ASSESSMENT OF FLORAL COMPOSITION, STRUCTURE AND NATURAL REGENERATION OF THE TANO OFFIN GLOBALLY SIGNIFICANT BIODIVERSITY AREA

ASSESSMENT OF FLORAL COMPOSITION, STRUCTURE AND NATURAL REGENERATION OF THE TANO OFFIN GLOBALLY SIGNIFICANT BIODIVERSITY AREA

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ABSTRACT

Tano Offin Globally Significant Biodiversity Area (GSBA - 178.34 km²) is a reserve within the Tano Offin Forest Reserve in the Ashanti Region of Ghana. The reserve which experiences occasional wildfires has been logged and mined for bauxite in the past. Current menace of forest degradation presents the need to generate information on the current state of the floristic patterns of the forest. To address conservational needs of the reserve, a study was carried out in the GSBA to assess the floristic composition, structure, natural regeneration, canopy closure as well as to evaluate the influence of elevation and invasive species on the aforementioned. Ten 50 m x 50 m plots were demarcated for the assessment of diameter and height of all trees, lianas and shrubs with dbh ≥ 10 cm as well as the identification of other plant life forms. These individual plots varied in invasiveness and altitude: ranging from 264 m – 623 m. A 10 m x 10 m plot was located within each of the 50 m x 50 m plots where diameter of trees and other plant life forms of dbh < 10 cm were identified and measured. Quadrats (1 m x 1 m) were laid at the corners of the 50 m x 50 m plots and its centre for canopy closure and natural regeneration assessments. Plant species (240) belonging to 59 families were identified. These comprised of 171 trees, 41 lianas, 11 shrubs, 7 herbs, 7 herbaceous climbers, 1 epiphyte, 1 grass and 1 fern. Fabaceae was the predominant family in terms of species richness. Species diversity () of the tree layer, shrub and herb layers were 2.55, 2.54 and 2.48 respectively. Trees and other plant life forms of dbh ≥ 10 cm were grouped into six diameter classes and four height classes; the number of plants in these groups was decreasing as the group size increased so that the highest-size group had the least number of plants. Basal area of the GSBA was 28.36 m²/ha and average tree height of the emergent layer was 46.19 m. Floristic diversity did not differ significantly (P > 0.05) in the tree, shrub and herb layers of all three forest types namely, lowland, transition and highland forests. Celtis mildbraedii was both the most significant species among trees and other plant life forms with dbh 10 cm in general, and in all three forests types. Rinorea welwitschii was also both the most significant species among trees and other plant life forms with dbh < 10 cm, and at the shrub layer of highland and lowland forests. Hymenostegia afzelii was the most significant species for the shrub layer at the transition area though Rinorea welwitschii was completely absent from this zone. Basal area for trees and other plant life forms with dbh ≥ 10 cm increased with increasing elevation. Generally, there was increase in the number of taller trees with increase in elevation. Areas with invasive species recorded lower mean basal area at both the tree and shrub layers. Invasive species were present only at the transition area which had the lowest percentage of shade bearers and non-pioneer light demanders but the highest with pioneers. A total of 75 plant species were found regenerating as saplings and seedlings. Eight species of the regeneration flora were absent from the adult community. For the lowland forests, 50.94 % of the adult tree population were regenerating while 46.74 % were regenerating in the transition area and 42.86 % at the highlands. Pioneer saplings were absent from lowland forests. Invasiveness had significant influence (P < 0.05) on the species diversity of seedlings. There was a decline in canopy closure with respect to decreasing altitude, measuring 89.06 %, 87.84 % and 84.9 % for the highlands, transition area and the lowland forests respectively. It is revealing that the GSBA is under exploitation especially in lowland forests. The information generated on species composition, structure and regeneration should be useful in designing conservation measures for the Tano Offin GSBA.


CHAPTER ONE

1.0         INTRODUCTION

1.1         Background

The total forest area in the world is about 4 billion hectares (FRA, 2010) with the tropical forests covering about 10 % of the earth surface (Lewis, 2006). The diversity of the tropical forest provides resources which are both tangible and intangible (Antwi, 1999) and these could be considered as ecological, environmental, cultural and socio-economic benefits (Abeney, 1999). In Ghana, the forestry sector contributes about 6 % to the GDP, accounts for 11 % of export earnings, employs a labour force of 100, 000 people, and it provides and supports livelihoods of about 2.5 million rural folks (FAO, 2002; FOSA, 2002).

Records reveal an 8.5 % increase in the rate of tropical deforestation from the 1990’s to the period of 2000 – 2005 (Butler, 2009). Reports by the 2010 Global Forests Resources Assessment indicates a 2 % annual forest loss from 1990 – 2000 in Ghana (FRA, 2010). Forests in Ghana are being depleted at an alarming rate through several factors including excessive logging, mining and quarrying, developmental projects, charcoal production & firewood collection, bushfires, and unsustainable agricultural practices (FAO, 2000).

The impact of human activities on the environment is evident throughout the world and it includes remarkable changes in species composition, abundance, and diversity of organisms in various ecosystems including the tropical forest such as the Tano Offin GSBA (FRA, 2010; Kim

&    Byrne, 2006). Thus information on the current state of the Tano Offin GSBA is vital in 1


maintaining healthy plant diversity which should results in enhanced productivity of the forest, greater prospects for economic development, medical discoveries and an increased resiliency to environmental challenges (Shah, 2009).

1.2          Problem Statement

The deforestation of tropical forest threatens the sustainability of its biodiversity, demanding the conservation of remnant species of flora lest these may be subjected to extinction in the near future (Myers et al., 2000). But how can the plant diversity of tropical forest be conserved if knowledge on the composition and abundance of the different kinds of species is lacking? In this regard, floristic assessment of the forest is indispensable in detecting the risk of extinction, arrival of invasive species and changes in plant diversity over time.

Relatively, limited work has been done in determining the floristic composition and structure of forests in Ghana (Hall & Swaine, 1981; Hawthorne, 1993; Vordzogbe et al., 2005; Anning et al., 2008; Addo-Fordjour et al., 2009b; Pappoe et al., 2010). In 1981, an extensive assessment of the distribution of vascular plants in Ghanaian forests was carried out by Hall and Swaine which included the Tano Offin Forest Reserve (Hall & Swaine, 1981). It has been decades now and thus the need to generate a carefully compiled up-to-date data on the floral composition of the reserve. Moreso, inventories by the Forestry Commission of Ghana in the reserve have focused mainly on timber species (Affum-Baffoe pers.comm) which is deficient for conservation purposes. Most imperatively on the GSBA, records indicate that no formal ecological research has been undertaken in the GSBA (FC, 2007; Affum-Baffoe pers.comm).

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Notably, although 44.5 % of the reserve has been designated as a Globally Significant Biodiversity Area (GSBA), reports about the reserve and even the GSBA indicate activities of illegal logging and other forms of human disturbances (FC, 2007). The situation is that serious to the extent that when the then Minister of Lands and Natural Resources payed an unannounced visit to the Tano Offin Forest Reserve in May 2011, some illegal chainsaw operators were caught right on the act (The Chronicle, 2011).

Depending on the level and rate of infliction, disturbances such as logging in forest affect the composition, structure and light availability to the forest, eventually influencing natural regeneration process (Jennings et al., 1999; Hale, 2004). In addition, canopy opening promotes growth of invasive species which alter the course of natural regeneration in the forest, restraining the resurgence of native trees (Vermeulen & Koziell 2002). Moreover, the Tano Offin GSBA has been a forest reserve of bauxite deposits which presents a potential threat to its forest diversity.

In addition, elevation acts as an important source of local variation and it determines rate of recovering, recruitment and eventually the floristic composition of a forest (Wang et al., 2002; FAO, 2002; Joseph et al., 2012). Thus, investigations on the influence of elevation on patterns of plant diversity is critical for this biodiversity rich area which sits on a mountainous range of the Nyinahin hills - Tano Offin forest reserve is cited as one of the three Upland Evergreen forests in Ghana (Hall & Swaine, 1981). A study with a focus on local patterns of plant diversity assists profoundly with local conservation practices and the management of localised biodiversity patterns (Kim & Bryne, 2006).

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It is in this light that attempt is being made in this study to generate information on the current state of floristic patterns of the Tano Offin GSBA as well as the influence of elevation and invasive species on the GSBA. Plant species composition and stand structure serve as important indicators in the formulation of conservation measures (Lindenmayer et al., 2000; Newton et al., 2003; Oteng-Yeboah et al., 2009). Thus the current work should be useful in enhancing conservation efforts of the GSBA as well as helpful in managing protected areas.

1.3         Main Objective

The general objective of the research is to determine the floristic patterns of the Tano Offin Forest Globally Significant Biodiversity Area (GSBA), assessing the floristic composition, structure, natural regeneration and canopy closure as well as the influence of elevation and invasive species on the aforementioned.

1.3.1     Specific Objectives

·         To assess the floristic composition of the Tano Offin GSBA


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