zonation and characteristics of the vegetation of mt

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 Museum1 and Department of Geography2, University of Helsinki

Abstract In this paper the vegetation of Mt. Kenya is briefly summarized, mostly by describing the zonation from the foothills 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 fairly young (a few millions of years) if compared to the ancient Eastern Arc mountains closer to the Indian Ocean coastline, 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 upheaval of 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 hundred metres 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, and anomalies 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


Figure 1. The vegetation zones of Mt. Kenya, according to Coe (1967), strongly modified.

Figure 2. The vegetation zones around Mt. Kenya interpreted from Landsat TM satellite scenes from 1976 and

the 1990s. The bamboo zone in the west is patchy and zone limits are not as clear than in the east.


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, the forest 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 are 30–40 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 on the 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 valuable timber trees, and in most areas they have been


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 soil erosion, 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 uniform thicket 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, in addition 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, the forest 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.


Figure 4. In the upper montane zone the trees are twisted and multi-stemmed and lichen epiphytes are abundant

and 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 shrubs there 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, Grevillea robusta, 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.


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 of Europe. 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 –10 degrees, 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 small leaves, 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 forming an 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 Senecio keniodendron. The thick trunk is not woody


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 and Senecio 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.

zonation and characteristics of the vegetation of mt

Documents