Factors Influencing Vegetation Types in Rwanda – Vegetation in Rwanda (Senior 4 Geography) | Ganzaa

Vegetation in Rwanda

Lessons

Senior 4 · Geography

Vegetation in Rwanda

3 lessons

Lesson 2 of 3

Factors Influencing Vegetation Types in Rwanda

25 min

Six belts of plants on one mountain

A visitor who climbs Karisimbi from the farms of Musanze walks through several different "countries" of plants in a single day. Figure 3 records them, using the altitude ranges published for the Volcanoes National Park.

Figure 3. Altitudinal zonation in the Volcanoes National Park. Drawn by Ganzaa from the zones described in the Wikipedia article "Volcanoes National Park" (CC BY-SA 4.0); summit height from REMA (2021).

Study the figure. Which zone covers the widest range of altitude? Why does the forest stop and give way to giant lobelias and then grass near the top? This lesson explains the factors that decide which vegetation grows where.

Definition

Altitudinal zonation: the arrangement of vegetation in belts (zones) at different heights on a mountain, because climate changes with altitude. Example: bamboo at 2,500–3,200 m on the Virunga volcanoes.

Environmental lapse rate: the average fall in air temperature with height, about $6.5\,^\circ\mathrm{C}$ per 1,000 m.

Rain shadow: an area on the sheltered side of high ground that receives less rain because the air has already lost much of its moisture.

Factors are grouped as climatic (rainfall, temperature), topographic (altitude, slope, aspect), edaphic (soil and drainage) and biotic (animals and people).

Worked example 1: how much colder is the summit?

Problem. Farmland gives way to the park's forest at about 2,400 m. Using a lapse rate of $6.5\,^\circ\mathrm{C}$ per 1,000 m, how much colder is the summit of Karisimbi (4,507 m)? How much does the temperature change across the bamboo zone (2,500–3,200 m)?

Solution.

\Delta T = 6.5 \times \frac{4\,507 - 2\,400}{1\,000} = 6.5 \times 2.107 = 13.7\,^\circ\mathrm{C}

\Delta T_{\text{bamboo}} = 6.5 \times \frac{3\,200 - 2\,500}{1\,000} = 4.6\,^\circ\mathrm{C}

Interpretation. Moving up 2.1 km has about the same effect on temperature as travelling thousands of kilometres towards the poles. Each zone is a band in which the temperature suits a particular group of plants; above about 4,200 m nights are too cold for trees, and only giant rosette plants and grasses survive.

Climatic factors: rainfall and temperature

Rwanda is close to the equator, but its height and its position behind the western highlands modify the climate. Meteo Rwanda describes the centre and east as "generally of semi-arid type owing to its position in the rainy shadow of the western highlands".

Region	Mean annual rainfall	Mean annual temperature	Natural vegetation
North-western volcanic highlands	about 1,500 mm	under 17 °C	Montane forest, bamboo, afro-alpine
West and Congo–Nile Ridge	about 1,400 mm (west)	under 17 °C on the ridge	Montane rainforest (Nyungwe, Gishwati, Mukura)
Central plateau	between the two	17.5–19 °C	Mostly cleared for crops, pasture and woodlots
Eastern lowlands	under 1,000 mm (about 900 mm in the south-east)	20–21 °C	Wooded and grass savannah, gallery forest
Bugarama valley (south-west)	see note	23–24 °C	Mostly farmland (rice) today

Sources: rainfall from REMA (2009) and RWB data reported by REMA (2021); temperature from Meteo Rwanda. Note: national rainfall averages about 1,200 mm.

Rain falls in two seasons (around March–May and September–December). Where rainfall is high and the dry seasons short, trees keep their leaves and form closed, layered forest. Where rainfall is lower, temperatures higher and the dry seasons longer, more water is lost by evaporation and only grasses and drought-tolerant, often thorny trees survive: savannah.

Worked example 2: why is the east savannah, not forest?

Question. Akagera lies at about the same latitude as the Volcanoes National Park, yet its natural vegetation is savannah. Explain.

Answer, as a chain of causes.

Moist air arriving from the west rises over the Congo–Nile Ridge and the volcanoes, cools and drops much of its moisture there (relief rain).
The east lies in the rain shadow: under 1,000 mm a year, against about 1,400–1,500 mm in the west and north-west.
The east is also lower and warmer (20–21 °C against under 17 °C), so evaporation is greater and the dry seasons are harsher.
Trees that need water all year cannot survive the dry season; grasses (which die back and regrow) and deep-rooted, drought-tolerant trees such as acacias can.
Dry-season bushfires, which REMA (2009) reports as frequent in Akagera, and grazing by wildlife kill tree seedlings and keep the landscape open.

Conclusion. Latitude is not the controlling factor in Rwanda; altitude and rain shadow are, with fire and grazing reinforcing the pattern.

Topographic and edaphic factors

Aspect. On the volcanoes, Hagenia–Hypericum forest grows mainly on the wetter south and west slopes; that is why it overlaps the bamboo zone in Figure 3.
Slope. Steep slopes have thin, easily eroded soils. In Rwanda they were also the last to be farmed, so they keep more natural forest.
Drainage. Valley floors where water stands for much of the year have waterlogged soils with little oxygen. Only plants adapted to them grow there, so the valleys are papyrus swamps (Rugezi, Akanyaru, Nyabarongo). Along rivers and lakes in the east, groundwater lets gallery forest grow in the middle of savannah.
Soil fertility. The rich volcanic soils of the north-west attract farmers, which is why the forest boundary of the Volcanoes National Park has been pushed uphill (lesson 3).

Biotic factors: animals and people

Animals. REMA (2009) reports that after elephants disappeared from Nyungwe, a climbing vine (Sericostachys scandens) that they used to eat spread and began to smother standing trees, and that without large grazing animals, ferns and grass spread after fires and slow the return of the forest.
Fire. Fires set by people, for example by honey collectors smoking bees, burn into Nyungwe in the dry season, and dense bracken fern (Pteridium aquilinum) takes over burned areas (REMA, 2021).
Cultivation and grazing. Most of the central plateau's natural vegetation has been replaced by crops, pasture and woodlots.
Protection. Laws and park boundaries decide where natural vegetation survives (Figure 2 of lesson 1).

Warning

Altitude does not "cause" vegetation directly. It acts through temperature, rainfall, cloud and soil. A strong answer names the chain: higher → colder (lapse rate) → shorter growth, frost at night → smaller plants. In the same way, write "the east is in a rain shadow, so it receives under 1,000 mm", not just "the east is dry".

Nature's Work: "Altitudinal zonation on Mount Meru". A short film of the vegetation belts on a volcano in northern Tanzania. Compare its zones with Figure 3: which belts do the two mountains share?

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Practice

Define altitudinal zonation and name, in order from bottom to top, four zones of the Virunga volcanoes.
A station at 1,800 m has a mean temperature of 18.0 °C. Using a lapse rate of 6.5 °C per 1,000 m, estimate the mean temperature at 3,200 m, the top of the bamboo zone.
Explain how the rain shadow of the western highlands affects the vegetation of eastern Rwanda.
Why are valley floors such as Rugezi covered by papyrus rather than forest?
Explain why Hagenia–Hypericum forest is found at the same altitudes as bamboo on some slopes but not on others.
Give two ways in which people and one way in which animals influence vegetation in Rwanda.
"Climate is the most important factor influencing Rwanda's vegetation today." Discuss.

Show answersHide answers

The arrangement of vegetation in belts at different heights because climate changes with altitude. Any four in order: farmland (former lower forest), Neoboutonia forest, bamboo, Hagenia–Hypericum forest, giant lobelia and Dendrosenecio, grassland.
$\Delta T = 6.5 \times \frac{3\,200 - 1\,800}{1\,000} = 9.1\,^\circ\mathrm{C}$ ; $18.0 - 9.1 = 8.9\,^\circ\mathrm{C}$ .
Moist air loses its moisture over the western highlands, so the east receives under 1,000 mm and, being lower, is warmer; the long dry season favours grasses and drought-tolerant trees, producing savannah.
The soil is waterlogged for much of the year and short of oxygen; papyrus and reeds are adapted to it, most trees are not.
Aspect: Hagenia–Hypericum forest grows mainly on the wetter south and west slopes; on drier slopes bamboo occupies those altitudes.
People: clearing for crops and pasture; fires (e.g. from honey collecting); planting eucalyptus; protecting parks. Animals: the loss of elephants let a vine spread in Nyungwe; grazers keep savannah open.
Climate sets the broad pattern (forest in the wet west, savannah in the dry east, zones on the volcanoes), but people now decide where natural vegetation survives at all: most of the land is farmed or planted. A good answer concludes that human activity is the dominant factor today, working within limits set by climate.

Quick check

Put these Virunga vegetation zones in order from the lowest to the highest.

Tap the cards in the right order.

Key points

On the Virunga volcanoes vegetation forms zones: farmland, Neoboutonia forest, bamboo (2,500–3,200 m), Hagenia–Hypericum (2,600–3,600 m), giant lobelia and Dendrosenecio (3,500–4,200 m), grassland near the summit.
Altitude acts through temperature (about 6.5 °C per 1,000 m), cloud and rainfall; aspect explains overlaps.
West and north-west: about 1,400–1,500 mm, cool → forest. East: under 1,000 mm, 20–21 °C, rain shadow → savannah.
Waterlogged valleys carry papyrus; groundwater supports gallery forest; fire, grazing, farming and protection decide much of today's pattern.

Sources and credits

Sources: Volcanoes National Park, Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa (vegetation zones and their altitudes); Climatology of Rwanda, Rwanda Meteorology Agency (temperatures, rain shadow, seasons); Rwanda State of Environment and Outlook Report, REMA, 2009 (rainfall range, elephants and vines in Nyungwe, bushfires); Rwanda State of Environment and Outlook Report 2021, REMA, citing the Rwanda Water Resources Board (rainfall by region, Karisimbi's height, fires in Nyungwe).

Figure 3 was drawn by Ganzaa.

Photos: Golden monkey (Cercopithecus kandti) eating, Charles J. Sharp, CC BY-SA 4.0; Mt Bisoke, Rwanda, MUGISHA Paul, CC BY-SA 4.0; via Wikimedia Commons.

Video: "Altitudinal zonation on Mount Meru", Nature's Work, embedded from YouTube. The rest of the lesson, the worked examples and the exercises were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Six belts of plants on one mountain

2Climatic factors: rainfall and temperature

3Topographic and edaphic factors

4Biotic factors: animals and people

Six belts of plants on one mountain

Climatic factors: rainfall and temperature

Topographic and edaphic factors

Biotic factors: animals and people