The present boundary of the national park is situated at an average of some 2500 metres above sea level. This has considerably reduced the diversity of the vegetation in the park as it is today. Trees such as Dombeya which used to be more widespread at lower altitudes, have now become rare in the park. The Congolese side of the Virunga forest has many more areas situated at lower altitudes. The boundary of the Virunga forest on the Congo side is generally at around 2000m.
On the steeper slopes and deep ravines of Mt. Sabyinyo, specific formations rich in Giant Heather can be the dominant vegetation type. On the most exposed and drier – slopes of Mt. Muhabura, one can also encounter a different a vegetation pattern, with characteristic plants of drier soils and Giant Heather again.
Open meadow areas are found mostly in the saddles between the volcanoes. True wetlands are not numerous in the park. Lake Ngezi, the crater lake of Visoke and another smaller lake in the crater of Mt. Muhabura are the only ones. They harbour specific plant communities as well as a few species of birds and insects linked to water. In addition to these small lakes, a few swamps and the swampy crater bottom of Mt. Gahinga are the only other permanent aquatic habitats in the national park.
Over 1200 species of plants have already been identified in the Volcanoes National Park of which about 120 species – an astonishing 10% or so are endemics of high altitude forests of the Albertine Rift.
Volcanoes National Park has distinct vegetation cover mainly because of large altitudinal variations. The lower montane forest straddles between 2400 and 2500 meters together with Neoboutonia forest. From 2500 to 3200 meters, you will find Arundinaria alpine also known bamboo forest and it spans up to 30% within the Volcanoes National Park. Hypericum forest on the other hand can be seen on the humid slopes in the South and West between 2600 and 3600 meters and it also spans to about 30% in the park. You will also find Hagenia Abyssinica and this covers the biggest portion of the park.
Between 3500 and 4200 meters above sea level, Volcanoes vegetation is characterized by L. lanurensis, senecio erici rosenii and Lobelia wollastonii and this takes almost 25% in the park’s area. The grassland can be found within altitude 4300 and 4500 meters above sea level while the marshes, secondary thicket, small lakes, meadows as well as swamps cover relatively small portion.
The vegetation types in the Volcanoes National Park are directly linked to the altitudinal ranges. Although other local parameters can also influence the plant, communities present (local soil conditions, the steepness of the slopes, the local microclimate and /or the exposition to the sun. Extensive illegal grazing that took place in the past). The altitudinal range remains dominant factor shaping plant life in the park.
The main vegetation types encountered in the Volcanoes National Park are from lower elevations to higher elevations:
- Bamboo forest (2300 to 2600m)
- Hagenia – Hypericum Forest (2600 to 3300m)
- Subalpine zone (3300 to 3000m)
- Afro – Alpine zone (above 4000m)
All vegetation types can sometimes partially mix with each other, especially at the contact altitudes.
The Bamboo Forest
The dominant bamboo species in Volcanoes National Park is Sinarundinaria Alpina. Bamboos are actually huge herbaceous plants (grasses). It can grow to about twenty meters, young bamboo shoots grow at an astonishing rate.
Bamboo Forests are used to extend over larger areas in the park, but a good part of them disappeared when its size was reduced. One can still find extended bamboo forest today, in the Karisimbi area, in the larger saddle area between the two volcano groups and in the saddles between Mt. Sabyinyo, Mt. Gahinga and Mt. Muhabura.
Bamboo forests grow typically on soft, rich and wet soils. They cover some 30% of the national park. They usually form monospecific stands from where most other plants are totally excluded ( mostly due to luck of light under the dense stands of bamboo plants ). Even the undergrowth is restricted, which makes walking in mature bamboo stands very easy.
Every 15 to 20 years or so ( in the Virunga ), whole patches of bamboo will flower and die. It takes many years before the bamboo will re-grow sufficiently and, in the meantime, all kinds of fast growing undergrowth plants will grow in profusion in the newly open patches.
Bamboo offers precious resources for many animals in the Virunga forest. It is especially during the bamboo shooting season (mainly September to November) that mountain gorillas tend to spend more time in the bamboo forest. The other primate species present in the Volcanoes National Park, the golden monkey, spends most of its time in the bamboo all year round.
Many areas in the Volcanoes National Park show a true mosaic of closed forest patches (mainly Hagenia-Hypericum forest) and open clearings.
Several areas at the lower altitudes in the park are also covered by regenerating-or secondary – forest, which means forest that is growing again after the original forest was cut, degraded or perturbed (be it by man or due to natural causes). This type of forest is characterized by very dense undergrowth and aprofusion of vines and vianas.
The most typical tree in regenerating forest is Neoboutonia macrocalyx, easy to identify thanks to the very large round leaves. It can grow up to some 25 m and, as a pioneer species, grows much faster than other larger trees in the Virunga forest.