Most of the park is situated in the central basin of democratic republic of Congo, although it is called the lower part, this actually consisted of steep mountains cut by deep valleys whose altitude varies from 600m to 1200m above sea level. Here the prevailing vegetation type is the equatorial rain forest, it is a typical tropical rain forest, and a great diversity of plant species is dominated by tall trees that branch only at the top from dense, ever green canopy 45 m above the ground. This canopy creates an atmosphere of half –shade and half-light that is barely sufficient for the growth of herbaceous plants and other understory species. These develop in the forest edge and its clearings, many species of big trees have buttress roots or in more humid places, still roots. The humidity in the forest is always high, The ground is often soaking wet and its humus layer is very thin because the conditions of high temperature and humidity are ideal for decomposing organisms. A leaf can be disgested within two months. In the mountainous parts of Kahuzi biega national park are the succession of the vegetation types from 900 to 3300m has been well studied. Six different primary vegetation types are distinguished, each one accompanied by its typical secondary form.
Mountain Rain forest and its secondary vegetation.
The mountain rain forest stretches from the altitude of 900-2300m, its dominating species are albizia gummifera, parinari excelsum and chrysophyllum gorungosanum. Between 900m and 1350 m the forest is low altitude forest with michelsonia and gilbertiodendrom. Towards 1800m, this is followed by the low mountain forest with pentadesma and lebrunia. This is a timber forest with sufficient light to allow growth of a layer of herbaceous plants. This type of forest is protected against fire as it cannot penetrate into it.
After humans have disturbed the equilibrium, this forest often becomes a shrub forest with a dense understory and many lianas. The best known secondary vegetation type is the hagenia forest. This vegetation type is the result of minor interventions such as the cutting of undergrowth to make pasture for livestock.
The existence of herbaceous savannas at this altitude is due to massive deforestation. All the places where light penetrates to the ground, such as all the clearings are colonised by the ubiquitions fern pteridium aquilinum which the livestock won’t eat.
Bamboo forest and its secondary vegetation
The Bamboo forests is practically monophytic , in Kahuzi biega national park its found between 2300 and 2600m , bamboo forests has minimum annual requirement of 2000 precipitation , this bamboo stands between 6-15m high and occasionally reaches even 20m . Every 30 to 40 years these plant flowers collectively over large areas and then die, clearing and cultivation of land enables the bamboo forests to spread over large areas. Therefore great stretches of bamboo forests seen today are the consequence of human activities. In the rainy seasons the young bamboo shoots constitute a favourite food for both the gorillas and the chimpanzee in Kahuzi Biega national Park.
The forest type develops in all places where the plants reproductive cycle takes place on soaking wet soils or where the soil has at least a layer of water on its surface during the dry season. There can be periodic or occasional floods. These conditions are not very favourable for the decompositions of organic materials which therefore accumulate. Many species have developed still roots, others aerial roots. Light can filter through to favour the development of lower shrubs layers; this vegetation is very difficult to penetrate.
High Altitude Rain Forests and its secondary Vegetation.
In general, the high altitude rain forest stretches from 1600m to 2700m. In the Kahuzi biega national park it is found above the 2300m and consists of the conifers podocarpus mahoni . Therefore this type of forest is termed primary podocarpus forest and its characterised by a relatively lower height of tree and of the upper canopy, ( its vegetation is therefore less high than the vegetation of lower altitude )
Moreover, the height if the vegetation tends to decrease with increasing altitude. Most of the evergreen plants follow this rule. The shrubs layer is well represented and epiphytes are abundant with increasing altitude lichens becomes more frequent with mosses, ferns and orchids. These remarkable trees ferns occur on lower slopes, lianas are rare or absent. In contrast to the soils in lowland equatorial forests , the soils contains a thick layer or litter and an even thicker layer of humus.