«ABSTRACT Dramatic changes are taking place in the glacier-covered high mountains of Africa. The glacier-covered area on Kilimanjaro is now only half ...»
African Study Monographs, Suppl.30: 195-212, March 2005 195
VEGETATION SUCCESSION IN RELATION TO GLACIAL
FLUCTUATION IN THE HIGH MOUNTAINS OF AFRICA
Graduate School of Asian and African Area Studies, Kyoto University
Dramatic changes are taking place in the glacier-covered high mountains of
Africa. The glacier-covered area on Kilimanjaro is now only half as large as it was in the 1970s. The Tyndall Glacier on Mt. Kenya, which retreated at approx. 3 m yr–1 from 1958 to 1997, retreated at ca. 10 m yr–1 from 1997 to 2002. Pioneer species such as Senecio keniophytum, Arabis alpina, mosses, lichen, and Agrostis trachyphylla have advanced over areas formerly covered by the glacier. The rate at which this vegetation migrated up the former bed of the glacier (2.1–4.6 m yr–1 from 1958 to 1997) is similar to the rate of glacial retreat (2.9 m yr–1). In the interval from 1997 to 2002, pioneer species advanced at a rapid rate of 6.4 –12.2 m yr–1 when the glacier retreated at 9.8 m yr–1. Rapid glacial retreat has been accompanied by rapid colonization by plants. Pioneer species improve soil conditions and make habitat suitable for other plants. If warming continues, alpine plant cover may extend all the way to mountain summits, and then eventually diminish as trees colonize the areas formerly occupied by the alpine plants. Larger woody plants such as Senecio keniodendron and Lobelia telekii, which showed no obvious advances before 1997, have advanced quickly since 1997.
Key Words: Vegetation; Deglaciation, Global warming; Environmental change; Alpine zone;
INTRODUCTIONVegetation at glacier fronts is commonly affected by glacial ﬂuctuations (Coe, 1967; Spence, 1989; Mizuno, 1998). Coe (1967) described vegetation zonation, plant colonization, and the distribution of individual plant species on the slopes below the Tyndall and Lewis Glaciers. Spence (1989) analyzed the advance of plant communities in response to the retreat of the Tyndall and Lewis Glaciers for the period 1958 to 1984. Mizuno (1998) addressed plant communities’ responses to more recent glacial retreat by conducting ﬁeld research in 1992, 1994, 1996, and 1997. These studies illustrated the link between ice-retreat and plant colonization near the Tyndall Glacier and Lewis Glacier. In addition, till age and substrate stability are critical controls on vegetation patterns around the glacier (Mizuno, 1998).
Numerous studies have been carried out on the glaciers of Mt. Kenya (Gregory, 1894, 1900; Mackinder, 1900; Troll & Wien, 1949; Charnley, 1959; Coe, 1964;
Kruss & Hastenrath, 1983; Hastenrath, 1983, 1984). Many of these studies dealt with glacial ﬂuctuations and deposits (Baker, 1967; Mahaney, 1979, 1982, 1984, 1989, 1990). Recently, mountain glaciers in Africa h
on glacial ﬂuctuations over the period 1997 to 2002; it clariﬁes the response of plant communities to recent glacier retreat, and discusses the effects of glacial retreat on ecosystems. The habitats of large woody plants such as Senecio keniodendron and Lobelia telekii, which are characteristic plants of tropical high mountains, are examined.
STUDY AREAS AND METHODSI. Study Area Mt. Kenya is an isolated, extinct, denuded volcano that lies on the equator (0°6’S, 37°18’E), approx. 150 km NNE of Nairobi. The summit, Batian, is 5,199 m above sea level (Fig. 1). The mountain was built up by intermittent volcanic eruptions between 3.1 and 2.6 million years ago (Bhatt 1991), and the volcanic plug was dated to 2.64 million years ago (Everden & Curtis, 1965; Mahaney, 1990). Rocks of the volcanic massif consist of basalt, phonolite, kenytes, agglomerates, trachyte, and syenite (Baker, 1967; Baker et al., 1972; Bhatt, 1991; Mahaney, 1990).
The Tyndall Glacier is the second largest glacier on Mt. Kenya, after the Lewis Glacier. Fluctuations of these glaciers have been recorded in detail (Gregory, 1894, 1896, 1900, 1921; Mackinder, 1900, 1901; McGregor Ross, 1911;
Dutton, 1929; Light, 1941; Howard, 1955; Hastenrath, 1984; Mahaney, 1990).
Mahaney (1984, 1990) subdivided Neoglacial deposits into two advances (Tyndall advance and Lewis advance) on the basis of several relative dating (RD) criteria, including topographic position, weathering characteristics, and degree of soil proﬁle expression.
The Lewis and Tyndall Moraines formed in front of the Tyndall Glacier (Fig.
1). The Lewis Till (the Lewis Moraine, ca. 100 yr BP) and Tyndall Till (the Tyndall Moraine, ca. 900 yr BP) are considered to be late Holocene in age, Fig. 1. Alpine zone of Mount Kenya.
Vegetation Succession in Relation to Glacial Fluctuation in the High Mountains 197 based on soil development and weathering features (Spence & Mahaney, 1988;
Mahaney, 1989, 1990; Mizuno, 1998). The Tyndall Moraine is divided into Tyndall Moraine I and Tyndall Moraine II on the basis of topographic position, weathering characteristics, and relative soil development (Mizuno, 1998, 2003a).
The elevations at which the annual minimum, mean, and maximum temperatures of the free atmosphere in East Africa are 0°C, are approx. 3,500 m, 4,750 m, and 6,000 m, respectively (Hastenrath, 1991). The precipitation is southeasterly maximum resulting from the classical monsoon, and secondary maximum on the western side (Mahaney, 1990). Annual precipitation is about 2,500 mm per year at 2,250 m on the southeast slopes of Mt. Kenya, grading to less than 1,000 mm per year at same altitude on the north slope (Hastenrath, 1991; Mahaney, 1984).
Annual rainfall is highest between 2,500 and 3,000 m on the south, west, and east slopes, and decreases towards the peak (900 mm at 4,500 m–4,800 m).
Above 4,500 m, most of the precipitation falls in the form of snow and hail.
Vegetation on Mt. Kenya has been classiﬁed into the Alpine Belt (3,600 m), the Ericaceous Belt (3,600 m to 3,400 m on the south slope, 2,900 m on the north slope), and the Montane Forest Belt (3,400 m; Hastenrath, 1984). The vertical distribution of Senecio keniodendron and Senecio brassica is used to distinguish the upper and lower alpine zones, although there is considerable overlap in their distribution (Hedberg, 1951). In the lower alpine zone, tussock grasses, Senecio brassica, and Lobelia keniensis occupy the wetter areas, and Alchemilletum predominates in dry areas. In the upper alpine zone, Senecio keniodendron is present up to 4,500 m, together with Carex monostachya, Agrostis spp., Cardus platyphyllus, Arabis alpina, Senecio keniophytum, and Lobelia telekii.
The position of Tyndall Glacier’s snout was established by measuring the distance from a sign at Tyndall Tarn. The leading edge of plant-cover was measured from the terminus of the glacier. Moraine positions were compiled on a topographic map (The Glaciers of Mount Kenya, 1:5,000, Hastenrath et al.,
1989) from ﬁeld surveys and aerial photographs (1:50,000).
Plant communities and their environments were surveyed at nine sites (Plots 1 to 9, each 2 m×2 m and representing different terrain conditions). In each survey site, surface materials, land surface stability, lichen coverage on exposed rock, vegetation coverage, and species composition were investigated. The particle-sizes in the surface rubble layer were measured by the long-axis of rubble (30 to 100 measurements at each quadrat). Substrate stability was established using the deﬂection of a painted line. Lichen cover was used as a crosscheck for identifying stability, and to estimate the elapsed time from glacier release. Lichen coverage is the percentage that lichen covers the exposed part of the debris. Soil proﬁles were surveyed at 12 sites (Plots a to l). A till age for each plot was estimated using its distance from the glacier front and established glacial retreat rates [2.9 m yr–1 (1958–1992); 3.8 m yr–1 (–1958); Charnley, 198 K. MIZUNO 1959]).
Habitats of large woody plants such as Senecio keniodendron and Lobelia telekii were investigated around Plot 6. The relationship between the clast size of surﬁcial material and the height of Senecio keniodendron and Lobelia telekii was studied at two sites (15 m×15 m): Plot A (4,390 m, on Tyndall Moraine I) and Plot B (4,390 m, on a debris ﬂow and outwash slope).
RESULTSI. Fluctuation of the Tyndall Glacier and Glacial Topography on Mt. Kenya The Lewis and Tyndall Moraines formed in front of the Tyndall Glacier (Figs.
2 & 3). The Lewis Till (the Lewis Moraine, ca. 100 yr BP) and Tyndall Till (the Tyndall Moraine, ca. 900 yr BP) are considered to be late Holocene in age, based on soil development and weathering features (Spence & Mahaney, 1988;
Mahaney, 1989; Mizuno 1998). The date of Tyndall Moraine corresponds to that of the leopard discovered from the snout of the Tyndall Glacier in 1997 (Fig. 4) (Mizuno, 2005; Mizuno & Nakamura, 1999). The Tyndall Moraine is divided into Tyndall Moraine I and Tyndall Moraine II on the basis of topographic position, weathering characteristics, and relative soil development. The climate ﬂuctuated between warm and cold periods prior to 100 yr BP, and was accompanied by moraine deposition. In the last 100 yr, however, the Tyndall Glacier has retreated constantly and no new moraine material has been deposited. Figs. 5, 6, and 7 shows the extent of the Tyndall Glacier in 1992, 1997, and 2002, during the time it retreated rapidly. This very rapid rate of retreat from 1997 to 2002 (ca. 10 m yr–1) contrasts with the average rate of ca. 3 m yr–1 for the period from 1958 to 1997 (Fig.8). Comparing the photos of 1997 (Fig. 9) and 2002 (Fig. 10) illustrates the very rapid recent retreat.
II. Plant Succession in Response to Deglaciation
Fig. 8 shows changes in the position of the glacier front and the leading edge of each advancing plant species (arrow inclination indicates speed of advancement). For example, in 2002, no plants were present within 12 m of the glacier front, and Senecio keniophytum and Arabis alpina were in areas 12 m away from the glacier front. Moss and lichen were present at distances of 27 m and more.
The ﬁrst species to colonize new till was Senecio keniophytum (Fig. 12b), which advanced at an average rate of 2.7 m yr–1 from 1958 to 1984, and 2.1 m yr–1 from 1984 to 1992. These rates of advance are similar to the rate of glacial retreat (2.9 m yr–1). Other pioneer species, such as Arabis alpina, moss, lichen, and Agrostis trachyphylla, advanced at rates between 2.1 m yr–1 and 4.6 m yr–1 in response to glacial retreat rates of 2.9 m yr–1. Senecio keniophytum advanced at 8.8 m yr–1 and Arabis alpina advanced at 12.2 m yr–1, in response to the Vegetation Succession in Relation to Glacial Fluctuation in the High Mountains 199 Fig. 2. The summit of Mt. Kenya (5,199 m) Fig. 4. Leopard remains discovered on the and the Tyndall Glacier (left). The upper slope Tyndall Glacier, Mt. Kenya, in 1997.
is the Lewis Moraine (white) and the lower slope is Tyndall Moraine I (black).
Fig. 3. Geomorphological map for the environs of the Tyndall Glacier, Mt. Kenya. Margins of the Tyndall Glacier for 1919, 1926 and 1963 are from Hastenrath (1983); for 1950 and 1958 from Charnley (1959). Lewis Moraine (Lewis Till) and Tyndall Moraine (Tyndall Till) are from Mahaney (1982, 1989) and Mahaney and Spence (1989).
glacial retreat of 9.8 m yr–1 for the interval from 1997 to 2002. Arabis alpina eventually got ahead of Senecio keniophytum: the leading edge of the area containing Arabis alpina was 11.56 m from the glacier front, whereas that of Senecio keniophytum was at 11.80 m. Mosses and lichen advanced at a rate of
10.2 m yr–1, and Agrostis trachyphylla also advanced at the rapid rate of 6.4 m yr–1. Large woody plants such as Senecio keniodendron and Lobelia telekii, which did not advance prior to 1997, advanced rapidly at 17.2 m yr–1 and
Fig. 8. Glacial ﬂuctuations and succession of alpine plants.
The horizontal axis: distance (m) from the margin of Tyndall Glacier to the front of each plant
distribution. The vertical axis: date (the length of the vertical axis indicates years). The arrow:
movement of the glacial margin or the front of each plant distribution (the inclination of the arrow indicates speed of movement).
such as ridges or banks, because the ﬁne material within the cracks retains water, and the bedrock slope is stable.
III. Plant Succession and Soil Development Plants change the environments they colonize when they advance into areas formerly covered by glacial ice. Fig. 11 shows the soil proﬁle and till ages (yr) for the study plots, or the time since release from glacial ice. This age, or time, is estimated using the distance between the glacier front and each plot, and the glacial retreat rates [2.9 m yr–1 (1958–1992); 3.8 m yr–1 (1926–1958)]. For example, the time since release from the glacier ice at Plots a, b, and c (i.e., the till ages) are estimated at 5–13 yr. Soil near the glacier is sandy (loamy sand, sandy loam, and sand) with much ﬁne gravel. Soils are immature, lacking humus content, and thus exhibits dark grayish yellow (2.5Y4/2), grayish olive (5Y4/2), and yellowish gray (2.5Y4/1) colors. In the area closest to the icefront, only Senecio keniophytum grows abundantly. At Plot e, where 79 years have elapsed since glacial release, soil is ﬁne-grained (e.g., silty clay), and its color is brownish-black (7.5YR2/2, 10YR2/2) because of a signiﬁcant humus content. Soils of this type can support growth of the large woody plant Senecio keniodendron.