A 22,000-YEAR POLLEN RECORD FROM THE PLATEAU OF ZAMBIA

Pollen analysis of a core from Ishiba Ngandu in Zambia reveals only one significant change in the pollen assemblage laid down over a period of 22,000 yr. About 3,000 B.P. a number of taxa characteristic of moist to dry evergreen forest at low, moderate, or high altitudes declined from very low levels to even lower ones. This decline was probably both relative and absolute. At the same time a small group of taxa that grow in both swamp forest and Savannah increased in a relative scnsc, although probably not in absolute deposition rate. Possibly this change represents the effects of agriculturalists on the vegetation. Neither montane forest nor moist forest of low and intermediate altitudes appears to have grown in this part of Zambia during the past 22,000 yr. The lack of dramatic changes in pollen cannot bc attributed with any assurance to a stable climate, however, bccausc it is unlikely that the vegetational changes to be cxpccted if the rainfall changed by as much as 50% from the present mean level would be recognizable by pollen analysis. INTRODUCTION Bakkcr and Clark 1962; Clark and Bakkcr The published evidence indicates great 1964), Bangweulu Swamp (Lawton 1963), changes in tropical African environments and a temperate lake ( Martin 1968)) but during Plcistocenc and recent time. In no long tropical profile is available from a many places, especially in the Rift Valley lake deposit, where the record is likely to of Kenya, lake levels have been much be most complete and where the difficulhigher th,a.n they are now (Nilsson 1949; ties of paleoccological reconstruction are Richardson 1969; Washbourn 1967). The least scvcre* glaciers #of high mountains spread to much This paper presents a pollen diagram lower altitudes several times during the from Ishiba Ngandu, a lake in the central Pleistocene ( osmaston 1965). The shipart Of northern Zambia. In this part of larity of bird faunas on the mountains of tropical Africa the vegetational changes east and west tropical Africa sugges.ts a during 22,000 yr have been too slight to lowering of vegetation belts with montanc decipher by palynolo,gical methods, and rain forest growing over vast intervening the diagram is more intcrcsting for what areas (Moreau 1963). Mountain pollen possible vegetation types can bc ruled out analyses have confirmed the expectation than for any positive information that it of change in high altitude vegetation (Bakgives about the vegetational history of kcr 1964; Coetzcc 1967; Livingstonc 1967), Zambia* but interpretation of the details and their I am indebted to Dr. J. L. Richardson environmental causes is open to dispute. for assistance in the field, to the late Sir The single pollen diagram from the intcrStewart Gore-Brown for hospitality and montanc plateau ( Kendall 1969) cannot the loan of boats, to Dr. Minzc Stuiver for bc reconciled with extensive low-altitude radiocarbon dating, and to Shirley Cavamontane forest during the last ice-age, but lierc and Alice Richardsoln for technical it abundantly confirms the general finding assistance. of profound environmental change. The research was supported by the John Guggenheim Memorial FounSouth of the equatorial region pollen dation and by several grants from the analysis has been applied to a number of National Science Foundation to Duke Unistream and spring deposits (Bakkcr 1955; vcrsity. Dr. Edna Lind provided a base LIMNOLOGY AND OCEANOGRAPIIY 349 MARCH 1971, V. 16(2) 350 D. A. LIVINGSTONE for field operations at the Botany Department of Makcrerc College, Kampala, TIIE LAKE AND ITS SURROUNDINGS Ishiba Ngandu, Shiwa Ngandu or Lake Young, as it was labeled on some early maps, is a lake about 1 x 5 km in area, with a maximum depth of 7 m. It appears to have been formed when gcntlc crustal warping dammed Mansha Stream., a tributary of the Congo, about 20 km from the Zambczi divide. The local terrain is underlain by Precambrian rocks (UNESCO 1963) and covered by Brachystegia (miombo) woodland. The Luangwa Valley, 60 km to the southcast, is under Colophospermum m,opane (mopanc) woodIand, and the Chambcshi Valley, 50 km to the northeast, supports a variety of vegetation types : Marquesia woodland, PterocarpusE rytlzrophleum ( chipya ) Savannah, and Hypurrhenia grassland, The Nyika Plateau, 200 km away in Malawi, is covcrcd with grassland and patches of evcrgrcen forest with Wicldringtonia, Podocnrpus, K7zaya, and Hagenia ( Fed. Rhodesia and Nyasaland 1960). Ishiba Ngandu is contained within Shiwa Estates, where citrus and cucnlyptus trees have been planted extensively during the past few decades. It is almost surrounded by marsh in which papyrus is important and which has about twice the surface arca of the water. The lake supports a rich growth of aquatic plants, including Blyxa radicans Ridl., Brasenia schreberi J, F. Smel., Limnophyton angolense Buch., Nymp7wzea caerulea Savigny, Ottelin muricatn (C. H. Wright) Dandy, Scripus confervoides Poir, and Utricularia reflexa Oliv. ( Richardson 1964a). Ishiba Ngandu is primarily a sodium bicarbonate water with about 10 ppm of total dissolved solids ( Ricardo 1939). Unpublished analysts OF lcachablc mud cations by A. E:. Richardson indicate that it has never been much more conccntratcd. The productivity is low, only 0.47 g 02 m-2 day-l when measured by Richardson