Tuesday, December 2, 2014

African Influence

Urban ecosystems are cosmopolitan, blending plants and animals, as well as people, from all over the world. In Central Arizona, there is a distinct African influence, especially in the wildly diverse succulent plants that are supremely adapted to hot, arid climates similar to the Sonoran Desert. In addition to fleshy leaves and stems, most succulent plants have thick waxy skin and specialized metabolism, traits that increase their ability to conserve water. Here is a small sample of the most common African succulents that you are likely to encounter in you daily life in the Phoenix suburbs.

Aloe (Aloe vera) has become an element of almost every American household because of its use in cosmetics, skin treatments, herbal medicine and food products, as well as for landscaping and for ornamental houseplants. There are over 500 species of Aloe, ranging from small rosettes to giant trees, all native to Africa and the Middle East. Many species have become naturalized in arid regions around the world wherever there are human settlements. (Agaves, which are native to arid lands in the Western hemisphere resemble aloes in form, so are sometimes called "American Aloe," but are not closely related.) The thick leaves of Aloe vera are filled with slimy gel that is extracted and marketed for everything from laxatives to sunburn treatments. The Bible references aloe as part of the embalming mixture used to wrap the body of Jesus Christ (John 19:39).

Moroccan Mound (Euphorbia resinifera) is one of hundreds of Euphorbias that have been imported from Africa for landscaping. Euphorbia is one of the most diverse plant genera in the world, with nearly 2000 species, including poinsettias and leafy spurge. Many African species of Euphorbia have spines as well as succulent stems, making them resemble cacti, which are indigenous to the Western hemisphere; this is a classic example of convergent evolution. If you look closely at the tiny flowers of Euphorbias like Moroccan Mound, you will see that they are completely different from showy cactus flowers. Most Euphorbias also have thick white sap, or latex; cacti do not. The sap of Moroccan Mound is called resiniferatoxin, and is renowned for being rated at 16 billion Scoville heat units, which is one thousand times hotter than pure capsicum, the spicy ingredient of chili peppers.  Handle with care!

Pencil Tree (Euphorbia tirucalli), also known as Firesticks because of the flaming red stem tips, is super easy to grow and can flourish into a large tree with little to no irrigation. Like most other members of the Euphorbia family, it oozes milky sap when a branch is broken or cut. (Natural rubber is made from the latex of another member of this plant family that is native to Brazil, Hevea brasiliensis.) Some have dubbed this plant a "miracle tree," claiming that the sap can be used to treat a broad suite of ailments ranging from warts to scorpion stings. However, beware! Many people suffer painful skin rashes and eye irritation when exposed to the sap or even just vapors from the plant. There are also reports that Nomadic hunters in Africa have used the toxic sap to poison their arrows for small game and to kill fish.

Elephant Food, Pencil tree and a columnar Euphorbia
grow together in a giant pot in front of a local grocery store

Elephant Bush (Portulacaria afra) was imported from South Africa, where wild populations of this evergreen succulent shrub are an important food source for elephants, as well as other wildlife, including tortoises. In Africa, they call it "spekboom." The cut stems of Elephant Bush easily regenerate into new plants, a characteristic that may have evolved as a symbiotic relationship with elephants, since they scatter plant fragments as they feed. There is some speculation by scientists that growing Elephant Bush on a large scale may be useful to store carbon dioxide in order to reduce atmospheric levels of the gas that contributes to global climate change. This would dovetail nicely with objectives to improve wildlife habitat in Africa.

To learn more about succulent species from all over the world, including cacti, visit your local chapter of the Cactus and Succulent Society, which holds monthly meetings at the Desert Botanical Garden in Phoenix.

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