Numerical interpretation of a high resolution

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Palaeogeography,Palaeoclimatology,Palaeoecology109 (1994) 357-370

Numerical interpretation of a high resolution Holocene pollen record from Burundi D. Jolly a, R. Bonnefille a, M. Roux b a Laboratoire de Gkologie du Quaternaire, CNRS, Facultk de Sciences de Luminy, Case 907, 13288 Marseille Cedex 09, France b Laboratoire de Biomathkmatiques, service 462, Facultk des Sci. et Tech. de St J&ome, 13397 Marseille Cedex 13, France

(Received May 10, 1993; revised and accepted September 1, 1993)

Abstract

The Holocene history of the tropical montane forest is reconstructed using a high resolution pollen record from a valley swamp (3°35'S, 29°41'E), at 2000 m elevation. The post-glacial establishment of the montane rain forest is evidenced at ca. 10,600 yr B.P. Numerical analysis combines the Correspondence Analysis with the Cluster Analysis of both 38 modern and 101 fossil pollen spectra including 125 identified pollen taxa. Four distinct successionnal stages of forest development have been identified. The past vegetation recorded during the early Holocene period ( 10,600-3800 yr B.P.) was more closely related to modern pollen spectra from secondary forest rather than to primary forest samples from Burundi. At 3800 yr B.P., a sharp decrease of arboreal pollen percentages and a noticeable increase of Celtis, a semi-deciduous tree indicate the opening of montane forest due to a drier climatic event. The beginning of the anthropogenic impact evidenced later at 3500 yr B.P., contemporaneously with the Early Iron Age in Burundi, follows the late Holocene climatic shift.

I. Introduction

In East and Central Africa, the montane forest belt occurs between 1800 and 2500 m elevation (Hedberg, 1954). Its vegetation history is now documented through a number of pollen diagrams which show successive changes in floristic composition through time, most of them from high-altitude sites (Livingstone, 1967; Coetzee, 1967; Hamilton, 1972; Taylor, 1992). It is now well established that an open vegetation occurred during glacial times, whereas the forest expanded during the Holocene (Bonnefille and Riollet, 1988; Taylor, 1990). But in these diagrams, the Holocene is either missing (Hamilton, 1982) or incomplete (Bonnefille and Riollet, 1988), or analyzed with insufficient strati0031-0182/94/$7.00© 1994ElsevierScienceB.V. All rights reserved SSDI 0031-0182(93)E0183-T

graphical resolution (Vincens, 1989; Jolly and Bonnefille, 1991 ). This study has been undertaken in order to document the past vegetational changes of the last 10,000 yr with a finer time resolution. The pollen data are interpreted in the light of modern pollen spectra from various types of forests in Burundi, which may provide analogues for past vegetation. Such procedure has been more thoroughly investigated by palynologists working in temperate regions. If a fossil pollen spectrum can be matched with one or more pollen spectra, the analyst may infer that the ecosystem around the modern sampling site is an analogue for the past ecosystem surrounding the fossil site (Overpeck et al., 1985). Reasoning by analogy between the past and the present requires a direct measure of

358

D. Jolly et al./Palaeogeography, Palaeoclimatology, Palaeoecology 109 (1994) 357-370

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Fig. 1. Location of the Kuruyange coring sites (3°35'00"S, 29°41'00"E, altitude 2000 m), Burundi.

D. Jolly et al./Palaeogeography, Palaeoclimatology, Palaeoecology 109 (1994) 357-370

the difference between multivariate samples such as quantitative pollen data. Several dissimilarity coefficients are available in multivariate statistical methods (Prentice, 1986). Among them, the signalto-noise coefficients are not strongly influenced by high values of pollen types and are not too sensitive to variations among rare types. They are considered to be the best for this kind of study (Overpeck et al., 1985). The chi-square coefficient is prefered here instead of the squared chord distance because both cluster anaysis and correspondence analysis are being undertaken using the same dissimilarity coefficient (Benzecri et al., 1976; Gordon, 1982).

359

sequence of Holocene sediment than Kul (Fig. 2). It was therefore decided to undertake high resolution pollen counts on core Ku2. In order to obtain a continuous history of past vegetation for the last 10,000 yr, the sediment was sampled at 10cm interval (101 samples) and analysed. The chemical preparation and pollen counting procedure follow Bonnefille et al. (1991). We have counted pollen grains of 125 identified pollen taxa included in the first publication of Kuruyange core Ku2 (Jolly and Bonnefille, 1991). The data presented here include both 101 fossil and 38 modern pollen samples, the latter being used for statistical analyses only.

1.1. Regional setting 1.2. Dating and chronology The pollen data presented here are from the Burundi highlands at the junction of the Lake Tanganyika and Lake Victoria catchments. The Kuruyange peatbog (3°35'S, 29°41'E) is situated in the watershed of the White Nile system. It lies upstream of the Nyagatika river at 2000 m elevation in a valley 2.5 km long and 200 m wide, located 3 km east of Gisozi (Fig. 1). At Gisozi, the mean annual rainfall averages 1400 mm/yr and mean annual temperature is 16°C (Ergo and Halleux, 1984). Precipitation occurs from June to September following the apparent movement of the Intertropical Convergence Zone (Bultot, 1972). From the botanical point of view the area belongs to the Afromontane phytogeographical region (White, 1983) and, more precisely, to the middle horizon of the montane forest belt, but it is now occupied by pasture land (Lewalle, 1972). Three cores were recorded from the Kuruyange swamp in 1982. Core Kul reached 11.85 m depth and comprised 10 m of an Holocene peat deposited on top of 1.5 m of an organic clay which is Late Pleistocene in age (Bonnefille et al., 1991). In order to obtain a more complete record of the lower Holocene, two additionnal cores were collected again in 1986, a few hundred meters upstream of core Kul. Core Ku2 presented here was terminated at 10.84 m depth by an unpenetrable sand layer. Although the last 1000 yr are missing because the upper layer of the peat has been exploited, the conventional radiocarbon dates obtained for core Ku2 showed a more complete

The radiocarbon dating was performed at the

Laboratoire de Gkologie du Quaternaire on the total organic matter content using conventional counting methods and at Gif/Yvette on the macroscopic plant fragments for the Accelerator Mass Spectrometra (AMS) date (Table 1). The oldest date (10,750__+200 yr B.P.) obtained for the level

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AGE 14C (years B.P.) Fig. 2. Radiocarbon chronology of the Kuruyange core Ku-2, Burundi.

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D. Jollyet al./Palaeogeography, Palaeoclimatology,Palaeoecology109 (1994) 357-370

Table 1 List of radiocarbon dates, performed on core Ku-2 at Kuruyange, Burundi Depth (cm)

Lab. No.

Age 14C (yr B.P.)

0157-170 0270-270.5 0427-440 0526-540 0561-575 0606-626 0708-720 0782-795 0920-934 050-1060 077-1090 106 1115

UQ1471 GIF 92086 UQ 1472 LGQ 234 LGQ205 LGQ235 UQ 1473 UQ 1474 UQ 1475 LGQ783 UQ 1 4 7 6 LGQ784

1650± 100 2800± 100 3750± 100 3960± 150 (481)±183 4580± 170 5300± 100 6600± 100 8300±100 10,010± 190 (9300)±100 10,750±200

LGQ: Lab. de G6ologie du Quaternaire, Marseille (France); UQ=G6otop, Montr6al (Canada); GIF=CNRS Gif/Yvette (France).

1106-1115cm indicates that the Holocene sequence overlies 1 m of organic clay deposited during the glacial period (Fig 3). The chronology of core Kul (Bonnefille et al., 1991) and that of core Ku2 presented here are rather consistent. In the Kuruyange swamp, peat accumulated rapidly at a sedimentation rate varying from 0.73 to 1.51 mm/yr during the last 11,000 yr. Both chronologies indicate that peat deposition occurred over an organic clay that had been deposited during the last glacial period. The change between organic clay and peat sedimentation occurred at 11.05 m depth. The comparison of the chronologies of cores Kul and Ku2 invalidates the conventional ~4C age of 7650__ 100 yr B.P. at 10 m depth in core K u l . This was definitely confirmed by an AMS date of 10,290 + 140 yr B.P. (Gif/Yvette: no. 92088) obtained on plant macrofossils remains at 1018-1019cm. The good agreement between the independent chronologies of the two cores obtained by different laboratories led to our choice of Kuruyange to provide a valuable detailed history of the montane forest in the equatorial region 4°S. This study focuses on results from core Ku2. The choosen 10 cm interval for sampling, and 1 cm thickness for each pollen sample, provides a time

series of pollen data in which each point (spectra) represents pollen deposition during 10-20 yr separated by time intervals averaging about 100 yr. This is valid for the period 10,750-5300 yr B.P. (Fig. 2). As the sedimentation rate increased to 1.51 mm/yr in the last 3500 yr, the time resolution is even better during that period. Beyond 1 m depth, the water content did not allow proper coring of this part. On the basis of extrapolation of the sedimentation rate, the top of the Ku2 sequence can be estimated at ca. 1400 yr B.P., probably because the uppermost sediments have been lost to peat cutting. This estimated age is also confirmed by the radiocarbon date of 1390+100 yr B.P. at 2 m depth on K u l where peat was not exploited. In the area where peat exploitation is now abandoned; sedges and Dissotis shrubs have colonized the soil and their roots penetrate into the peat deposited about 1000 yr ago. This would invalidate any AMS dates that could have been carried on plant macrofossils in the uppermost 50 cm at Ku2. Additional information for the last 1000 yr could be provided in the future by short cores taken from upstream.

2. Vegetational history In the Ku2 pollen diagram, three main pollen zones can be distinguished by eye and by an automatic zonation without stratigraphical constraints (Goeury, 1988) carried on pollen taxa (Fig. 3). Local taxa (Gramineae and Compositae tubuliflorae) and swampy taxa are excluded from the total pollen sum.

2.1. Pollen zones Pollen zone III (1184-1095 cm, > 10,600 yr B. P. ) This lower pollen zone predates 10,750 yr B.P. The high percentages of arboreal taxa (90%), mainly Cliffortia (