Dave Taylor's African Safari: Trophic Level I: The Producers - Trees

Trophic Level I: The Producers
Trees
Page 7

Summary

Trees are found in three different communities. The riverine forest community is found along rivers and suitable lake shores. The bush thicket community is found where sufficient rainfall and soil conditions permit. Acacia woodlands are found in drier areas where droughts are a major abiotic factor.

The battle between trees and grasses for dominance is an ongoing one. If the factors that support the grassland change, as happened in the late 1800s, the forest regains its hold on the land. Right now, the forest is declining throughout most of the Serengeti-Mara ecosystem. There are only a few scattered places where it is increasing or holding its own.

Serengeti-Mara Forests

Open woodland covers much of the northern and western portions of the ecosystem. The trees are commonly between 9 and 12 meters (29 to 39 feet) in height and most have thorns for protection against herbivores. In some places, their canopy may shade up to 20% of the ground. In others, as little as 5 to 10% may be shaded. In either case, there is plenty of sunlight reaching the ground to support the growth of grass. Grasses here commonly reach 1 meter (1 yard or 3 feet) in height. Most trees here are acacias. These forests are often confined to ridges and hills.

Tracts of whistling thorns (A. drepanolobium) are found here. These dense, bushy stands are associated with poor drainage. Forests consisting solely of these trees may stretch over several hectares.

Thickets are found scattered across parts of the north. Usually they are not very extensive, perhaps up to 30 meters (100 feet) wide, but where conditions are right, they may stretch for several kilometers (miles). They appear impenetrable but are riddled by paths made by buffalo, elephant and (in the past) rhinos. Venturing into these thickets on foot can be a dangerous thing as it is difficult to get off the path should you meet one of the larger game animals coming your way! Trees here reach about 8 meters (26 feet) in height.

Kopjes have their own flora. The candelabra tree or euphorbia tree (Candelabra euphorbia) is often found growing on them, gaining a foothold in the crevices among the granite rocks. It has dark green, succulent leaves and often reminds visitors of a cactus. Like a cactus, this tree is covered with short spines that protect it. It also gives off a sticky poisonous latex that deters animals from feeding on it.

Other trees grow on the kopjes too. They provide shade for the many species of mammals and birds that live here. Indeed, on the southern plains they may provide the only shade for some distance. Lions will rest here during the heat of the day. Often, cubs will be hidden among the rocks and dense bush of the kopje.

Riverine forest lines the banks of most of the rivers. The tallest trees of the ecosystem are found here, reaching 10 to 25 meters (32 to 82 feet) in height. The vegetation is often impenetrable! Saplings, shrubs, vines and creepers grow thickly. Fever trees (A. xanthophloea), sausage trees ( Kigelia aethiopica) and several species of fig trees (Ficus spp.) are found here.

Fires seldom reach these trees due to the amount of moisture associated with these areas. Even in the dry season, ground water is usually present to supply the trees’ needs.

Symbiotic Relationships

Most acacia trees produce seed pods which are thin and papery. These dry out during the dry season and the seeds are scattered on the ground. The pods themselves are eaten but, for the most part, the seeds are ignored. This is not always the case; at least one type of acacia tree has developed a more ingenious method of dispersing its seeds.

Acacia tortilis has a special relationship with the impala. It produces thick, fleshy pods which are rich in carbohydrates. When the dry season arrives, the heavy pods fall to the ground where they give off a strong aroma. This attracts impala, grass rats and other animals to the pod. The seed pods of this tree are a favorite of the impala at this time of year and are readily eaten. If they are not eaten, bacteria kills the seeds and they fail to germinate. However, those seeds that are consumed by the impala, survive. The gastric juices of the impala's digestive system kill the harmful bacteria and soften the seeds’ hard outer layer. Once the seeds pass out with the impala's droppings, they are ready to germinate. An added advantage is that they have been carried away from their "mother" tree and may well have landed in an ideal spot where there is little competition.

Grass rats and other rodents also eat the seed pods but they consume the seed. Pods eaten by these rodents are not going to produce any seedlings.

Termites also help feed impala. These ground dwelling insects build huge mounds around dead thorn trees which are their staple diet. Such mounds are often found in the country where Acacia tortilis grows. On old, inactive mounds, star grass grows. This grass is also a favorite of impala.

Of All the Gall

The whistling thorn tree has growths on it that resemble black balls. These are called galls. Inside these galls live colonies of small ants (Crematogaster sp.). Each leaf stem of the tree has a nectar outlet whose sole purpose seems to be to produce food for the ants. In return, the ants protect the tree from browsing giraffe and antelope. When the browser begins feeding on the tree's leaves, the ants sense the disturbance and rush to the site. There, they bite the lips and tongue of the plant-predator. Their bites sting and soon convince the browser to move on, ensuring that the tree is not overbrowsed.

However, as with the Acacia tortilis tree, the whistling thorn requires that its seeds be eaten if they are to successfully germinate. Once again, browsers fulfill the tree’s needs. The problem is that the ants normally would stop browsers from feeding on the tree’s flowers and buds. This would prevent the plant from successfully passing its seeds on. The problem is solved by the flowers and seed pods which produce a chemical signal that inhibits the ants from attacking the browser. Once the seed pods are taken, the ants return to attacking their host’s predators.

Most of Africa’s savanna trees have thorns that discourage browsing. Some browsers such as the giraffe take the thorns in their stride and feed regularly on the trees. Indeed, the umbrella shape of many acacia trees is caused by giraffes. Still, a careful watcher would note that giraffe seldom browse on a tree for long before leaving. Further observation reveals that they will ignore other trees that look just as tasty in favor of trees a considerable distance away. Why?

Chemical Warfare

The acacia tree that is being browsed by the giraffe produces distasteful chemicals in its leaves. As the chemical change in the leaf builds up, it becomes less palatable to the giraffe. After a while, the animal is forced to move on. At the same time, the tree has released ethylene gas into the air which spreads to nearby trees. These trees respond by producing the same chemical. It is only when the giraffe moves to trees that were not warned by the gas, that it can begin to browse. The distance is about 50 meters (165 feet).

If this is so effective, why don’t all the trees produce such defenses? It costs energy and reserves to produce the chemical and the gas. The tree must divert some of its production of carbohydrates and sugars and convert the production to repellents. This is only cost-effective if the tree is browsed.

Trees also deter the attack of bugs and insects in the same way. As a result, most bugs tend to specialize in feeding on a few trees or plants for which they have developed a tolerance. The ants that feed on one species of acacia probably could not feed on another species.

Drought and Fire Resistant Acacias

Acacia trees are drought resistant and survive for long periods without water. They are also fire resistant. Fires will kill young saplings by severely damaging their leaves and bark. If, however, the tree survives beyond the sapling stage, its bark protects the tree from the fast moving grassfires. Then the tree will prosper. Conditions needed for this to happen include few grazers and little grass. Such conditions result after prolonged droughts.

When such conditions occur, a stand of acacia bush appears on the plain. It may be isolated and small in size or it may stretch over hundreds of hectares. Such areas are typically made up of trees the same age.

Eventually, such a stand will attract large browsers like elephants who will start to convert the area back to grasslands by stripping, barking, pushing over and killing trees.

Acacia Trees and Legumes

Acacias are trees that belong to the legume family. There are 1,200 species of acacias, most of which are native to Africa or Australia. Most have bipinnate leaves. Acacias have some economic importance. They are a source of edible seeds and valuable timber and gum.

Acacias belong to the family Fabaceae.

Legume is the common name for a plant family. They make up the third largest order of flowering plants. There are some 18,000 species. Legumes are the second most important group of plants economically. Only grasses are more important to human beings.

Legumes are found worldwide but the greatest numbers are found in tropical and subtropical regions. There is a great range and variety of plant forms within this group. Some are woody and grow for years. There are also numerous herbaceous forms and even a few aquatic legumes.

The family is best characterized by the fruits it produces. It is a single-chambered, flattened seedpod with two sutures. When put under pressure or sufficiently dried out, the legume normally splits open along the two sutures. The pods of the common pea are a well-known type of legume. Legume seeds are attached along one of the sutures.

Not all legumes split. The peanut, which matures underground, is an example of a type that does not split. Others will explode when touched, throwing their seeds outward.

The seedpod may range from only a few millimeters long (less than an inch) to more than 30 cm (more than 12 inches).

Legumes are important to the ecosystem because they possess a feature that allows them to fix nitrogen. The root nodules contain bacteria (genus Rhizobium) that are capable of converting atmospheric nitrogen into a form which can be used by plants.

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Dave Taylor's African Safari - Book 2: Trophic Level I: The Producers (Standard Version)
Copyright � 1999 Dave Taylor & James Cash