A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria

A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria

Quaternary International xxx (2013) 1e7 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate...

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Quaternary International xxx (2013) 1e7

Contents lists available at ScienceDirect

Quaternary International journal homepage: www.elsevier.com/locate/quaint

A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria Reginald C. Njokuocha a, Izuchukwu M. Akaegbobi b, * a b

Department of Plant Science and Biotechnology, University of Nigeria, Nsukka, Enugu State, Nigeria Department of Geology, University of Ibadan, Ibadan, Oyo State, Nigeria

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

The pollen record from Ohe Pond (N 06 50.156, E 007 21.837) in Nsukka, Enugu State, southeastern Nigeria, provided strong evidence of existence of a Southern Guinea savanna in the mid-Holocene. Prior to 6417  67 BP, there was a closed forest with strong Guinea savanna affinities with a more humid condition. However, after 6417  67 BP, Southern Guinea savanna woodland became well established in the area. Considerable periods of high precipitation alternating with warm and dry climatic phases may have favoured the establishment and maintenance of this complex heterogeneous vegetation structure after 6388  71 BP. Frequent natural fires, indicated by charred plant tissues throughout the mid Holocene, probably were important in promoting the expansion of savanna species in the vegetation. Furthermore, human impact associated with iron smelting from about 4005  40 BP in the area may have contributed to depletion of the forest vegetation. The transition from the Southern Guinea savanna to derived forest-savanna mosaic vegetation became apparent at a latter part of the late Holocene around 1470  79 BP. Although climate may have played a significant role in shaping the structure of the vegetation, human activities certainly played an important role in the late Holocene vegetation transformation. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

(Njokuocha, 2012). The sudden disappearance of Podocarpus, a montane plant, was seen as an indication of a drier climate condition. Possible degradation and opening of the forest by agricultural and other anthropogenic activities were also reported in the palynological studies by Agwu (1986) and Njokuocha (2012) in locations from Nsukka and elsewhere in southeast Nigeria. Further evidence of anthropogenic activities was recorded from archeological excavations in Opi and Lajja in Nsukka and around Okigwe cuesta in southeast Nigeria. Artifacts of material culture and metal technology (Ibeanu and Umeji, 2001; Eze-Uzomaka, 2009) were recovered, with the oldest dated 4005  40 BP. According to Ballouche and Neumann (1995), the emergence of metal technology and food production may have had some effects on the vegetation of the West African subregions. This perhaps could have contributed to the fragmentation and degradation of original forests in southeastern Nigeria, into patches, secondary forests as well as patches of secondary grassland and woodland thickets (White, 1983; Agwu, 1986). In the Niger Delta and southwestern Nigeria, distinct climatic and vegetation changes were reported by Sowunmi (1981, 2004), and Alibi (1999) from pollen, archeo-botanical, and geomorphological analyses covering the late Pleistocene to the late Holocene. Prior to ca. 3109  26 BP, Sowunmi (2004) reported significant

A palynological study of a core from Ohe pod, Nsukka, southeast Nigeria was conducted as part of an effort to conceptually understand the history of the establishment of the mosaic lowland rainforest and secondary grassland vegetation, which was originally believed to be rainforest vegetation. In this region, there is a paucity of palynological and archeological research in relation to the Holocene vegetation changes. Agwu (1986) attempted a preliminary history of the climate and vegetation of east central Nigeria using pollen data of surface sediments. The findings showed an indication of a dynamic vegetation development which could be discerned as possible vegetation and climatic change, or possibly change in the floristic composition. A pollen diagram from Lake Obayi, Nsukka showed a gradual transition from a forest dominated vegetation to that of mosaic lowland rainforest and secondary grassland dominated by grasses and savanna components. This became prominent from before 1845  41 BP to present

* Corresponding author. E-mail addresses: [email protected] (R.C. Njokuocha), [email protected] yahoo.com (I.M. Akaegbobi). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.07.036

Please cite this article in press as: Njokuocha, R.C., Akaegbobi, I.M., A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.07.036

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changes in the vegetation of the southwest coast on the Nigeriane Benin border, and attributed the changes primarily to a slight shift to drier climate and marine regression. She reported that this period was marked by the disappearance of Rhizophora-mangrove vegetation which was replaced by fresh water swamp, with the expansion of fresh water swamp forest as well as the coastal savanna. According to her this led to the establishment of the mosaic open forest and savanna vegetation currently existing today in the area. In the Niger Delta, considerable fluctuations in magnitude of wet and dry climatic phases occurred around 7500e4200 BP (Sowunmi, 1981). These fluctuations were reported to have controlled the establishment, expansion, and contraction of the inland rainforest and swamp forest vis-à-vis the mangrove vegetation. Human activities were suggested to have probably become an additional causative factor that accelerated the cause of vegetation change in the late Holocene. The increased expansion in Elaeis guineensis beyond the limits maintained by natural forces relative to other rainforest components, as well as presence of weeds of waste places, indicates possible human interference in the vegetation. Additionally, palynological investigations from northeast Nigeria indicate the rise in extreme climatic conditions primarily resulted in the transformation of the vegetation of that sub-region (Salzmann and Waller, 1998; Salzmann et al., 2002; Waller et al., 2007). They inferred that the increase in the intensity of aridity from about 3800 BP to present appears to have led to the establishment of the modern vegetation types. However, based on palynological and anthracological studies from Burkina Faso and Gajiganna, northeast Nigeria, Ballouche and Neumann (1995) attributed the major changes in vegetation of northeast Nigeria to the intensity of human impact from about 3000 BP. It is therefore evident in Nigeria that prior to about 3800 BP most studies attributed changes in vegetation to climatic factors due largely to lack of distinct evidence of human impact from both palynological and archeological records (Salzmann et al., 2002; Alibi, 2004; Waller et al., 2007). However, from 3800 BP onwards, evidence of human impact was recorded in most palynological and archeological studies across Nigeria (Sowunmi, 1981; Ballouche and Neumann, 1995; Ibeanu and Umeji, 2001; Njokuocha, 2012). This study was therefore undertaken as part of a concerted effort to contribute to the baseline data for the trace of the history of Holocene vegetation and climate changes in Nigeria through palynological investigation on a 100 cm core from Ohe Pond, Nsukka. 2. Study area Ohe Pond is about 3.0 m deep, but may exceed this depth during the rainy season. It is one of the natural water bodies formed in the poorly permeable shaly Nsukka and Ajali lithological formations, and is located at N 06 50.156, E 007 21.837, in a valley at an altitude of 1502 m above sea level. It is a stagnant pond, annually flooded by rainfall and is maintained at shallow level during the dry season by underground percolation. The region is climatically subject to two major seasons with prevailing northeast trade winds during the dry season and a humid southwest monsoon during the rainy season. The dry season lasts for about five months from November to March, with less than 50 mm monthly precipitation (White, 1983). The dry season desiccation is intensified by the dry harmattam wind (NE Trade) that blows strongly from December to February. The rainy season starts in April and attains two peaks in July and September, with an annual rainfall in Nsukka ranging from 986 mm to 2098.2 mm. The present rainfall and dry season pattern is controlled by the intertropical front (ITCZ) which shifts over Africa

between w22 N (Rainy season position) and w4 N (Dry season position) during the year (Inyang, 1978). The monthly temperatures fluctuate between 24  C and 29  C. The highest temperature (33  C) occurs in January and the lowest in (19  C) in August (Agwu, 1997). The vegetation of Nsukka is part of the mosaic lowland rainforest and secondary grassland (White, 1983) which stretches east to west across the latitudinal zones of Nigeria. Presently, the vegetation has been degraded by intensive cultivation and bush burning among other anthropogenic activities into a typical forestsavanna mosaic with small, usually severely degraded patches of the original forests, secondary thickets, and woodland forests (White, 1983; Agwu, 1986). The body of the pond is presently dominated by a cushion of grasses, mostly Imperata cylindrica, sedges, and a luxuriant community of ferns among other herbaceous plants such as Jussiaea sp. and Commelina diffusa. Surrounding the pond is a narrow zone of degraded vegetation, composed mainly of riparian, semi-deciduous and pioneer forest taxa as well as savanna communities dominated by Alchornea cordifolia, Berlinia grandiflora, Baphia nitida, Canarium schweinfurthii, Afzelia africana, Ficus sp., Anthocleista djalonensis, Tabernaemontana sp., Phyllanthus muellerianus and Dacryodes klaineana and isolated clusters of E. guineensis. Associated with these are some woody shrubs and climbers such as Icacina trichantha, Sphenocentrum jollyanum, Cnestis ferruginea, Landolphia dulcis, Cyphostema quadrangularis, Bridelia ferruginea, Combretum recemosum, Barteria nigritiana, Voacanga africana, Rauvolfia vomitoria, Hugonia sp., Paullinia pinnata, Napoleona imperialis, Asparagus africana, and Lonchocarpus cyanescens among others. At the time of sampling, most of the surrounding vegetation had been cleared and burned for crop cultivation. The commonly cultivated crops include Solanum sp., Manihot utilissima, Cajanus cajan, Abelmoschus esculentus, Zea mays, Phaseolus sp. and Capsicum annum. Growing with the crops and in the surrounding pond are weeds of farmlands such as Venonia spp., Emilia sonchifolia, Aspilia africana, Ageratum conyzoides, Chromolaena odorata, Luffa cylindrica, Dissotis spp., C. diffusa, Amaranthus spp., Chenopodium spp. and roadside weeds such as Sida acuta, Senna mimosoides, Triumfetta rhomboidea and Desmodium adscendens (Figs. 1 and 2). 3. Geology of the study area The Nsukka area consists of three major geologic formations: the Mamu, Ajali and Nsukka formations (Fig. 3). The Mamu and Nsukka lithology are very similar, consisting of alternating successions of sandstone, dark shale and sandy shale with thin coal seams at various horizons (Ezeh and Ugwu, 2010; Umeji et al., 2012). The Ajali lithology consists of poorly sorted sandstones, characteristically white but occasionally iron-stained. The Ajali Formation overlies the Mamu Formation, while the Nsukka Formation overlies the Ajali Formation conformably. Surface waters are absent on the permeable members of the Ajali sandstone, while lakes, streams, rivers and marshlands are highly developed on the poorly permeable shaly Nsukka and Mamu sandstones (Egboka, 1983). The study also is characterized by two types of land formations: residual hills and dry valleys. The vegetation and valleys are related to the rock types. The residual hills are remnants of the Nsukka Formation and in some places form outliers on the Ajali sandtone separated by dry valley. The Nsukka Formation and Ajali sandstone are capped by thick deposits of red earth and laterites (Raiment, 1965; Nwachukwu, 1978). 4. Materials and methods Twenty subsamples were analysed from a 100 cm core. Subsamples for analysis were taken at intervals of 2.5 cm through 0e

Please cite this article in press as: Njokuocha, R.C., Akaegbobi, I.M., A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.07.036

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Fig. 1. Vegetation zones of Nigeria.

100 cm. The subsamples were processed in the Palynology and Environment Research Unit, Department of Botany, University of Nigeria, Nsukka following standard procedures with KOH, HCL, HF and ZCl2 (Moore and Webb, 1978). The polliniferous materials were stored in vial bottles in a suspension of glycerol. Routine counting and photomicrographs were done with a WESO trinocular compound microscope and digital camera attached (Nikon COOLPIX P5000). Pollen percentages were based only on total pollen counts. The fern spores and charred plant tissues were calculated separately. An average of 350 pollen grains was counted per sample with exception of some subsamples with very low pollen count. Identification of some pollen was hampered by corrosion, crumbling and breakage of grains. The unknown grains were lumped under indeterminate.

Identification of palynomorphs was aided by reference slides as well as pollen atlas publications by APLF (1974), Y’bert (1979) and Bonnefille and Riollet (1980). The term ‘Moraceae complex’ refers to pollen of five Moraceae species, i.e. Milicia excelsa, Antiaris toxicaria, Treculia africana, Musanga cecropioides and Ficus which are lumped together because their tiny, diporate/triporate, smooth pollen grains were not clearly distinguishable (cf. Sowunmi, 1991). The charred plant particles were prepared as part of the routine pollen preparation methods. The absolute pollen-slide charcoal counts were implored as in the pollen grains and the percentage frequency determined. This method was adopted because of the low density of charred plant particles in the sediment samples. Radiocarbon dates were obtained from Stable Light Isotope Laboratory run on a Thermo GasBench coupled to a Finnigan MAT 252

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Fig. 2. Satellite image of Ohe Pond showing the landscape and sampling point. Source: 2012 Google Earth.

mass spectrometer, Archaeology Department at the University of Cape Town. South Africa. The pollen diagram was prepared using the Tilia and Tilia graph computer software (Grimm, 1991).

Table 1 Isotopic fractionation

13

C/12C dates for Ohe pond, Nsukka, Southeastern, Nigeria. 13

C/12C BP

Depths of sample (cm)

Lab code

Age

31e30 58e60 90e91

SLT-X SLT-Y SLT-Z

1470  79 6388  71 6417  67

Cal BP 1321e1477 7262e7398 7288e7408

Zone 1 (100e90 cm): This zone is characterized by high values of A. cordifolia (45%) and Moraceae complex (25%), both constituting the major lowland rainforest taxa recorded. The values of the fresh water taxa (25%e15%) and ferns (58%e50%) are high at the lower and upper part of this zone, while the Guinea savanna taxa show low values. There is a low number of plant taxa from the LRF, Guinea savanna and freshwater taxa. Poaceae increased in values from 10% to 45% in the uppermost part of this zone. Zone II (90e30 cm): This zone is marked by an increase in a diversity of taxa from the rainforest, Southern Guinea savanna, and fresh water taxa. The rainforest pollen is characterized by high values of A. cordifolia (25e48%) and Moraceae complex (10e25%) especially up to the middle zone. Above, A. cordifolia decreased to a considerably lower level (5%). The values of Poaceae decreased (20%) after an initial peak (48%) and decreased (10%) at the end of the zone after a second peak (36%). The values of the Guinea savanna pollen fluctuated considerably in this zone with the component species occurring sporadically. However, P. muellerianus, Zypophyllum, Lophira lanceolata and CombretaceaeeMelastomataceae are the major wood savanna taxa. The frequency of the freshwater is higher than in zone I. Cyperaceae and Nymphaea lotus are more or less constantly present, with higher values at (20%) and near (10%) the top of this zone. The values of ferns are consistently high (30e50%) throughout this zone. Charred plant particles are recorded consistently in most of this zone. Herbaceous taxa such as Amaranthaceae/ Chenopodiaceae and Asteraceae are present in very low frequencies. Zone III (30e10 cm): In this zone the values of rainforest (25e 45%) and derived savanna (5e18%) declined at the beginning of this zone but progressively increased towards the end of the zone. There is a considerable decrease in Poaceae (40%e15%) and A. cordifolia (10-2%), while Moraceae complex (20e35%), E. guineensis (0e10%) and P. muellerianus (0e10%) increased slightly. Freshwater taxa such as Cyperaceae and N. lotus decreased to low values, while fern increased to high values (60%). However, in the mid-zone the sedges (Cyperaceae) increased sharply to higher values (5e20%). 6. Interpretation

5. Results The pollen data are presented as percentages of the total pollen sum (Fig. 4). The diagram is divided into three zones based on the fluctuations in pollen curves.

Fig. 3. Geology of study region.

6.1. Mid-Holocene vegetation (>6417  67e<6388  71 BP) zone I, II In the mid-Holocene, a vastly diversified Southern Guinea forest-savanna woodland was established in the area. The highest percentage of the component taxa and the maximum number of diversity of the taxa was recorded in zone II. Trees, shrubs, and woody climbers attained prominence in the mid-Holocene, including fresh water taxa as well as ferns. The pollen diagram contains some characteristic mosaic forest-savanna trees (Moraceae, Dracaena sp, E. guineensis, L. lanceolata, Syzygium guineense) as well as shrubs and woody climbers (A. cordifolia, P. muellerianus, Zypophyllum, Smilax kraussiana, A. africana, Combretaceae/ Melastomataceae). This trend towards increased diversity may be indicative of potentially forest-savanna vegetation around the pond, which may be an extension of the widespread Guinean savanna prevalent in the region. The decline in Poaceae percentage over a considerable period of time may be attributed to the establishment of more trees and shrubs in the area that create more cover. It could also be attributed to fire which lowers the total pollen productivity of grasses, such that reduced grass production showed no change in the percentage in the pollen diagram (Livingstone, 1971). Detailed interpretation of the trees and shrubs diversity in the area was hampered by the mostly zoogamous and amphiphilous tree and shrub taxa in the forest-savanna vegetation (Salzmann and Waller, 1998; Salzmann, 2000). However, there is an indication that

Please cite this article in press as: Njokuocha, R.C., Akaegbobi, I.M., A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.07.036

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Fig. 4. Pollen diagram of selected taxa, Ohe pond, Nsukka, Eastern Nigeria.

both the forest species (such as Moraceae, A. cordifolia, Dracaena and B. grandiflora) and characteristic savanna taxa (such as P. muellerianus, L. lanceolata, S. kraussiana, S. guineense and A. africana) were represented in the vegetation structure. In tropical Africa, relative abundances of Moraceae, Dracaena, Raphia and ferns are suggestive of moist evergreen or semi-deciduous forest. Changes in fresh water plants are readily associated with normal lake development. The increase in diversity of freshwater plants and abundance of ferns is an indication of low lake level, with high sedimentation rate that permits the establishment of abundant aquatic species. 6.2. Late Holocene (1470  79 BP) zone III Prior to the beginning of the late Holocene, a Southern Guinea savanna was established in the Nsukka region with a floristic affinity similar to that of the late Holocene vegetation. After 1470  79 BP, the lowland rainforest taxa declined slightly, while the savanna elements increased slightly, became more diverse, but occurred sporadically. The sharp decline in Poaceae and A. cordifolia shown in the curve is indicative of a drier climatic condition, but humid enough to support the growth of members of Moraceae and possibly other moisture loving plants. It is also possible that reduction in these two important taxa may not only be attributed to dry climate but other causes such as human impact. The abundance of Cyperaceae and the fern community not only indicates lowering of lake level but increase in sedimentation in the pond (Salzmann, 2000). Charred plant particles, a possible human impact indicator which would have provided information on human activities during this period, were not continuous in the diagram. However, another indicator of human presence would be an increase in trees of potential economic importance. Some members of the Moraceae complex, such as M. excelsa, A. toxicaria are valuable sources of timber, while T. africana seeds are major sources of food. Oil palm tree (E. guineensis), which is one of the most prominent and protected economic trees in the lowland rainforest and secondary grassland zone, became more visible in the late Holocene. At the end of the late Holocene, both the lowland rainforest components represented by the Moraceace complex and E. guineensis and the savanna taxa such as P. muellerianus, A. africana, S. guineense and CombretaceaeeMelastomataceae were on the increase, although the savanna components still showed

more diversity. The Southern Guinea savanna established prior to 6417  67 BP prevailed until some time before 1470  79 BP, when the vegetation transitioned into derived savanna vegetation with more open character. Although evidence of human impact was not well pronounced in this zone, it may be too presumptuous to attribute the open character of the vegetation to only the effects of climatic change. 7. Discussion The overall vegetation pattern indicates a closed Guinea savanna woodland that gradually transited into a derived savanna woodland and open secondary forest vegetation. Changes in vegetation structure and physiognomy during some stages in the development of the vegetation were found to occur from a time prior to 6417  67 BP. The period 6417  67 BP until a time before 1470  79 BP marked a period in the establishment of Southern Guinea savanna in this part of the region. However, from some time before 1470  79 BP, the vegetation transitioned into derived savanna woodland and secondary forest vegetation. Prior to 6417  67 BP, the diagram points to the existence of a forest vegetation with Guinean savanna affinities (Moraceae complex, A. cordifolia, P. muellerianus, Zygophyllum) around the pond. The ephemeral appearance of a fresh water fern, Polypodium, and freshwater swamp forest taxa Raphia and Scyphocephalium not only supports the existence of forest but indicates a period of wet conditions and probably an increase in pond level. Hydrological evidence in other parts of West Africa (Lézine and Casanova, 1989; Ballouche and Neumann, 1995) points to the existence of a humid condition from a period after 9000 BP. The sharp increase in Poaceae and the corresponding decrease in A. cordifolia and Moraceae complex around 6417  67 BP suggest a possible shift in climate. The high rise in ferns, Scyphocephalium, and the appearance of AmaranthaceaeeChenopodiaceae, N. lotus and Cyperaceae indicates increase in sedimentation and lowering of lake levels, possibly due to drier climatic conditions (Ballouche and Neumann, 1995; Salzmann and Waller, 1998). From 6417  67 BP untila period after 6388  71 BP, Southern Guinea savanna vegetation became established (Moraceae complex, A. cordifolia, P. muellerianus, L. lanceolata, S. guineense, A. africana). Patches of dry forests may have existed alongside the Guinean savanna vegetation. The floristic composition in this midHolocene vegetation closely corresponds to that of the present day

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mosaic lowland rainforest and secondary grassland found in Nsukka (Igbozurike, 1978; White, 1983; Njokuocha, 2012). Similar trends in vegetation development have been reported for the early and mid-Holocene pollen sequences at lake Tilla Biu, NE Nigeria (Salzmann, 2000), Senegal (Lézine, 1989) and Manga Grassland, NE Nigeria (Salzmann, 1996; Salzmann and Waller, 1998). It is most probable that considerable periods of high precipitation alternating with warm and dry climatic phases may have favoured the establishment and maintenance of this complex heterogeneous vegetation structure. The expansion in forest elements as well as the persistence of Cyperaceae and ferns that grow additionally in swamps, marshes and humid upland areas (Dupont and Agwu, 1991; Jahns, 1996; Umeji et al., 2012) could have been favoured by moist conditions created by rainfall during this period. Fluctuations between the wet and dry phases were reported to have occurred around the early-mid to late Holocene across locations in Nigeria (Sowunmi, 1981; Salzmann and Waller, 1998; Sowunmi, 2004). Reports of palynological studies in Manga Grasslands (Salzmann and Waller, 1998) and Lake Tilla (Salzmann et al., 2002) of northeastern Nigeria showed that humid conditions prevailed in the earlyemid-Holocene. This was suggested to have contributed to the establishment and sustenance of swamp forest vegetation in the interdune depression in Manga Grassland (10,000eca. 3300 BP) and the establishment of a dense Guinean savanna in Tilla (8500e7000 14C BP). The fluctuating curve of charred plant particles in the diagram points to the frequency and regular occurrences of savanna fires and intensity in the Nsukka Plateau as well as the surroundings throughout the mid-Holocene. Such fluctuations can be attributed to an increase in biomass of the fringing vegetation and or the surrounding woodland savanna (Salzmann, 2000). This probably was responsible for maintaining the Poaceae at its constant level during the major part of the mid Holocene in the area. The occurrences of these fires (in the late Holocene) predates all records of human impact in Nigeria and possibly in the West African subregion, and as such are attributed more to natural causes. From a period before 1470  79 BP onwards, the pollen diagram showed a gradual change from the dense Guinean savanna to more open forest-savanna mosaic vegetation, hence indicating drier climatic conditions. Around 1470  79 BP, a change in vegetation around the periphery of the pond points to the establishment of derived savanna mosaic vegetation, similar to the present day assemblage in Nsukka. This vegetation today is part of the ecotonal regional vegetation which stretches east-west across the latitudinal zones of Nigeria with its broadest north-south extension through Nsukka. It is located between the true Guinea savanna in the north and the tropical rainforest belt in southern Nigeria (Agwu, 1997). In nearly all West African palaeoecological studies, major vegetational changes were recorded at the beginning of the late Holocene (Maley, 1991; Kadomura and Kiyonaga, 1994; Ballouche and Neumann, 1995; Salzmann, 2000; Sowunmi, 2004). During the last phase in the late Holocene, there was a return of the humid conditions indicated by renewed spread of the lowland rainforest (Moraceae complex, E. guineensis) and savanna elements (e.g. Phyllanthus meullerianus, CombretaceaeeMelastomataceae) (cf. Sowunmi, 2004). However, the phenomenal decline in Poaceae and A. cordifolia, pioneer secondary taxa from this period, may not be attributed to climatic factors alone. The other feasible explanation may be persistent cultivation and consequent expansion of agricultural lands, associated with frequent clearance and weeding of farmland areas, as well as plantations of economic tress such as E. guineensis and Irvingia gabonensis. Presently, such plantations are frequently maintained and cleared of weeds and grasses.

8. Anthropogenic impact Anthropogenic interferences could have contributed in the maintenance of the forest-savanna mosaic vegetation by creating an enabling environment for the spread of savanna components. The presence of charred plant tissues and weeds of farmlands indicate possible agricultural activities and bush fires (likely anthropogenic) which could have contributed in maintaining the open character of the vegetation. The sharp decline in Poaceae after about 1470  79 BP cannot be attributed to natural bush fires alone (Livingstone, 1971), particularly because of the near absence of charred plant particles in the later part of the late Holocene. The relative presence and subsequent increase in E. guineensis, a plant of extreme economic importance and conservation in the region, as well as the drastic decline in A. cordifolia suggest the possible role of anthropogenic activities in creating derived savanna vegetation (Sowunmi, 2004). Another significant factor in the environment which could have affected the vegetation is the activities associated with the iron smelting revolution which has been dated to 4005  40 BP in Lejja Nsukka (Eze-Uzomaka, 2009). According to Okafor (1993) and Umeji et al. (2012), smelters moved from one location to another, attracted mostly by the presence and availability of iron rich lateritic deposits which cap most of the extensive plateau in southeastern Nigeria. The ores which are rich in Al2O3 have significant levels of TiO2, and because of that much fuel wood was needed to attain the high temperatures required for smelting the ores (Okafor, 1993). The implication is systematic deforestation caused by indiscriminate felling of hardwood trees to raise adequate charcoal through the length and breadth of the areas where the industries were sited. It is also possible that while the vegetation was opened up by ore smelting activities, subsistence agricultural activities and hunting games could have become common practice in the opened lands. Nevertheless, while it is believed that human activities may have had its toll on the vegetation, the expansion in Southern Guinea savanna elements from 6417  67 BP to a period after 6388  03 BP may be attributed more to climatic changes than to human impact. This is because the impact of prehistoric agriculture could not have led to the transformation observed in the vegetation during that period. Rather, the total transformation and consequent maintenance of the mosaic lowland rainforest and secondary grassland (derived savanna) could have started from a period before 1477  60 to present. This is because the oldest direct dating (4691  78 BP) of human influence in the natural vegetation of southeastern Nigeria was obtained from charcoal recovered from a cave in Uhuchukwu, Okigwe (Ibeanu, 2000; Umeji et al., 2012). This was followed by dates obtained from different iron smelting sites in Nuskka, with the oldest being 4005  40 BP in Lejja Nsukka (EzeUzomaka, 2009). These periods of increased human impact in the natural vegetation of southeastern Nigeria are more or less synchronized with archeological and pollen findings by Sowunmi (1999, 2004) and Alibi (1999) in southwestern Nigeria and Ghana as well as by Ballouche and Neumann (1995) in Oursi, Burkina Faso and northeast Nigeria, and in the Lake Chad basin (Salzmann and Waller, 1998). Such human activities could have contributed in maintaining the present derived savanna vegetation in the region. 9. Conclusion The results of the study, though limited in pollen counts, showed indication of vegetation change from a closed Southern Guinea savanna to that of an open mosaic lowland rainforest and secondary grassland vegetation. Prior to 6417  67 BP, forest vegetation with Guinea savanna affinities existed around the pond,

Please cite this article in press as: Njokuocha, R.C., Akaegbobi, I.M., A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.07.036

R.C. Njokuocha, I.M. Akaegbobi / Quaternary International xxx (2013) 1e7

supported by a humid climatic condition. However, from about 6417  67 BP to after 6388  71 BP, Southern Guinea savanna vegetation became established along with patches of dry forests. It is most probable that considerable periods of high precipitation alternating with warm and dry climatic phases may have favoured the establishment and maintenance of this complex heterogeneous vegetation structure. Around 1470  79 BP, the climate became drier and a distinct vegetation change showing a transition from Southern Guinea savanna vegetation to derived savanna mosaic vegetation similar to the present day existing vegetation in the area. From palynological findings in the area around 1470  79 BP, there is an indication that human activities contributed not only to the rise in E. guineensis, but maintained the derived savanna vegetation of the areas.

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Please cite this article in press as: Njokuocha, R.C., Akaegbobi, I.M., A contribution to the Holocene vegetation history of Nigeria: Pollen from Ohe Pond Nsukka, southeastern Nigeria, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.07.036