ASPECTS OF HOLOCENE VEGETATION RECONSTRUCTION OF SOUTH EASTERN NIGERIA POLLEN RECORD FROM OZZI LAKE OPI NSUKKA ENUGU STATE

ABSRACT

A palynological study of the Palaeo-vegetation of Opi Nsukka, north eastern Enugu State was investigation through a palyno-analysis of 1.5 m lake sediment core collected from Ozzi Lake, Ugwu Ozioko Opi- Agu Nsukka Enugu State. The pollen flora showed that a total of 52 pollen

types belonging to 30 families were recorded. Fungal and fern spores in addition to charred plant particles were also recorded from the sediment profile. The pollen diagram was divided into four zones based on the pollen curves which suggest possible changes in vegetation. The pollen profile provided evidence of a southern guinea savannah in the late Holocene. Prior to about

3800 ± 30 BP and mid part of zone iv, there was paucity of both the lowland rainforest and savannah elements in the vegetation, but with a relative increase in grass. Also the decrease in the members of aquatic species and hydrophytic plants indicate that the period must have been drier than humid. This can also be substantiated by the rise  in the peaks of Poaceae curve. Prior to 1910 ± 25 BP to about 340 ± 25 BP, there were subtle floristic changes in the vegetation from that of closed forest-savannah vegetation to an open vegetation. This is attributed to successional changes between the lowland rainforest and the savannah species due to alternating fluctuations between the dry and humid conditions. The paucity of pollen of both the savannah and the lowland rainforest species at the later part of 340 ± 25 BP is attributed highly to increase in anthropogenic activities.

CHAPTER ONE INTRODUCTION

1.1  BACKGROUND OF THE STUDY

Palynology, the study of pollen grains and spores is one of the most effective tools used to reconstruct past environment. Because exines, the tough or resistant outer shell of pollen grains of various species are unique and can survive under favourable conditions for thousands of years, they have served as important tools for identifying many plants that were present in the past. Preserved pollen and spores are extracted from sediment of different environment and used to reveal what plants were associated with a particular landscape in the past. On a large scale, palynological studies can identify broad environmental trends. It is commonly stated that the present condition of the biosphere is the result of the action of ecological and historical factors of the past and the relative importance of each of these agents in shaping present day world can be evaluated (Qian et al., 2007; Emerson and Gillespie, 2008; Alvarez et al., 2009).

Palaeoecology is  defined  as  “the  ecology of  the  past”  (Birks  and  Birks,  1980).  It  is  the reconstruction and study of past ecosystems, including the relationships between organisms and their  environment  (Robert,  1998).  The  most  fundamental palaeoecological tenets,  Hutton’s principle of uniformitarianism emerged from the field of geology and the most common is that “The present is the key to the past” (Tomkeieff, 1962). This means that  natural processes operating in the past are the same as those that can be observed in the present, though rates can vary. Palynological investigations during the late Quaternary indicate that the vegetation of West Africa has undergone considerable changes (Agwu, 1986; Sowunmi, 2004; Njokuocha 2012 ). This is believed to be as a result of climate variability, which influences the development and distribution of vegetation in time. Early to mid Holocene conditions in the deserts of Africa and

Asia were reported to be significantly wetter than in the late Holocene, as revealed by micro and macro fossil remains of plants and animals from lake sediment (Agwu and Beug, 1982; Jahns,

1996).

This investigation on the Holocene vegetation was conducted by means of a sediment profile from Ozzi Lake in Opi, Nsukka Local Government of Enugu State. Nsukka is located between latitudes 6o  181 N and 7o  071 E  and  between longitudes 6o  521 E  and 7o  5411 N  in the southeastern geopolitical zone of Enugu State. Nsukka area is underlain by sedimentary rock formations whose geologic age ranges from Coniacian to Paleocene (Chima and Okpe, 2008). The temperature range in Nsukka is between 21oC – 32oC. It has a daily maximum temperature which  is  usually  more  than  27oC  while  the  mean  daily  minimum  is  21oC.  The  highest temperature of 33oC is usually recorded in January while the lowest 19oC occurs in August (Agwu, 1997). The climate has two distinct alternating dry and rainy seasons occurring within the months of November – April and May – October respectively. Nsukka plateau in particular is characterised by a derived savannah which is located between the true Guinea savannah in the north and lowland tropical rainforest belt in the southern part of Nigeria (Keay, 1959). It forms a part of the mosaic of lowland rainforest and secondary grassland (White, 1983) which stretches from east to west across the country with its widest north south extension located in the Nsukka plateau area (Agwu, 1997).

1.2 DESCRIPTION OF THE STUDY SITE

The study was conducted in Ozzi Lake, Opi, Nsukka Local Government Area. Ozzi Lake is located at Ugwu – Ozioko in Neke village (Fig. 1). It is more than 9 m deep in the centre and 7 km long (Ezeh, 2006). Ozzi Lake is a deep dystrophic water (Plate 1).   Its depth and width

enables the smooth movement of locally made canoe from one end to another. The lake is surrounded by stands of riparian (Uapaca togoensis, Pandanus sp. and Berlinia grandiflora) and transitional forest species (Alchornia cordifolia). Others are savannah components (Dalbergia spp., Bridelia ferruginea, Baphia nitida, Nauclea latifolia, Daniella oliveri, Vitex simplifolia, Annona senegalensis, Hymenocardia acida, Terminalia superba, Combretum spp., Cussonia barteri and Anthocleista vogelii) as well as remnants of lowland rainforest such as Elaeis guineensis, Vitex doniana, Anthiaris toxicaria, Irvingia gabonensis, Treculia africana, Newbouldia laevis, Pentaclethra macrophylla, Triplochiton scleroxylon, Napoleona imperialis, Icacina tricantha. The lake is fed by underground water and rainfall. It has no inlets.  During the dry season, the lake level goes down due to prolonged aridity. On the bands of the lake are members of Cyperaceae, fern community and aroids (Cryptospermum senegalensis). There is also an abundance of Nymphaea lotus floating on the surface of the water. On the raised sandy part of the lake extending outwards are grass communities made up of Andropogon gayanus, Imperata cylindrica, Panicum maximum, Sporobolus pyramidalis, Axonopus compressus, Hyparrhenia barteri, Elusine indica, Cynodon dactylon, Chloris pilosa, Pennisetum polystachion and continuous presence of Bambusa vulgaris.

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