Congo Basin forest structure
Rainforest of the Congo Basin share many characteristics with the Amazon rainforest and other tropical forests worldwide: high diversity, dense vegetation, and multiple layers of forest structure. The forest canopy is usually about 30 meters tall, with emergent trees such as the great maobi reaching up to 50-60 meters. The most humid, western portions of the Congo basin forests remain evergreen throughout the year, while interior forests are semi-deciduous, and many species drop leaves for short periods of time. Peripheral forests of the Congo basin with less precipitation become even more deciduous, although trees usually grow foliage on some branches before other branches lose their leaves.
Forests of the Congo Basin are characterized by fewer species of trees than the Amazon rainforest. In the interior of the Congo, forests are dominated by heavy seeded, shade tolerant trees of the Caesalpinoid subfamily of the legume family, such as Julbernardia and Cynometra species. Other areas of mature forest include the monodominant stands of Gilbertiodendron dewevrei, also a member of the legume family. Heavy seeded and shade tolerant trees such as these often have greater success seeding and surviving in the shade of the mature forest. In open, more disturbed habitats of secondary forest however, sun loving, light seeded trees dominate, such as Entandrophragma and Khaya species, both members of the African mahogany group, as well as species of Albizia. In younger secondary forests, early pioneers such as Musanga cecropiodes are more common.
In mature forest, understory plants are dominanted by shade tolerant families such as Maranthaceae and Zingiberaceae. Although not as well documented as in the Amazon, studies have found lianas to be a significant component of forest structure and diversity, comprising approximately 25% of the total diversity of woody species. In one study of the Ituri forest in northeastern DRC, liana diversity is dominated by Manniophyton fulvum (Euphorbiaceae). Epiphytes such as orchids and mosses are also much less common in the Congo forest as compared to the Amazon rainforest, likely due to the dry climatic history of the Congo Basin. Diverse epiphyte families of the Neotropics such as the cactus and the bromeliads are largely absent from central Africa, but at least 2,400 species of orchids are still present throughout the humid forests of Africa. Other native herbaceous plants include numerous species of Begonia and Impatiens.
In rainforest regions such as the Congo Basin, trees exhibit shallow and/or buttressed roots, for maximum nutrient absorption and stability in wet soils. Bark on tropical trees is usually smooth and thin, instead of the thick textured bark of dry or temperate forests. Smooth bark prevents epiphytes and lianas from growing on the tree surface, as well as reducing the risk of fungal infection. Leaves are often waxy with drip tips to shed excess water and prevent nutrient loss. Plants develop chemical defenses to protect themselves from insects and other herbivores. Ant mutualisms are especially common in the Acacia trees of the forest savannahs and grasslands outside of the Congo Basin.
In the interior of the forest, especially in the understory, little wind is available to pollinate flowers and disperse seeds, so many plants have developed elaborate relationships with insects and other animals. Various flower types exhibit colors, aromas, and nectar rewards to attract pollinators such as bees, butterflies, and moths. One interesting difference between the Amazon and the Congo rainforests is the prevalence of hummingbirds and hummingbird pollinated plants in the Neotropics, which are absent in Africa. Filling this ecological niche in Africa are nectar feeding sunbirds. Trees like the coral tree (Erythrina spp.) have evolved horizontal facing flowers for perching for the sunbirds to pollinate them, whereas Neotropical hummingbird plants have vertically oriented free standing flowers adapted for visit by the hovering hummingbirds.
Plants have also evolved to create edible fruits and seed covers to facilitate dispersal with rainforest animals. In the Congo Basin, the Moabi tree (Baillonella toxisperma), an important resource for timber and medicine, is large seeded and dispersed only by large mammals such as elephants and gorillas. In Gabon, Cola lizae, a member of the Kola nut group (and an original ingredient in Coca-cola) is a dominant tree and observed to be dispersed largely by gorillas. In the Albertine montane forests of the eastern Congo Basin, a recent study (Biotropica) discovered rodent dispersal of the common tree Carapa grandiflora. Interestingly, these characteristics, as well as the frequent mild disturbance of events such as treefalls are thought to be one of the major factors leading to the great biodiversity of tropical rainforests.
Congo Basin soils
Also characteristic of tropical forests, the soils of the Congo Basin forests are generally nutrient poor, weathered, and acidic. Studies find that humid forests of the interior are most lacking in soil phosphorous, whereas the forest savannah ecosystems are more lacking in nitrogen, thus giving rise to the numerous nitrogen fixing trees such as Acacia. Nutrient poor soils in the Congo Basin are generally unproductive for permanent agriculture; native peoples instead practice shifting cultivation in order to manage soil fertility. Recently, some areas of central Africa have been discovered as carbon rich peat soils (see a news story on NPR).
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