Unforgettable Acacias

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Noteworthy Plants For August 1999

The Unforgettable Acacias

A Large Genus Of Trees & Shrubs

Modified From: Zoonooz Volume 71 (8): 28-31.
Aug. 1998 by Original Author W. P. Armstrong. 

The genus Acacia is well-represented in southern California parks and gardens with dozens of different species. In fact, it is one of the largest genera of trees and shrubs in the world, with nearly 800 species. It is rivaled in size by only a few woody genera, such as Ficus (1,000), Eucalyptus (500) and Cassia (500). Acacias are commonly cultivated throughout temperate, tropical and desert regions, and include numerous distinctive growth forms with a profusion of yellow or white flower clusters. Acacias also provide us with many useful products, from beautiful hardwoods to water-soluble gums used as a thickening agent in your favorite frozen dessert. In the wild, native acacias provide the food and habitats for a variety of animals, from hoofed mammals and birds to countless species of insects. In fact, giraffe-browsed acacias produce the picturesque, flat-topped tree silhouettes on South African savannas. Swollen thorn acacias of Central America and South Africa form one of nature's most remarkable symbiotic associations with ants.

The enormous genus Acacia has evolved into two main groups. One group includes species bearing phyllodes (broadened leaf petioles functioning like leaf blades). Another group includes species with fernlike, pinnate leaves divided into small leaflets. Fourteen of the nearly 800 species of phyllode and pinnate-leaf acacias: 1. Acacia baileyana, 2. A. cyanophylla, 3. A. xanthophloea, 4. A. farnesiana, 5. A. cornigera, 6. A. collinsii, 7. A. podalyriifolia, 8. A. pendula, 9. A. melanoxylon, 10. A. longifolia, 11. A. dealbata, 12. A. cultriformis, 13. A. greggii, and 14. A. redolens. 

Seed Dispersal By Ants

Seed pods of Acacia cyclops (A. cyclopis) showing the bright red attachment stalk coiled around each seed. The enlarged photo insert (red arrow) shows a seed with the attachment stalk uncoiled. A brightly colored stalk presumably aids in seed dispersal by birds. The hard black seeds pass through the bird's digestive system unharmed and are dispersed to new locations. This Australian shrub is naturalized in coastal dunes of California and is commonly planted along highways. Recent research indicates that the attachment stalks of a related species (A. verticillata) are actually lipid-rich structures called elaiosomes. The seeds are carried by ants to their nest where the elaiosomes are eaten and the seeds are discarded. 

Central American Thorn Acacias With Symbiotic Ants
Giraffe Browsing On Spiny South African Acacia

Thousands of plant species from throughout the world produce seeds with an attached fat body called an elaiosome. [Note: Some botanists refer to the fleshy attached structure on a seed as an aril.] The elaiosome is rich in lipids (elaio=oil) and may be variously shaped, depending on the species of plant. In some Acacia species, such as the Australian A. verticillata, it resembles a thickened, coiled stalk attached to the seed. According to S.N. Handel and A. J. Beattie (Scientific American Volume 263 (August 1990), elaiosomes attract ants, which carry both the seed and its elaiosome back to their nest. There the colony eats the nutritious elaiosome and discards the seed unharmed. This type of dispersal is termed myrmecochory from the Greek "ant" (myrmex) and "dispersal" (kore). The interaction does not sacrifice any seeds to predation and appears to be mutualistic to both the ant and plant. The plant benefits from the relationship because its seeds are dispersed to favorable germination sites, and are also planted (carried underground) by the ants.

In addition to nutritious food morsels gathered by ants, the elaiosome may elicit other innate behaviors in ants. For example, oleic acid is known to stimulate corpse-carrying bevavior (necrophoresis) in some ants. This may explain why the ants carry the elaiosome as well as the attached seed, instead of removing the seed which they apparently do not use.

Acacias belong to the legume family (Fabaceae), the third largest family of flowering plants, including three subfamilies, 630 genera and 18,000 described species. All three subfamilies produce typical legume seed pods that either split open or remained closed at maturity, but their flowers are quite different. Acacia blossoms are not pea-like, and for this reason the genus is placed in the subfamily Mimosoideae, along with silk tree (Albizia), fairy duster (Calliandra) and mesquite (Prosopis). The flowers consist of an inconspicuous calyx and greatly reduced or no petals, with numerous, showy stamens. Acacia flowers are clustered together in small yellow or white globose heads, or in cylindrical spikes. In some species (A. baileyana) the flower clusters are produced in spectacular yellow masses, and in others (A. farnesiana) they are very fragrant, attracting numerous insect pollinators. The latter species is a spiny shrub native to the southwestern United States and Mexico. The flowers contain a essential oil used for perfumery in France. At the San Diego Wild Animal Park it grows in the Baja Garden and along Kilimanjaro Trail.

Three beautiful Acacia hardwoods from Australia, Hawaii and Africa: Top, blackwood acacia (A. melanoxylon); Left, Hawaiian koa (A. koa; right, knobthorn acacia (A. nigrescens).

One of the most intriguing taxonomic features of the genus Acacia is its divergence into two major groups with entirely different leaf types. One group has fern-like, bipinnate leaves subdivided into numerous minute leaflets. It includes hundreds of species throughout Australia, Africa and the Americas. Another group has "simple" leaves that are not divided into leaflets. The leaves of this group are called phyllodes, and they are actually expanded or broadened petioles (leaf stalks) which have lost the upper pinnate portion. Seedlings of this group produce the ancestral pinnate leaf, gradually replaced by phyllodes. Pruned branches of some species often develop phyllodes bearing bipinnate leaves at their tips. The phyllode group also contains hundreds of species distributed throughout Australia and the Pacific Islands. In fact, one of these species is the magnificent "koa" tree (Acacia koa) native to Hawaii. The following chart shows the vegetative divergence in the genus Acacia:

Phyllodes of (a) black acacia (Acacia melanoxylon) compared with (b) Sydney golden wattle (A. longifolia). The black acacia phyllodes have pinnate leaves at their tips, showing that they are modified petioles. 

Many of the pinnate-leaf acacias produce sharp spines, although some Australian phyllode acacias also have spines. The spines typically occur in pairs and are actually modified leaf appendages called stipules at the base of the leaves. Since "spines" are modified leaf stipules, they are referred to as stipular spines. Technically thorns are modified stems, but most botanists use the names thorn and spine interchangeably. The paired spines can be two-three inches long in the fever tree (A. xanthophloea), or short and curved in the knobthorn (A. nigrescens). Some species, such as the cats-claw (A. greggii) also produce thorns or prickles that arise directly from the bark as in roses. Cat's claw is also called "wait-a-minute bush" because clothing readily becomes entangled in the hooked prickles. This interesting shrub is native to the Colorado Desert of North America and also grows in the Baja Garden at the San Diego Wild Animal Park.

Long, white stipular spines and smooth, greenish-yellow bark of the African fever tree (Acacia xanthophloea), a common tree of lowland, swampy savannahs in South Africa. 

The distinctive trunk of knobthorn acacia (Acacia nigrescens) has large woody "knobs" each tipped with a sharp, curved thorn. 

The "swollen-thorn" acacias of Central America and South Africa are truly remarkable trees. In the wild, their enlarged, hollowed-out stipular spines are occupied by fiercely biting-stinging ants that protect them from browsing herbivores and epiphytic plants that might shade them out. The swollen thorns are not galls, they are not produced in response to chemical or physical stimuli from invasive insects imbedded in their tissues. The amazing thorns are genetically-programmed structures that are formed with or without the presence of symbiotic ants.

Central American Thorn Acacias With Symbiotic Ants
Giraffe Browsing On Spiny South African Acacia

Paired thorns of the Central American bullhorn acacias (Acacia cornigera and A. collinsii) resemble the horns of steers. They are commonly strung into unusual necklaces and belts, and in El Salvador, provide the legs of small ballerina seed dolls which can be worn as decorative pins. On wild trees they are inhabited by colonies of Pseudomyrmex ferruginea, very aggressive ants with a painful sting. Disturbed ants release an alarm pheromone and rush out of their thorn "barracks" in great numbers. According to Daniel Janzen (Costa Rican Natural History, 1983), livestock can apparently smell the pheromone and avoid these acacias day and night. In addition to protecting A. collinsii from leaf-cutting ants and other unwanted herbivores, the ants clear away invasive seedlings around the base of the tree that might overgrow it and block out vital sunlight. According to Dr. Janzen (Smithsonian Contributions to Botany Number 13, 1974), the Central American swollen-thorn acacias lack the chemical defenses of most other acacias to deal with their predators and competition, and symbiotic ants have taken over this vital role. For example, A. cornigera is apparently unable to synthesize bitter alkaloids that would normally discourage ravaging insects. This competitive disadvantage is particularly true in wetter areas where competing vegetation grows very rapidly. The acacias reward their ant helpmates with thorn "condos" to live in, carbohydrate-rich nectar from glands on the leaf stalks, and nourishing, protein-lipid Beltian bodies on the leaflet tips. There is no known function for Beltian bodies, except to provide food for symbiotic ants. Although they attack bird nests initially, the ants may grow accustomed to nests of orioles and kiskadees, and may even protect the nests from climbing predators.

A necklace made from a Central American swollen-thorn acacia. Thorns (paired stipular spines) such as these are occupied in the wild by symbiotic stinging ants which protect the trees.

The legs of this seed doll from El Salvador are made from the stipular spines of an acacia (possibly Acacia cornigera). 

Central American Thorn Acacias With Symbiotic Ants
Giraffe Browsing On Spiny South African Acacia

Prior to settling on a thorn acacia, the winged virgin queen ant goes on a mating flight to the highest treetop or nearby hill. Here she gets inseminated by a winged male and then hunts for an acacia in which to lay her eggs. This "hilltopping" phenomenon also occurs in San Diego County, where sexually mature male and female butterflies fly upslope to a rendezvous point at the summit. The queen ant cuts an entrance hole into a green thorn, hollows it out and then deposits her eggs. Subsequent entrance holes are cut by the new generation of worker ants.

Hilltopping In Southern California

The whistling thorn of Kenya (Acacia drepanolobium) has bulbous thorns occupied by stinging ants of the genus Crematogaster. Like the Central American acacias, the ants hollow out the soft green thorns for living quarters. Giraffes and other herbivores normally eat thorny acacia foliage, but leave the whistling thorn alone. In fact, for most South African acacias, spines alone are no deterrent to giraffes. They simply reach high into the branches and carefully wrap their long prehensile tongue around the tender foliage, carefully avoiding the vicious spines. At the San Diego Zoo you can watch a giraffe delicately removing tender leaves from the winter thorn (A. albida) near the edge of its enclosure on Horn and Hoof Mesa. Their feeding actually produces the characteristic flat-topped shapes of many acacias on the vast African savannahs. In the case of the whistling thorn acacia, a mouthful of stinging ants is an excellent deterrent to hungry herbivores. Old thorns which have been vacated by the ants make a whistling sound as wind blows across the entrance hole.

Large, inflated thorns of the whistling-thorn acacia (Acacia drepanolobium), an African swollen-thorn acacia. In the wild, the thorns are occupied by symbiotic stinging ants of the genus Crematogaster that protect the tree from destructive herbivores. The bulbous thorns are not galls and will develop with or without their symbiotic ants partners.

Whistling Thorn Symbiosis May Be One-Sided:
Additional complexities about Africa's whistling thorn (Acacia drepanolobium) are discussed in a recent article by Maureen Stanton and Truman Young in Natural History Volume 108 (November 1999). Studies by Stanton and Young reveal that four species of stinging ants live symbiotically on A. drepanolobium: Crematogaster mimosae, C. nigriceps, C. sjostedti, and Tetraponera penzigi. Their studies also reveal that the symbiotic relationship between some of these ant species and their host acacia may not be equally beneficial to both partners. Since these different species of ants are rival enemies, they occupy separate trees. If acacia branches containing rival ant colonies make contact, the different species of ants will fight each other, with the loser being evicted from its tree. Unlike the Central American thorn acacias that provide their ant warriors with protein-lipid Beltian bodies on leaflet tips, the whistling thorn provides no such service. This forces the ants to leave the tree to forage for insects and other protein-rich foods which they bring back to the developing ant larvae living inside the swollen thorns. According to Stanton and Young, refuse pushed out of the thorn nests may help to fertilize the tree.

The relationship between some of these ant species and their acacia is not completely mutualistic because it may harm the acacia tree. Crematogaster mimosae and C. sjostedti both tend scale insects that feed on the tree's vascular system, milking the aphid-like insects for their nutritive honeydew secretions. In fact, C. sjostedti pays little attention to herbivores that attempt to feed on the tree. The latter species also nests in hollow spaces within dead and dying branches, rather than in the swollen thorns. Colonies even thrive in the stumps of dead trees. To make matters worse, this species of ant often comes out the winner in battles with other ant species over the possession of a tree. According to Stanton and Young, the balance in a once mutualistic relationship has shifted in favor of one partner (the ant) at the expense of the other (the tree).

Although not as extreme as Crematogaster sjostedti, the relationship between C. nigriceps and its acacia host is also one-sided in favor of the ant. This ant species chews off the tips of growing shoots, including leaf and flower buds, thus pruning and sterilizing the tree. New branches are allowed to grow mainly in the proximity of swollen thorns, thus ensuring nectar-rich petiolar nectaries (glands) on new leaves easily accessible to worker ants inside the thorns. Over time, this pruning by ants changes the growth rate and shape of the tree canopy, compared with trees occupied by other ant species. In addition, pruning is more radical on sides adjacent to rival ant trees, thus reducing the chance of contact with branches of rival trees containing more aggressive ant species.
The San Diego Wild Animal Park has some marvelous examples of African thorn acacias. A splendid knobthorn acacia (Acacia nigrescens), also listed in some references as A. pallens, grows in the planter in front of the Administration Building. The trunk is covered with large, woody knobs each tipped with a sharp, curved thorn. The knobthorn is a common tree in Kruger National Park and produces a dark, heavy "ironwood" that sinks in water. Its density rivals the "quebracho" (axe-breaker) Tabebuia serratifolia of South America. At the San Diego Wild Animal Park, beautiful specimens of the fever tree (A. xanthophloea) grow at Thorn Tree Terrace and around Mombasa Lagoon. This acacia is unmistakable with its greenish-yellow bark and long, white stipular spines. Its native range is the lowland savannahs of South Africa and northward, often in swampy surroundings. Because its range coincides with regions where malaria is prevalent, it is called "fever tree." Some people actually believe that it may transmit malaria, but we know the vector is a mosquito.

The African fever tree (Acacia xanthophloea) has long, white stipular spines and fragrant yellow flowers in globose heads.

Another common acacia at the San Diego Wild Animal Park is the paperbark (A. woodii), so named because the bark flakes off in papery sheets. In South Africa this is a striking indigenous tree with a large, flat-topped crown. In Natal it is known as "camel thorn," although that name applies to another species A. giraffae. Fever trees and paperbark acacias can also be seen at the Heart of Africa, along with dark-trunked mesquite trees (Prosopis glandulosa). West of the Trading Village is a small whistling thorn acacia, complete with its bulbous stipular spines, but without its stinging ants. This remarkable acacia also grows near the exit gate at the San Diego Zoo.

The distinctive paperbark acacia (Acacia woodii) has thick, exfoliating bark and long, stipular spines. 

Phyllode acacias are common at the San Diego Zoo and Wild Animal Park, especially the Australian Rain Forest and Horn and Hoof Mesa. They include dense hedges (Acacia longifolia and A. veticillata), ground covers (A. redolens), black acacia shade trees (A. melanoxylon), everblooming A. retinodes, silvery weeping acacia (A. pendula), pearl acacia (A. podalyriifolia) and the unmistakable knife-leaf acacia (A. cultriformis). In Australia these fast-growing trees and shrubs are called "wattles." In their native habitats, some of them become huge forest trees over 100 feet tall with trunks four feet in diameter, including the black acacia and A. bakeri. Some of these huge phyllode acacias provide beautiful hardwood lumber for furniture, cabinets, boats and musical instruments. Wood of the Hawaiian koa is well known for the finest ukeleles and hand-carved canoes. Some Australian wattles have bipinnate leaves, such as the spectacular flowering A. baileyana and the beautiful silver wattle (A. dealbata).

Seed pod, phyllodes and beautiful wood of Acacia koa, a native species in the Hawaiian Islands. Most of the original koa forests were cut down and burned by people.

Native koa tree (Acacia koa) in a natural koa forest near the edge of Waiamea Canyon on the island of Kauai.

When acacia stems are cut or injured, they often exude a clear, gummy substance which helps to seal the wound and prevent infection by fungi and bacteria. Chemically, this natural gum is a water-soluble polysaccharide composed of many sugar molecules joined together. Natural plant gums are used as thickening agents and emulsifiers in food products, cosmetics and pharmaceuticals. Gums produce the thick, rich consistency of sauces and syrups, prevent the formation of ice crystals in frozen desserts, and stabilize the foaming in beer. One of the most important natural plant gums is gum arabic, obtained from the spiny, shrubby Acacia senegal of northeastern Africa. In addition to its use in foods, hand lotions and soaps, it is used in fine water colors, inks and confections. It also produces the water-soluble adhesive on postage stamps and the "lace curtain" on the sides of your beer glass. Other spiny African acacias have been tapped for their gums, including A. nilotica and A. seyal. The latter species (also called shittim) is especially interesting because biblical scholars believe its wood was used to construct the Ark of the Covenant.

Gum arabic, a natural thickening agent and emulsifier from the powdered sap of Acacia senegal. It is used in many products, from foods, cosmetics and pharmaceuticals, to sticky adhesives. 

Lace curtain of foam on a mug of root beer.

Acacias are one of the largest and most diverse groups of trees and shrubs on earth. From picturesque silhouettes on African grasslands and lovely Australian shade trees covered with blossoms, to spiny, fragrant shrubs of the Colorado Desert, acacias are truly unforgettable.

-References About Acacias:

Armstrong, W.P. 1999. "The Unforgettable Acacias." Zoonooz 71 (8): 28-31.

Armstrong, W.P. 1995. "To Be or Not to Be a Gall." Pacific Horticulture 56 (4): 39-45.

Janzen, D.H. (Editor). 1983. Costa Rican Natural History. The University of Chicago Press, Chicago.

Janzen, D.H. 1974. "Swollen-Thorn Acacias of Central America." Smithsonian Contributions to Botany 13: 1-131.

New, T.R. 1984. A Biology of Acacias. Oxford University Press, Melbourne.

Palmer, E. and N. Pitman. 1961. Trees of South Africa. A.A. Balkema, Cape Town.

Stanton, M. and T. Young. 1999. "Thorny Relationships." Natural History 108 (9): 28-31.

Tame, T. 1992. Acacias of Southeast Australia. Kangaroo Press, Kenthurst, Australia.