholic priest in the Hawaiian Islands, planted a tree that he had raised from a seed he had brought with him from Paris. The tree was later determined to be Prosopis pallida. In August 1832, the tree was found to be hearing fruit. By 1840, progeny of the tree had become the principal shade trees of Honolulu and were already spreading to dry, leeward plains on all of the islands (3,8,12).
Kiawe is native to the drier parts of Peru, Colombia, and Ecuador, especially near the coast. It is naturalized in Hawaii and Puerto Rico (10).
In Hawaii, kiawe is most common in leeward coastal areas that have an annual rainfall of 250 to 760 mm (10 to 30 in) and a mean annual temperature of 24° C (75° F) with a range of 13° to 35° C (55° to 95° F). In a California study (6), kiawe trees less than 3 years old survived -2° C (29° F), but were killed at -6.1° C (21° F). Kiawe rarely extends above 150 m (500 ft) in elevation because higher rainfall and lower temperature give other species competitive advantage. In a few locations, however, it reaches an elevation of about 610 in (2,000 ft). In its native habitat, it is reported to grow from sea level to 300 m (990 ft) in annual rainfall from 250 to 1240 mm (10 to 49 in) (11). On the Islands of Lanai, Kahoolawe, and Niihau, which are in the lee of larger islands, kiawe occupies both windward and leeward shores extensively. It is present also as a shrub on some windward coasts of all islands.
Many old kiawe trees have been saved as garden and park trees during land development and have grown to large sizes with irrigation. Although it is a coastal species, kiawe is defoliated by windblown salt spray of winter storms (3).
Kiawe grows well on the soils that form on the and or semiarid coastal lands of leeward Hawaii. It is also very tolerant of saline soils (11). The soils where it grows in Hawaii are primarily Inceptisols, Mollisols, and Vertisols. The Inceptisols are in the great group Eutrandepts (reddish-brown latosols) developed from basaltic rock or ash. The Mollisols are Haplustolls (low humic latosols) developed from basaltic material and alluvium. The Vertisols are Chromusterts (dark magnesium clays) developed from alluvium and coral limestone and Torrerts developed from olivine basalt. These soils are mostly neutral to moderately alkaline in reaction, frequently stony to very stony, and occupy level to moderately steep coastal lands. In the driest locations, such as the eastern slopes of the Island of Lanai, kiawe is confined to the gully bottoms. Elsewhere, it extends onto ridges.
Kiawe is usually found in association with koa haole (Leucaena leucocephala), opiuma (Pithecellobium dulce), and klu (Acacia farnesiana). It is sometimes associated with the native tree, wiliwili (Erythrina sandwicensis), the native shrub, a'alii (Dodonaea viscosa), and the introduced shrub, lantana (Lantana camara).
Flowering and Fruiting- In Hawaii, kiawe begins to flower when 3 to 4 years old. The tree can flower at any time of the year and frequently flowers twice a year. Usually, it flowers from January to March, but in some years with wet summers it also flowers heavily during September and October. The numerous small perfect flowers are borne in pale yellow spikes 7 to 10 cm (3 to 4 in) long and about 13 min (0.5 in) in diameter. Styles protrude from the corolla just before it opens, but when it is opened the style and the 10 stamens are about the same length. Flowers are insect pollinated. Kiawe is an excellent honey tree (3).
About 6 months after flowering and pollination, the pods ripen and fall to the ground in April and July, or in normally dry years, only in July. The pods are straight or slightly curved yellowish beans 7 to 20 cm (3 to 8 in) long by 8 min (0.3 in) wide; there are 350/kg (160/lb) (18).
Seed Production and Dissemination- Pods do not open on drying to release their seed as some other legumes do. Instead, seeds are released either by natural decomposition of the pods or by passage of the seeds through the digestive system of an animal. From 10 to 20 light brown seeds per pod are encased in a sticky, sugary pulp. Seeds are difficult to extract from the pods. It is possible, however, to obtain seed by running pods through a commercial meat grinder with 1-cm (0.4-in) holes (5). There are about 28,500 to 32,000 seeds per kilogram (13,000 to 14,500/lb). The seeds can be stored at ambient room temperature, protected from insects, for 9 months with little loss of viability (18). Germination is improved slightly by scarifying the seeds with hot water before sowing. Scarification with concentrated sulfuric acid for 10 minutes improved germination from 64 percent (without treatment) to 88 percent (18).
Many seeds are destroyed by insect pests. In Hawaii, a black beetle, Mimosestes amicus, bores into the pods that have fallen to the ground (16). In Puerto Rico, a Bruchid weevil attacks seed on the tree, causing seed from green pods on the tree to have a germination rate of only 59 percent, yellow pods on the tree only 40 percent, and pods on the ground only 6 percent (18).
Seedling Development- Germination is epigeal. Seedlings are usually found associated with animal droppings during and after rainy periods. In this highly fertile situation, seedlings grow rapidly, reaching 0.3 in (1 ft) in 3 to 4 months. Even in the absence of browsing animals, kiawe reproduces prolifically on abandoned city house lots containing older trees. Seedlings in such a situation can grow more than 1 in (3 ft) tall in the first year if rainfall is adequate. Seedling survival depends primarily on sufficient rainfall during 4 to 6 weeks after germination. The seedlings are also intolerant of shade.
Vegetative Reproduction- Kiawe stumps often sprout after being cut. Some thornless trees have been propagated by air-layering of the mature branches, but only on an experimental scale. Kiawe cuttings can also be rooted under mist (5).
Growth and Yield- The oldest kiawe trees in Hawaii of known age are at the entrance to Punahou School in Honolulu. This area was a treeless field in 1848. In 1918, trees on this site that were about 70 years old ranged from 61 to 104 cm (24 to 41 in) in d.b.h. and from 20 to 26 m (65 to 85 ft) in total height (9). This is a relatively wet area for kiawe, with an annual rainfall of 940 mm. (37 in) and abundant groundwater from a nearby spring.
At Waianae, Oahu, an area with 510 mm (20 in) annual rainfall, a tract of kiawe trees of unknown age yielded 226.8 m³ per ha (3,240 ft³/acre or 36 cords/acre). On Maui, a 2.4-ha (6-acre) area with 380 mm (15 in) rainfall yielded 365.4 m³ per ha (5,220 ft³/acre or 58 cords/acre) (9).
In Puerto Rico, on a dry, gravelly site (760 mm or 30 in rainfall), a 21-year-old planting had trees 25 to 36 cm (10 to 14 in) d.b.h. and 4.6 to 7.6 in (15 to 25 ft) tall (18).
In Honolulu, the original tree, when measured in 1916, at 88 years of age, was 99 cm (39 in) in diameter (8). But the champion kiawe tree in Hawaii is at Puako, Island of Hawaii, and measures 130 cm (51 in) d.b.h. and 27.7 in (91 ft) tall (1).
In a study of biomass production of other Prosopis spp. in southern California (5), several species, mostly from South America, were grown for 3 years at 1.2 in (4 ft) spacing and three levels of irrigation. These trees produced an annual average of 8.5 t/ha (3.8 tons/acre) of fresh biomass. Another study in Texas (19) determined that Prosopis natural stands yielded 19.3 t/ha (8.6 tons/acre) on deep upland soils and 36.1 t/ha (16.1 tons/acre) on deep bottom land.
In Puerto Rico, direct seeding in dry areas (760 mm or 30 in rainfall) gave poor survival, but planting of seedlings produced 67 percent survival despite a severe 6-month drought (18).
On windy or dry sites, kiawe grows as a shrub, or a small twisted tree only 3 to 5 in (10 to 16 ft) tall. It is usually layered where it grows in strong trade winds and lies along the slopes as a rounded bush.
Rooting Habit- The genus Prosopis is noted for its ability to root deeply (10). A Prosopis uelutina tree in Arizona was found with roots extending downwards 53 in (174 ft) into the ground (10). Kiawe seedlings produce strong, rapidly growing taproots that appear to be capable of deep extension and may share the deep rooting characteristic common to the genus. The species can grow on windy, dry sites, which suggests that its roots penetrate deeply to reach moisture. Trees on such sites are small. Trees on coastal plains where abundant, shallow groundwater is available reach large size, but are shallow-rooted and subject to windthrow (3,9).
Reaction to Competition- Kiawe is an intolerant tree and is shaded out by faster growing plants on wet sites. On dry sites the grasses, koa haole, and opuima, with which it commonly associates, remain so sparse that abundant sunlight reaches the kiawe.
No silviculture is practiced with kiawe, but the species' characteristics indicate that it could be managed as a fuelwood crop. It fixes nitrogen, so requires less fertilizer than nonleguminous trees, and it coppices after cutting, so does not require replanting at each rotation. It is a short-boled, crooked tree even when grown at close spacing and appears unsuitable for use as a timber tree.
Damaging Agents- Kiawe trees are severely defoliated by the introduced caterpillar Melipotis indomita but quickly leaf out again after defoliation (17). They are also sometimes partially defoliated by the Blackburn butterfly, Vaga blackburni, an insect that usually does more damage to other legumes (16). In California, a psyllid, Alphalaroida spp., caused leafroll of new leaflets in plants grown in a glasshouse (6). The kiawe roundheaded borer, Placosternus crinicornis, infects trees under stress, and recently cut firewood, boring under the bark and into the sapwood (18). Carpenter bees seem to have a particular affinity for the sapwood of kiawe fence posts.
The tree grows in areas where fire hazard is often extreme. It is usually killed outright by fire and burned trees almost never sprout.
Kiawe is used principally as a cover tree for erosion control on and land. As recently as 1915, it was considered the most valuable tree in Hawaii for a variety of other reasons. Its pods and seed are nutritious fodder. The pods alone contain 9 percent protein and the seeds 34 percent, one of the highest levels for any legume. But because the seed coats are indigestible, the seeds must be ground, if animals are to recover this protein. In 1935, Hawaii shipped more than 200 tons of kiawe honey. A small honey industry in Hawaii continues to depend on kiawe (3,6,9).
The wood is extremely hard and heavy. It is used directly for fuel and also is made into charcoal. The heartwood is durable and kiawe is preferred for fence posts despite its crooked form. Mesquite, a related species in the United States, is the traditional wood used for boatbuilders' calking mallets and, in Hawaii, kiawe is referred for cement floats.
Because the entire population of kiawe in Hawaii is originally from one tree, inbreeding has been intensive. One possible recessive characteristic of the population is thornlessness. Although most kiawe trees have thorns with strong spines often 2.5 cm (1 in) long, an estimated 25 percent of the mature trees produce only small, hard stipules rather than long, spikelike spines at the twig nodes. The thornless characteristic has been noted for years, and as early as 1937, Hawaii shipped seed from thornless kiawe trees to Cuba, Arabia, Australia, Fiji, and South Africa (3). Attempts have been made to breed for thornlessness, but have so far been unsuccessful. Thornless trees can be propagated by air-layering of mature twigs (13). Some other Prosopis spp. also exhibit thornlessness among individuals in the populations (10). Thomlessness can be seen in some or all of these other species when they are only 3 to 4 months old (4).
One report states that Prosopis spp. bear self-incompatible flowers (15). This is obviously not true of Prosopis pallida, or at least of that one individual Prosopis pallida originally introduced into Hawaii.
Another report mentions that in 1920, the U.S. Department of Agriculture's Experiment Station in Honolulu had imported seed and was growing seedlings of a number of other Prosopis spp. in an attempt to determine the identity of the tree common to Hawaii by comparison (14). No record exists of the disposition of these seedlings, but a possibility exists that they may have been outplanted. No hybrids are known in Hawaii, however.
In addition to kiawe, at least five other Prosopis species deserve consideration for use in and lands for firewood, forage, and cover (10). One of these, Prosopis alba, is the backbone of the Arizona nursery shade tree industry (4).