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NFT HighlightsNFTA 87-05, October 1987A quick guide to useful nitrogen fixing trees from around the world |
Prosopis pallida - Pioneer Species for Dry, Saline Shores
Some trees must be valued simply because 'they are there,' filling important vacuums in nature. The hostile environment this scraggly tree occupies is hot, dry and saline. Under these conditions, Prosopis pallida and its sister species produce nutrient rich pods for animals and the world's finest charcoal. They also fix nitrogen to enrich the soil around them.
Native to incredibly dry coastlines of Peru and Ecuador, P. pallida
was spread in the 19th century to African and Pacific nations and later
throughout Asia. It often dominates arid, saline coastlines, while other
saline-tolerant trees like the casuarinas are riverine or restricted to
shores with subsoil moisture. The tiny leaflets presumably intercept fog
to obtain much of their moisture along S. American coasts.
| BOTONY:
P. pallida (H. & B. ex Willd.) H.B.K. is one of about 40 species of this genus many of them quite similar. It is much like P. juliflora (Sw.) DC., a smaller Central American tree now widespread internationally and often weedy (Burkart 1978). Common names include 'kiawe' (Hawaii) and 'algarrobo' (S. America). Its better known temperate relatives are called "mesquite" it is a wide-spreading tree to 18 m in height and 80 cm in diameter with sinewy branches reminiscent of twisted ropes. On poor sites it is shrubby. Spikes of small pale-yellow mimosoid flowers are followed by abundant tan pods 12-24 cm in length. Most trees have sharp spines, but about 12% of Hawaii's trees are thornless. |
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FUELWOOD AND CHARCOAL:
Prosopis spp. are justly famous for charcoal and fuelwood (marketed
as "mesquite" from related temperate species). Kiawe's wood is very dense
(old trees to .85 specific gravity) and is challenging to cut but burns
slowly almost smokeless and with little ash (NAS 1980). Its charcoal is
superb.
WOOD:
Heartwood of kiawe is dark reddish-brown and sapwood is light yellow.
It is used for fence posts that are durable and decay-resistant but susceptible
to termite attack. Although dense and strong the wood is relatively easy
to work (eggs mallets).
FODDER:
Mature pods of P. pallida produced annually, are sweet and high
in energy with adequate digestibility and protein (12%). Pods can thus
form a significant dry season fodder after grasses have dried down. When
pods are fed as an exclusive diet, however cattle become malnourished and
lose weight. Trees are largely cross-pollinated, and produce few pods when
grown in isolation. The sparse thorny foliage is not a good fodder, and
seeds are indigestible.
HONEY AND FOOD:
Kiawe is a source of nectar that makes an excellent honey that was
once a major commercial product of Hawaii (Neal 1965). Trees flower seasonally
in dry areas, but may flower for longer periods when rooted in subsoil
moisture. Pods of kiawe are sweeter than most other species. They can be
made into a syrup, “algarrobina,” as a natural thickening for foods and
beverages. They are reported also to serve as a medicinal for dysentery.
OTHER USES:
The principle use of kiawe is as cover for erosion control on arid
land and shifting dunes. Thornless trees should be carefully chosen for
beach parks; a thornless composite is being produced in Hawaii. Bark has
been used for tanning, and exuded stem gums for glue and varnish.
SALT TOLERANCE:
P. pallida grows in highly saline soils along coastlines. It
reputedly tolerates irrigation water containing up to half sea water (NAS
1980).
ECOLOGY:
Most Prosopis spp. tolerate drought and thrive under 250-600
mm of annual rainfall, although growth improves greatly with more moisture.
Rooting is commonly shallow and spreading but can be deep in arid soils.
Trees do not tolerate frost.
PRODUCTION:
Kiawe has not been widely planted. Seeds should be scarified (nicking,
hot water) for best germination. Seedling growth is very slow, to 1 m annually.
Natural reseeding of the species is common in low-rainfall regions with
saline soils. Trees survive coppicing, but may grow back as many-branched
shrubs.
PESTS AND PROBLEMS:
The bruchid beetle (Algarobius prosopis) often invades kiawe
pods. Several other insects including psyllids, can defoliate trees. Posts
of kiawe are durable, but are subject to attack by teredos, a shipworm,
when used as pilings in the water. Rooting is shallow in many soils and
windstorms can topple the trees. Growth is very slow compared to other
trees under adequate moisture conditions. Trees rarely survive slow fires.
Trees appear highly variable genetically and are almost uniformly poor
in form. Taxonomy is debated, and the relationships of P. pallid
and P. juliflora needs clarification.
A publication of the Forest, Farm, and Community Tree Network (FACT Net)
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