- DIVISION: Tracheophyta (vascular plants)
- CLASS: Magnoliopsida (dicotyledons)
- ORDER: Fabales
- FAMILY: Fabaceae (legumes)
- GENERA: Acacia, Acaciella, Mariosousa, Senegalia and Vachellia
- SPECIES: more than 1,400
They're an icon of the African savanna, but various species of acacias occur in many other warm, tropical, and desert-like regions of the world, too. Until recently, they all belonged to the same genus, Acacia. Today botanists have renamed many of the species, grouping them in five separate genera. In addition to the genus Acacia—which still includes more than 1,000 species, mostly from Australia—acacias include the following recent genera:
- Senegalia includes more than 200 species, mostly from the Americas, Africa, and Asia
- Vachellia includes more than 160 species from the Americas, Africa, Asia, Australia, and the Pacific
- Mariosousa includes 13 species from the southwestern US through Costa Rica
- Acaciella includes 15 species, all from the Americas.
Acacias can be tall trees or low-growing shrubs. People admire the feathery, compound leaves of some acacias; small leaflets are arranged in rows, one on each side a leaf's midrib, and each of those leaflets is further divided in the same fashion. This gives the leaves of these acacias a delicate, fern-like look. In many varieties from dry areas, though, compound leaves are replaced by flattened leaf stalks, called phyllodes, that look like long, simple leaves. The reduced surface area of a phyllode is an adaptation for conserving water. Even acacias with phyllodes, though, start out with lacy, compound leaves when they are young.
In many acacias, long, sharp thorns hide among the leaves. While thorns may discourage some predators, they're no match for the talented tongue of a giraffe. With its long, prehensile tongue, a giraffe avoids acacia thorns to grasp even the most delicate of leaves.
Acacias bloom in clusters of small, yellow or whitish flowers. Fertilized flowers produce tough seed pods that dry and eventually burst, releasing the flat, bean-like seeds inside.
Examine the ingredients list of your favorite gum, candy, or soft drink, and you might see the term "gum arabic." This water-soluble emulsifier and stabilizer comes from the hardened sap of various African acacias, and it's just one of the many products we get from acacias. This group of trees and shrubs also yields flavorings, wood pulp, cellulose, perfumes, cut flowers, oils, tannins, dyes, fodder, timber, and fuel.
Acacias are fast growing, so they are often planted for land rehabilitation. Various species show up in suburban and urban landscapes, too, and they are cultivated in areas where they're not native.
The Zoo and Safari Park have long cultivated acacias for their beauty and their shade—and because they are popular browse for many of the hoofed mammals! We grow 38 taxa of acacias and they are one of our accredited plant collections. Here are some to look for:
- A knobthorn acacia Senegalia nigrescens stands guard at Safari Base Camp at the Safari Park. Look for the big thorns on the trunk's woody knobs.
Monkey Thorn Acacia
San Diego Zoo, Elephant Odyssey
Monkey thorn acacia is a fast-growing, deciduous tree that reaches nearly 100 feet tall. In spring and summer on the savanna, kudu, elephants, and giraffes browse on its leaves.
San Diego Zoo, Hawaiian native plant garden
Expansive forests of koa trees once blanketed mountainous areas of Hawaii. Long before Californians were riding waves, the Hawaiians invented surfing as a sport, and some of the first surfboards were made of koa! Hawaiians also crafted ocean canoes, musical instruments, and many day-to-day items from koa wood. Much koa habitat has been cleared to make way for cattle ranches; there are only a few upcountry areas where these ancient trees still can be found. Reforesting projects are underway in Hawaii.
Safari Park, Okavango Outpost and Thorntree Terrace
The thorn acacia is prominent in many African vistas, as well as at the Safari Park. You can identify it by its yellow-green trunk and branches—which participate in photosynthesis. The thorn acacia is also called the "fever tree" due to its association with malaria. Of course, we now know that mosquitoes carry the disease, but these trees have been guilty by association—they often grow in low-lying, marshy and swampy areas and along streams, where mosquitoes breed.
Safari Park, Thorntree Terrace and Mombasa Lagoon
The thick bark of a paperbark acacia flakes off in papery sheets.
Safari Park Nativescapes Garden.
It's easy to see how this acacia got its name: its branches bristle with sharp, curving thorns.