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In: Pellikka, P., J. Ylhäisi & B. Clark (eds.) Taita Hills and Kenya, 2004 – seminar, reports and journal of a field excursion to Kenya.

Expedition reports of the Department of Geography, University of Helsinki 40, 14-20. Helsinki 2004, ISBN 952-10-2077-6, 148 pp.

Zonation and characteristics of the vegetation of Mt. Kenya

Tuomo Niemelä1 & Petri Pellikka2 Botanical Museum

1 and Department of Geography

2, University of Helsinki

Abstract

In this paper the vegetation of Mt. Kenya is briefly summarized, mostly by describing the zonation from thefoothills upwards: Montane rain forests, Bamboo zone, Upper montane forest, Ericaceous zone, Páramo, and

Nival zone. The peculiar giant rosette plants of the upper zones and their ecology are outlined. Mt. Kenya belongs to the series of volcanoes that occur along the fault lines of the Rift Valley system; they are fairlyyoung (a few millions of years) if compared to the ancient Eastern Arc mountains closer to the Indian Oceancoastline, whose origins date back to 20 my or more.

Introduction

The predominantly gentle East African terrain is

here and there interrupted by isolated

mountains. They are of two origins.

Closer to the eastern coastline are ancient

mountains of the Eastern Arc: Taita Hills in

Kenya, and in Tanzania within sight distance

the Pare and Usambara Mountains, and several

others further south. The faulting and upheavalof these granitic mountains commenced in the

Miocene some 20 million years (my) ago or

earlier still, and their forests are remnants of the

Pan-African tropical forest belt that covered

equatorial Africa from the Atlantic coast to the

Indian Ocean before the formation of the upland

and gorge system of the Rift Valley. Eastern

Arc forests are characterized by a very high

degree of endemism, harbouring endemic tree

species and even genera. For the most part,

these forests lie well below the altitude of 2000

metres, with the foothills just a few hundredmetres above sea level.

Along the Rift Valley fault lines there are

volcanoes, some of them being active, like

Oldoinyo Lengai in northwestern Tanzania (a

major eruption in mid-1960s), or dormant, like

Kilimanjaro and Mt. Meru 70 km west of it, or

extinct, like Mt. Kenya. As compared to the

Eastern Arc mountains, these volcanoes are

fairly young: the first lavas of Kilimanjaro 1

my, Ngorongoro 2.5 my and Mt. Kenya 3 my

old. Their age explains the lower degree of

endemism if compared to Eastern Arc. On the

other hand, their foothills lie at the altitude of

1500 to 2000 m above sea level and peaks of

many of them exceed 5000 m. High altitude

makes these volcanoes characteristic in their

vegetation, and altitudinal zonation is seen

clearly on their slopes.

Even though general features are the same, East

African volcanoes differ from each other in

many details of their vegetation. This is

explained partly by their different ages, andanomalies in rainfall, but also by the distances

from one mountain to the next, and hence the

degree of isolation, or probability of seed

exchange. The higher up we go, the more

specialized vegetation we encounter.

The vegetation zones are not similar on each

slope of the mountain. Since eastern and

southern slopes receive more rainfall than the

other slopes, the lower montane zone reaches

lower there than on the other slopes. In addition,

there is no bamboo zone on the northern slope(Figure 1). The vegetation zones are also

modified by human interference. The lower

montane forest is taken for cultivation in the

lower elevations. This has been distinguished on

a Landsat TM satellite scene. There is a sharp

edge between forest and cultivations in each

slope of the massif (Figure 2). The Landsat

MSS satellite scene from 1976 reveals that the

upper montane forest was a continuous belt

around Mt. Kenya. As the northern slope is

lacking a bamboo zone, the agricultural fields

are nowadays practically next to upper

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Figure 3. Bamboo thicket on the SW-slope of Mt. Kenya (P. Pellikka).

montane forest. This zone is very favourable for

wheat cultivation, for example. In addition, theforest reached lower elevations during the 1970s

and did not form such a sharp border with

cultivated land. The upper montane forest zone

and the ericaceous zone are affected by humans

as well. The forest and grassland ARE burned in

order to favour growth of flowering plants for

bees and honey making.

Montane rain forest of Mt. Kenya

The foothills of Mt. Kenya are (or at least were

originally) covered by dense evergreen forests,extending from about 2200 to 3500 metres.

Their lowermost section, the Lower montane

forests, also called Montane rain forests, are

very similar in structure and general appearance

to the lowland rain forests of the Congo basin.

At the species level, however, they are different:

even though a few tree genera are the same,

lowland rain forests and montane forests do not

share common species. This is mostly due to

cooler climate in upland areas, even though

there are no frosts in the montane rainforest

belt.

Montane rain forests are the most luxuriant ones

in the country. The trees of the upper canopy are30–40 metres tall, with heavy-branched and

wide-spreading crowns, and very thick and

pole-shaped trunks, often supported by

buttresses. The forest structure is layered:

beneath the emergents there are trees of middle

stratum, 15–30 m tall, which are more shade-

tolerant and have narrower crowns. Smaller

trees make a lower canopy, but shrubs are few,

and the herb layer is usually sparse, except

along roadsides where there is more light.

Lianes and strangling figs abound, and

epiphytes make thick and speciose colonies onthe branches of larger trees.

The wealth of forest vegetation in this zone is

maintained by abundant rainfall, 1500–2500

mm per annum depending on the exposure of

the slopes, by frequent mists, and rich volcanic

soil. However, there is a dry season of a few

months. Some of the emergent trees shed their

leaves for a short time, but the forest as a whole

is evergreen.

These forests have harboured many valuabletimber trees, and in most areas they have been

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heavily logged in the past. Also, along our

ascent of the mountain via the eastern slope, the

forest structure was obviously disturbed:

emergent trees were much fewer than in a

natural state. Anyhow, we saw, for instance,

giant-size examples of East African camphor

(Ocotea usambarensis), Wild olive (Olea

capensis), and, on drier slopes and further up,

the coniferous trees Podo ( Podocarpus falcatus)

and East African cedar ( Juniperus procera).

The high rainfall, luxuriant and complex

vegetation, and the abundance of mossy and

other epiphytes make montane rain forests very

important catchment areas. Water is effectively

absorbed in the vegetation, peatlands, and

humid soil, and runoff is even throughout the

year. Therefore, and in order to avoid soilerosion, the protection of these forests is of

paramount importance.

Bamboo zone

Above the montane rain forests there is a

speciality of tropical African mountains, the

Bamboo zone. While it is fragmentary on some

mountains, like Mt. Kilimanjaro and Meru, it is

spectacularly well developed on Mt. Kenya.

Climbing uphill, the forests end quite abruptly,

and the landscape changes into a uniformthicket of Mountain bamboo ( Arundinaria

alpina). The stands of bamboo culms are so

dense that passage is only possible along roads

and elephant paths (Figure 3). Tree seedlings

usually succumb to the lack of light.

Bamboo thickets make a sickle-shaped belt on

the moister eastern, southeastern and southern

sides of the mountain, while the plant is almost

absent from the drier northwestern slope. The

zone is several kilometres wide at an altitude of

2200–3200 metres a.s.l. The plant needs, inaddition to plentiful rains, also rich and deep

soil and fairly gentle terrain.

Mountain bamboo is a perennial grass, over 10

m tall, and its 3–7 cm thick culms arise from

massive rootstocks. This species is endemic to

the East African mountains. New shoots

develop almost continuously from the rootstock,

while older ones die but stay standing for many

years. No flowers develop, however, until the

bamboo population reaches the age of 7–9 years

(on some mountains 15 years or even 30 years

in Uganda). Then flowering starts

simultaneously in areas of several hectares: all

the plants burst into flowers, shed the seeds, and

die. A new generation of bamboo develops from

seedlings. The few trees found amongst bamboo

originate from such periods when the bamboo

vegetation temporarily disappears, and enough

light is available close to the ground. It is not

known what triggers the simultaneous flower-

ing, but it is understandable that a strict rhythm

has developed: bamboo individuals with erratic

flowering cycle would have no possibilities to

produce viable seedlings in the deep shadow of

mature bamboo vegetation.

Upper montane forest

Higher up, above the bamboo zone, there are

still forests. Even if a few tree species of the

montane rainforest zone reach so high up – for

instance East African cedar ( Juniperus procera)

– the high altitude Upper montane forests are

different in their characteristics and most of the

tree species.

Trees are mostly low, 12–20 metres tall, and

their trunks are twisted and repeatedly

branched, with cupola-shaped crowns. Even

though the tree stands may be fairly dense, theforest structure is not layered, and enough light

reaches ground level for a thick grass and herb

vegetation to develop (Figure 4). The so-called

Elephant grass ( Eleusine jaegeri) is a common,

robust plant growing in big tussocks. Forests

often have a park-like appearance. Rainfall up

here at the altitude of 2500–3500 metres is

lower than in the montane rainforest zone, but

also evaporation is smaller, and frequent heavy

mists contribute to the humidity. Hence the

upper montane forests are often called Cloud

forests or Mist forests.

Along the trail from Mt. Kenya Bandas upward

we saw the tree-like Giant St. John’s wort ( H-

pericum revolutum) with large yellow flowers;

it is common in all East African mountains high

enough to maintain this forest zone. Robust

Kosso trees ( Hagenia abyssinica, Rosaceae; in

Kikuyu Muthithiku) were plentiful around the

Mt. Kenya Bandas and along the path upwards,

growing together with single thick East African

cedars.

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Figure 4. In the upper montane zone the trees are twisted and multi-stemmed and lichen epiphytes are abundantand conspicuous (P. Pellikka).

Ericaceous zone

True forests end approximately at the altitude

where night frosts become frequent, ca. 3500

metres a.s.l. Above this, the Ericaceous zone

starts (Figure 5). The demarcation between

forest and Erica heath is abrupt, but it does not

make a straight line: because of the hilly nature

of the slopes of Mt. Kenya, the transition is

mosaic-like, forests growing in shaded valleys,

and Erica heathland on exposed slopes, until,

further up, forests end completely.

The zone is named according to Giant heather

( Erica arborea, also other Erica species), a

woody erect shrub, half-metre tall on dry and

upper slopes but 1–2 metres tall throughout

most of the zone. It has scaly leaves of spring-

green colour, and little, pink, urn-shaped

flowers. Erica and Muhatu (in Kikuyu; Stoebe

kilimandscharica) with a pastel colour of bluish

green, make an almost continuous heathland as

far as can be seen (Figure 5). Sugar bush

( Protea kilimandscharica) is similar in size, but

it has larger ovate leaves and striking, large,

pale yellow flowers (in fact capitulum

inflorescences). Among these woody shrubsthere is a plenty of lilies and other herbs with

bright coloured flowers.

The Heather family (Ericaceae) seems to have a

Laurasian origin, and could spread from the

Mediterranean to eastern and southern Africa

along mountain chains after the continents

touched some 18 my ago. It became extremely

speciose in South Africa. The Protea family

(Proteaceae) is a typical East Gondwana

element, common in South Africa and also

Australia (for instance Silver oak, Grevillearobusta, cultivated in Kenya) and neighbouring

islands.

The ericaceous zone is very well developed on

the eastern slope of Mt. Kenya. In its upper

limit the shrubs remain lower and more spaced.

The areas in between are occupied by tussock

grasses and bright-flowered dwarf shrubs,

characteristic of the next altitudinal belt. This

transition is caused by ever more frequent and

severe night frosts, and the rapid decline in

rainfall at the altitude of ca. 4000 metres.

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Páramo

The term Páramo is favoured by most plant

geographers working in the tropics; the more

colloquial term ‘Alpine zone’ is often used but

not recommended as it refers to the Alps ofEurope. Páramo starts from the transition of the

ericaceous zone ca. 4000 m a.s.l, and its upper

limit is somewhere at 4800–5000 metres,

depending on the steepness, exposure and soil

characteristics of the slope.

Páramo is characterized by low vegetation of

10–50 cm tall dwarf shrubs, often growing as

hemispherical cushions; tussock grasses and

sedges; and herbs arising just a few centimetres

from ground level (Figure 6). All the plants are

very well adapted to night frosts of –5 to –10degrees, very cold moraine soils, soil movement

due to the development of needle ice crystals at

night, and dry winds and extreme solar radiation

during daytime, when air temperature rises up to

+10–15 degrees or more. Rainfall is low, and

often comes down as snow, which melts within

a day or two. The soil’s coldness hampers water

uptake of roots, even in areas where the soil is

wet or there is running water nearby. However,

evaporation from leaves is high, and plants have

many mechanisms and structures to avoid dry-

up: dense hairiness, compact growth, very smallleaves, and thick cuticle.

The most common and striking plants are

Everlastings (genus Helichrysum with many

species), cushion-shaped, with cream or pink

coloured flowers. Adenocarpus mannii has

silvery leaves and yellow flowers, typical of the

Pea family (Fabaceae). Euryops brownei is an

erect dwarf shrub with green needle-like leaves

and bright yellow flowers like in Dandelion.

Lady’s mantle ( Alchemilla) species are creeping

dwarf shrubs with woody stems, unlike the

common herbaceous ones in Northern Europe.

Festuca pilgeri is the commonest grass, and

Carex monostachya the dominant sedge; they

both grow in tall tussocks.

Further up from páramo the term Nival zone

(Cold desert belt) is sometimes used: there is no

continuous vegetation anymore, but soil is

mostly bare moraine, gravel and stones. Here

and there, even close to the glaciers, small

groups of plants can be found in places protected by larger stones, which warm up in

daytime sunshine, and store some warmth for

the cold nights.

Giant rosette plants

In the ericaceous zone, and even more in páramo, the giant rosette plants rise above all

the others (Figure 6). Although looking like

small trees, they are not trees in a biological

sense, but a special life form of tropical–

subtropical high mountains. They are

adaptations to an environment where no seasons

(winter) exist, soil is cold, days are warm, and

night frosts of –5 to –10 degrees are frequent,

but last only a few hours at a time.

Figure 5. Erica and Stoebe kilimandscharica formingan endless heathland on the slopes of Mt. Kenya (P.

Pellikka).

Giant rosette plants, or megaphytes, are found in

all continents that have high tropical mountains.

In Africa such plants belong to two genera,

Senecio (groundsels, Asteraceae family) and

Lobelia (Lobeliaceae). Their taxonomy is

difficult because populations on each mountain

differ from the others; some authors

acknowledge many species while others

consider just a few, variable species to exist.

A striking feature of the megaphytes are their

massive leaf rosettes, situated terminally on

pole-like ( Lobelia) or branched (Senecio) stems.

An unfolded rosette is 1–2 metres across,

containing 80–100 leaves, which have a thick

woolly underside. When temperature falls after

sunset, the leaves turn tightly inside, making a

ball around the apical bud, and protecting it

from freezing. Dead leaves often stay attached

for decades, so making an insulating cover over

younger parts of the trunk, like in Seneciokeniodendron. The thick trunk is not woody

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throughout, but its large pith contains

parenchymatous, water-storing tissue, like in

succulents. This water reservoir is needed in

early morning, when sunshine warms up the

leaves and evaporation starts, but roots are in

the still-frozen soil.

Figure 6. Senecio brassica growing on a swamp andSenecio keniodenrdon species behind in the páramo

zone (P. Pellikka).

Lobelia telekii has a fairly short stem, but over a

metre-tall, and candle-like inflorescence, in

which flowers are protected behind dense, hairy,

fur-like leaves. Lobelia plants die after

flowering, and on the slopes all the time both

growing and flowering individuals can be seen.

They are pollinated by sunbirds. Senecio species

are slow-growing, long-living (up to 200 years),

and the trunk divides into two equal branches

after each flowering. They seldom burst into

flower, usually once in 10–20 years, but

simultaneously over the whole mountain. Their

inflorescences are large, richly branched, and

contain hundreds of small flowers, white or

yellow according to the species. They are

pollinated by flies and other insects.

Careless smoking and frequent grass fires are

very harmful to the slow-growing Senecio

individuals. Charred and half-dead plants are

seen along the paths, and the Senecio stands no

more have the splendour that can be seen from

old photographs. Also other high-mountain

vegetation (e.g. Erica) would benefit from a

more strict control of handling fire along tourist

trails.

Origins of the high-altitude vegetation

A striking feature for a European on the high

African mountains is the abundance of familiar

plant genera in the ericaceous zone and páramo,while they are almost totally unfamiliar further

down in the forest. This dichotomy has caused

much speculation.

Some plant geographers have proposed a well-

founded theory that African mountains were

almost void of vegetation above the timberline,

until land contact was established between the

European and African continental plates in the

Middle Miocene about 18 my ago, and again 13

my ago. Then a mass migration of dozens of

cool-climate-tolerant plant genera started,resulting in a colonisation of the African high-

altitude areas. This process took place fairly

recently in an evolutionary sense, and the

temperate Eurasian plant genera are still easily

recognizable. Almost all the species, however,

are already locally developed African endemics.

Further reading

Coe, M.J. (1967). The ecology of the alpine zone of Mount Kenya. 136 p. Junk, The Hague.Friis, I. (1992). Forests and forest trees of northeast tropical Africa. Kew Bull. Add. Ser . 15, 1–396.

Hedberg, O. (1964). Features of Afroalpine plant ecology. Acta Phytogeographica Suecica 49, 1–

144.

Lind, E.M. & M.E.S. Morrison (1974). East African vegetation. 257 p. Longman, London.

Niemelä, T. (1988). Itä-Afrikan vuoristojen metsät ja sademetsät. In: Erkkilä, A. & T. Kuuluvainen

(eds.) Tropiikin metsät. Silva Carelica 12, 57-72.

Rundel, P.W., A.P. Smith & F.C. Meinzer (eds.) (1994). Tropical alpine environments. Plant form

and function. 376 p. Cambridge University Press, Cambridge.

Vuilleumier, F. & M. Monasterio (eds.) (1986). High altitude tropical biogeography. 649 p. Oxford

University Press, New York & Oxford.

White, F. (1983). The vegetation of Africa. 356 p. + 4 maps. UNESCO, Paris.

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