BIOMES OF THE EARTH - GRASSLANDS Part 5 docx

The most common grasses in the South American llanos

are species of Trachypogon and Axonopus: they have no com￾mon names. In some places T. plumosum and A. canescens

account for more than 80 percent of the vegetation.

Trachypogon grasses have long, narrow leaves and culms that

grow one to 6.5 feet (0.3–2.0 m) tall. Axonopus species grow

about six to 40 inches (15–100 cm) tall. Trachypogon plumosus

appears to recover from fire better than Axonopus canescens

does, but where fire is prevented, A. canescens often replaces

T. plumosus.

98 GRASSLANDS

Savanna baboons

(Papio cynocephalus)

live in troops of up to

150 individuals. They

spend most of their time

on the ground but

climb acacia trees in

search of food. They

eat grass, seeds, fruit,

insects, and small

mammals. (Courtesy

of Fogstock)

LIFE ON THE GRASSLANDS 99

Grassland trees and shrubs

Pictures of the African savanna show a flat landscape stretch￾ing into the far distance, but with the monotony broken by

scattered trees that have a very characteristic flat-topped

shape. These are acacia or thorn trees (Acacia species). The

most typical thorn tree is the umbrella thorn (A. tortilis). As

the name suggests, it is an umbrella-shaped tree. It grows

13–50 feet (4–15 m) tall and is distributed throughout most

of Africa and the Middle East.

There are about 1,200 species of acacias, and most are able

to survive prolonged drought. This is a necessary characteris￾tic for any savanna plant, but most woody plants—trees and

shrubs—lack it. Consequently, acacias are often the only

trees to be seen in the savanna. In the drier parts of the

savanna they are widely scattered, because each tree needs a

large volume of soil in which to find water. Many animals

feed on the leaves, young shoots, and seedlings of trees, and

their isolation leaves acacias very exposed. Thorn trees pro￾tect themselves by means of the big, fearsomely sharp thorns

that give them their name.

Some acacias, including the whistling thorn or ant-galled

acacia (A. drepanolobium) found on the African savanna, have

recruited ants as allies. These trees have a pair of swollen

thorns at the base of each leaf. Ants hollow out the thorns

and then live inside them, feeding on nectar from nectaries at

the base of each leaf stalk (petiole) and on oils and proteins

produced in sausage-shaped organs called Beltian bodies at the

tips of the leaves. Worker ants swarm all over the tree,

defending their own territories and biting and stinging any

animal within their reach. The ants also cut away any part of

a neighboring plant that touches their own tree. This pre￾vents any other plant from shading their acacia, thus allow￾ing it to grow rapidly. It is a very successful alliance that ben￾efits both parties.

About half of all acacia species are native to Australia.

There they are often known as wattles, because early settlers

used their wood to build huts they then plastered with mud,

a building technique known as wattle-and-daub.

Thorn trees have an attractive shape, but the candelabra

tree (Euphorbia candelabrum) has an extraordinary one. Its