by René Dommain
If somebody asks you about Africa’s nature you will probably think first of its large herds of wild animals. Peatlands are not the first things to come in mind, because peatlands are generally rare on the African continent.
The summits of the East African high mountains are some of the places in tropical Africa where peat formation is possible. Although peatlands are mentioned in most descriptions of these mountains (e.g. Hedberg 1964), we still have a poor knowledge of them. The existing studies on these mountain peatlands concern paleoecological research that describes vegetation development during the Pleistocene and Holocene. Landscape ecological studies into peatland development, conditions of peat formation, and hydrological functioning are so far lacking.
From 15th to 29th February 2004, I spent two weeks on Mt. Elgon, together with my co-worker and friend Mathias Rieck to study peatlands in the framework of the project “Landscape-ecological inventory of peatlands in the Ugandan part of Mount Elgon”, a cooperation initiative of the University of Greifswald (Germany) and the Uganda Wildlife Authority (UWA).
Uganda is an East African country characterized by savannah
covered plains, rifts, large lakes (e.g. Lake Victoria), and scattered high
mountains. Except for the Rwenzori Range, these high mountains are volcanoes
of Tertiary origin. Their formation relates to the development of the East African
Rift Valley System (EARS).
A clear characteristic of the East-African mountains is the altitudinal vegetation zonation that reflects the changes in climatic conditions. At least seven mountains in East-Africa are high enough to exhibit the unique afroalpine vegetation belt (e.g. Kilimanjaro, Mt Kenya, Ethiopian Highlands). The term “afroalpine vegetation” describes plant communities that grow in the uppermost regions under conditions that Hedberg (1964) described as “winter every night and summer every day”. Daily climate variations are typically larger than the yearly variations. Several morphological adaptations allow the typical plant species to survive under these harsh climate conditions. Peatlands are another characteristic of the afroalpine zone.
Mount Elgon
Mt. Elgon is the oldest and largest solitary volcano in East Africa. It reaches a height of 4321 m a.s.l. and covers an area of around 3500 km2. The North-South extent is about 70 km, the East-West extent about 50 km. The border between Uganda and Kenya runs straight through Mt. Elgon.
The volcano is of Miocene origin (eruptions starting 22 Ma BP) and recently inactive. The eruption of alkaline lava formed smoothly inclined flanks resulting in a shape and structure that is commonly called a shield volcano. Because of its slight slopes Mt. Elgon is rather easy to climb. An important feature of the volcano is its central caldera. After the magma had erupted from the sub-surface magma chamber, its roof collapsed causing the formation of an up to 400 meter deep and eight kilometre wide caldera. Before that mega-event, Mt. Elgon may have been the highest mountain of Africa. During the Pleistocene also Africa experienced lower temperatures and from the Late Glacial Maximum 18.000 BP up to 11.000 BP the highest mountains, also Elgon, were glaciated (Hamilton & Perrot 1978). Currently, glaciers are still occurring in the Rwenzori Mts, on Mt. Kenya, and on Mt. Kilimandjaro. The glaciers on Elgon have shaped its summit region substantially, resulting in typical glacial landforms, like U-shaped valleys, moraines, and denudated basins. Several palaeoecological studies offer insight into the vegetation development after deglaciation.
On the top of Mount Elgon night frosts are common, whereas the temperature at noon can reach more than 20 °C. With exception of a dry season between January and March and a shorter one in September, rainfall is regularly distributed over the year. From the foot of the mountain (with 1600 mm of annual precipitation) going upwards, precipitation first increases to decrease again to a mere 900mm/y at the top (Wesche 2002). These altitudinal climatic differences are reflected in the distinct vegetation belts. In the Ugandan part of Mt. Elgon the area up to 2400 m consists of agricultural land (mainly for crop production of bananas, coffee, potatoes), followed by montane rainforest, a bamboo zone, a montane forest zone, the Ericaceaous belt, and the afroalpine zone respectively.
Mt. Elgon is the home of several tribes, who inhabit the lower slopes. In former times the “Elgonys” inhabited even the upper parts of the mountain, where they kept cattle. The Elgonys used to take fires into the Ericaceaous and afroalpine zones to enlarge their pastures. During the Ugandan civil war in the 1980s they were resettled to the lower slopes as a nature conservation activity. That time was characterized by violent conflicts among different groups. Attacks on villages and tourists continue until these days.
In 1993, the Mount Elgon National Park, covering an area of 1100 km2, was established on the Ugandan side, whereas on the Kenyan side a smaller National Park exists since 1967. In spite of the strong protection status, slash burning and wood and bamboo cutting are still wide-spread and some lower parts of the park have been taken into cultivation. This issue is not surprising, considering the population density of about 500 people/km2 outside the national park.
Study of peatlands
Accompanied by two rangers and three porters, we climbed Mt. Elgon along the Saasa Trail on the western flank. Our primary goal was to search for peatlands and describe them from a landscape-ecological point of view. In particular we aimed at classifying the mires into the ecological and hydrogenetical mire typologies of Succow and Joosten (2001, see also Joosten & Clarke 2002). The field investigations included vegetation analysis and coring along transects in order to describe the peat types and their degree of humification (von Post scale) and to determine peat thickness. An assessment was made of the extent of every peatland, its geographical position and altitude, the topographical relief, and the surface profile of the peatlands. All noticeable human impact was noted. Peat samples of different depths were collected for future analyses of macrofossils and chemical parameters.
The main areas of study were the surroundings of the Saasa trail above an altitude of 3300 m and the inner part of the caldera. After three days of climbing we found the first wetland. In all we studied seven sites, four peatlands and three wetlands with only sparse peat deposits (a spring, a swamp, and the margins of a brook).
The wetlands of the Ericaceous belt (3400m – 3700m) have no peat or only very shallow peat deposits. The topsoil is rich in humus and below that we found clay. Such wetlands have a vegetation of Cyperus species with scattered Ericaceae shrubs (e.g. Erica trimera). Either they lay along streams and are influenced by floods or in depressions with interrupted drainage.
At the higher parts within the afroalpine vegetation belt two real peatlands were found at 3900 m. They are situated at the outer rim of the caldera in basins among hills. Both peatlands are connected to lakes, although the latter were nearly or completely dry at the period of study. The groundwater level of the peatlands was up to half a meter below the surface –clearly caused by the dry weather conditions. The well decomposed sedge peats (H 5-7) underline the occurrence of fluctuating water levels. Clay below the up to two meter thick peat deposits prevents water losses. Probably during heavy rainfall these mires will be flooded. Therefore, I think that these peatlands can be classified as “water rise mires” sensu Joosten & Clarke (2002). The recent vegetation is dominated by the tussock forming sedge Carex runssoroensis, the main peat former of the Mt. Elgon mires. This species is endemic for the East African high mountains and grows both on wet and dry areas. Further conspicuous plants are typical afroalpine elements including the dwarf Alchemilla johnstonii on drier parts, the endemic Giant Lobelia Lobelia deckenii ssp. elgonensis in wet hollows, and particularly at the edge of the mires the impressive Giant Groundsel Dendrosenecio elgonensis ssp. elgonensis.
At the 21st of February we climbed the Wagagai summit, with 4321 meter
the highest point of Mount Elgon. From the top of that mountain we discovered
large mires inside the caldera. The mires are located in denudation terraces
on different levels. They are surrounded by the steep slopes of the caldera
rim that are covered by amazing open woodlands of Dendrosenecio elgonensis
ssp. barbatipes. The terraces result of the former activity of glaciers,
that cut hollows in the lava layers of different resistance. Currently these
depressions are filled with peat or lakes.
Within the time available, we could study
two peatlands, which are located next to the Wagagai. The upper one, at an elevation
of 4220 meter, has a size of 3,5 hectare and the lower one, at an altitude of
4150 meter, is at least 3 hectare large. Sloping fens connect the peat filled
basins to each other. In contrast to the peatlands at the outer side of the
caldera 
these mires were completely water logged (open
water in hollows and small pools) and therefore certainly peat accumulating.
The peat layers predominantly consist of
slightly decomposed coarse sedge peat (H3-5), but radicel peat and - in one
core - brownmoss peat are present as well. The peat thickness of the upper mire
varies between two and four meter deep, whereas the lower peatland is 7,10m
thick. Also there, clay is present below the peat. Both peatlands are dominated
by an open vegetation of Carex runssoroensis. Typical other plants are
Lobelia deckenii, Crassula granvikii, and Ranunculus volkensii
in hollows, whereas on drier parts especially Alchemilla elgonensis and
Deschampsia caespitosa are growing. Brownmosses cover the ground in more
or less clear hummocks and hollows. Sphagnum is totally absent. The sloping
fens between the individual peatlands have a maximum depth of one meter of strongly
humified (H 7-9) peat. The presence of streams along the sloping fens that interconnect
the mires on various terraces indicate that these peatlands have sufficient
water supply during the dry seasons. The less decomposed peats underline this.
A possible reason of the clear hydrological distinction
to the drier peatlands at the outer flanks can be found
in the functioning of the caldera as a pool of cold air where at night condensation
water supplies enough water to maintain a high water level in the basins. Another
possibility is that the mires are fed by groundwater exfiltrating from cracks
in the bedrock, because the uppermost part of Mount Elgon is geologically strongly
disturbed. We have, however, no further evidence for that hypothesis and hydrogeological
investigations would be necessary to test ti. Because of lacking insight into
the hydrological regime it is not possible the classify the caldera peatlands
unambiguously. Perhaps, they are percolation mires because they are slightly
sloping, perhaps they function according to the kettle hole principle and grow
upward because precipitation of humus colloids from the peat seals off the mineral
soil on the transition of mire and basin. This raises the drainage level and
leads to higher water levels in the basins that result in peat growth and in
humus precipitation on progressively higher levels (Gaudig et al. submitted).
That these peatlands are bogs, as they are called in nearly every relevant publication
is definitely a mistake.
The absence of Sphagnum species and of a dome shape disproves this interpretation.
Three main geological features facilitate mire development in the uppermost part of Mount Elgon. These include
- the occurrence of the caldera causing the a wetter climate in contrast to the outer flanks,
- the rim of the caldera next to the peatlands that functions as a catchment area,
- and the former glaciation that scooped out basins in the different rock layers that prevent water loss.
Peat formation outside the caldera is strongly influenced by dry periods which cause a high degree of humification. Consequently, the hydraulic storage coefficient of the peat is small (resulting a large water level fluctuations) and mire surface oscillation is restricted (Joosten & Clarke 2002). Therefore, the peat accumulation rates are very small and real peatlands are rare on the outer slopes of Mt. Elgon. They occur only in depressions where enough water is assembling during the rain season.
There
is a clear relation between the distribution of peatlands and the vegetation
zones. In contrast to what Hedberg (1964) states, peatlands on the Ugandan side
of Mt. Elgon are restricted to the afroalpine zone and no genuine mire was found
in the Ericaceous belt or at lower zones. The extreme climate conditions within
the afroalpine zone especially the night frosts, are decisive in preventing
fast microbial decay of dead plant material.
Further chemical analysis will reveal the ecological properties of the mires. Up to now only Rejmankova & Rejmanek (1995) conducted soil chemical investigations of peatlands and measured a C/N ratio of 16 and a pH of 6,15 in a Mount Elgon mire. According to Succow & Joosten (2001) this mire belongs to the eutrophic calcareous mire type.
A lot of work still needs to be done, but the first steps towards the understanding of these unique ecosystems on the East African high mountains have been made. Surely the harsh climatic conditions are deterrent for researchers. Nevertheless it was impressive for me to core while it was snowing on an African volcano at an altitude of over 4000 meter!
Use the chance to become acquainted with African mires by participating in the IMCG Field Symposium in Southern Africa!
References
Gaudig, G., Couwenberg, J. & Joosten, H. (Submitted): Peat accumulation in kettle holes: bottom up or top down. Wetlands.
Hamilton, A.C. & Perrott, R.A. (1978): Date of deglacierisation of Mount Elgon. Nature 273: 49.
Hedberg, O. (1964): Features of afro-alpine plant ecology. Acta Phytogeographica Suecica (49): 1 – 144, Uppsala.
Joosten, H. & Clarke, D. (2002): Wise use of mires and peatlands – Background and principles including a framework for decision-making. International Mire Conservation Group / International Peat Society, 304 pp.
Körner, C. (2003): Alpine plant life – Funtional ecology of high mountain ecosystems (2nd edition). Springer, Heidelberg/New York, 344pp.
Rejmankova, E. & Rejmanek, M. (1995): A comparison of Carex runssoroensis fens on Ruwenzori Mountains and Mount Elgon, Uganda. Biotropica 27: 37 – 46.
Succow, M. & Joosten, H. (Eds.)(2001): Landschaftsökologische Moorkunde. 2nd Edition –Schweitzerbart, Stuttgart, 622 pp.
Wesche, K. (2002): The high-altitude Environment of Mt. Elgon (Uganda, Kenya): Climate, Vegetation, and the impact of fire. Ecotropical Monographs No. 2., 253 pp.
author: René Dommain
Botanical Institute Greifswald (Germany)
Email: rene.dommain@gmx.de