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*Corresponding author: E-mail: jeminiwaoluremi@yahoo.com, Jeminiwaoluremi@gmail.com;
Asian Journal of Research in Agriculture and Forestry
2(1): 1-12, 2018; Article no.AJRAF.42848
Tree Species Diversity and Structure of Eda Forest
Reserve, Ekiti State, Nigeria
S. O. Olajuyigbe
1
and M. S. Jeminiwa
2*
1
Department of Forest Production and Products, University of Ibadan, Oyo State, Nigeria.
2
Southern Guinea Savanna Research Station, Forestry Research Institute of Nigeria, P.O.Box 209,
Mokwa, Niger State, Nigeria.
Authors’ contributions
The work was carried out in collaboration between both authors. Author SOO designed the study,
performed the statistical analysis and wrote the protocol. Author MSJ wrote the first draft of the
manuscript, managed the analyses of the study and managed the literature searches. Both authors
read and approved the final manuscript.
Article Information
DOI: 10.9734/AJRAF/2018/42848
Editor(s):
(1)
Dr. Md. Abiar Rahman, Department of Agroforestry and Environment, Bangabandhu Sheikh Mujibur Rahman- Agricultural
University (BSMRAU), Bangladesh.
Reviewers:
(1) Faridah-Hanum Ibrahim, Universiti Putra Malaysia, Malaysia.
(2)
Elly Josephat Ligate, Fujian Agriculture and Forestry University, P.R. China.
Complete Peer review History:
http://www.sciencedomain.org/review-history/26279
Received 21 June 2018
Accepted 11 September 2018
Published 19 September 2018
ABSTRACT
Tropical rainforest is continuously threatened by timber exploitation and conversion to other land
uses. In this study, tree species diversity and forest structure of Eda Forest Reserve in Ekiti State,
Nigeria, were assessed using systematic line transect and purposive sampling techniques for plot
demarcation and data collection. Two transects (2000m long) were laid in secondary forest and
encroached farmland in the reserve, while the primary forest fragments were sampled purposively.
Twenty sample plots (20m×20m) were laid out on each of the vegetation types. All trees >10cm
diameter at breast height (dbh) were identified to species level and enumerated for total height and
dbh. Data were analyzed using descriptive statistics such as tables, charts, frequency, percentages
and diversity index analysis using paleontological statistics software (PAST 2.14). There were 60
species from 22 families, with Sterculiaceae, Caesalpiniaceae and Moraceae being the most
abundant families. Individual tree populations were 380 trees/ha, 280 trees/ha and 137 trees/ha in
the primary forest, secondary forest and encroached farmland, respectively. Species composition
comprised 39, 38 and 19 species in primary forest, secondary forest and encroached farmland,
Original Research Article
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
2
respectively. Khaya ivorensis had the highest relative density in the three vegetation types
(19.74%, 24.53% and 27.74%) respectively, while Ceiba pentandra had the highest height
(53.87m). The mean basal area ranged from 0.36m
2
/ha (encroach farmland) to 3.18m
2
/ha (primary
forest). Shannon-Wiener Indices were 3.22, 3.14 and 2.51 for the primary forest, secondary forest
and encroached farmland, respectively. Eda forest reserve is a heterogeneous ecosystem that had
variations in tree population due to anthropogenic activities. The secondary forest and encroached
farmland have great potential for recovery if conservation efforts are put in place.
Keywords: Eda forest reserve; tree species diversity; forest structure; alpha diversity; beta diversity.
1. INTRODUCTION
Approximately, one-third of the earth’s land area
is covered with forests and nearly 50% of this
ecosystem is found in the tropical environments
of the world [1]. These rainforests are complex
ecosystems mostly dominated by diverse tree
species of various sizes. The tropical rainforests
also contain a high level of diversity of other flora
and fauna which provide a wide variety of food,
fodder, fibre and raw materials for people living in
and around the forests. They help maintain
biological diversity, ameliorate microclimates,
influence hydrological processes and nutrient
cycling; support soil conservation, as well as
improve air and water quality, while serving as
habitats for wildlife [1,2]. In Nigeria, 20-25 % of
the rainforest zone had been placed under
reservation since the late 1920s and ‘30s. Over
the years, the forest reservations have protected
natural ecosystems, conserved biodiversity,
preserved ecological processes, enhanced
scientific research and education, while
maintaining genetic resources of flora and fauna
[3,4]. However, increased anthropogenic
activities in the primary forests of the reserves
have resulted in serious deforestation and
degradation. Consequently, timber harvesting,
forestland encroachment for farming, and the
establishment of tree crop plantations are
threatening the continued existence of most
rainforests [5,4]. The situation is further
compounded by the paucity of information on
tree species composition and diversity in most of
these in–situ conservation areas.
Eda forest reserve is one of the 10 forest
reserves in Ekiti state, Nigeria. It is endowed with
an array of renewable natural resources that
have been subjected to high levels of exploitation
through legal and illegal means [6]. A section of
the forest reserve had been converted to
farmland, exotic and indigenous tree species
plantations, while 57.7% is still covered by
primary and secondary forests [7]. However,
there is limited information on the tree species
composition of the remaining primary forest as
well as the recovering secondary forest in this
forest reserve. Therefore, this study assessed
the tree species diversity and forest structure of
the encroached farmland, secondary and primary
forest areas in Eda forest reserve, Ekiti State,
Nigeria.
2. MATERIALS AND METHODS
2.1 Study Area
Eda forest reserve was gazetted in 1941 (gazette
number 37) with the objective of actualizing
biological diversity conservation and
environmental protection. This tropical humid
forest is a high forest located along latitude
7°41'3''N and 7°47'5''N and longitude 5°'36'1''E
and 5°37'6''E, at an altitude ranging from 497 to
560 m above sea level (Fig. 1). The reserve is
bordered by four towns: Orin/Ara Ekiti (North),
Eda-Ile Ekiti (West), Omuo Ekiti (East) and
Isinbode Ekiti (South). This 906 ha forest reserve
is divided, administratively, into two parts: the
318ha plantation compartment (Eda I), and the
508ha natural forest (Eda II). The natural forest
had been initially protected from exploitation but
has recently been encroached by subsistence
farmers and timber harvesters. The natural forest
was highly stocked with many economic tree
species and this is evidenced by the level of
exploitation that had taken place, resulting in
secondary forest regrowth [8]. The forest reserve
has an undulating terrain, which is gently sloped
in Northeast direction and as ultisol and oxisol
soil types. The bedrock material is underlain with
basement complex and contains undifferentiated
igneous rocks, laterites and white sand. The
reserve experiences two seasons with the wet
season occurring from April to October while the
dry season occurs from November to March.
Hence, the average annual temperature ranges
from 21°C - 28°C, average precipitation is 1800
mm, while the relative humidity ranges from 56%
and 85%. The fragmented primary forest is
dense with tree species forming continuous
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
3
multilayered canopies, while the lower canopies
contain climbers, shrubs and herbaceous plant
[7,6].
2.2 Sampling Technique and Data
Collection
Systematic line transect technique was used to
lay sample plots in secondary forest and
encroached farmlands, while purposive sampling
was used to lay plots in the primary forest,
following the method of Duran et al. [9]. Thus,
two transects (2,000 m long and 1m wide) were
laid in each of the secondary forest and
encroached farmland. While, the fragmented
nature of the primary forest, resulted in the use of
purposive sampling technique for selection of
plots. Twenty sample plots (20 m×20 m) were
laid in alternate positions along each transect at
an interval of 200 m (Fig. 2), while the same
number of plots were purposively selected and
evaluated in the primary forests. All woody plants
with diameter at breast height (dbh) > 10 cm
were identified and their total height and dbh
measured following the method of Adekunle et
al. [10].
Fig. 1. Map of Eda forest reserve in Ekiti state, Nigeria
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
4
Fig. 2. Sampling procedure for identification and enumeration in the study area
Where: T = Transects, p = Sample plot
2.3 Data Analysis
2.3.1 Tree basal area and volume estimation
The basal area (BA m
2
) of trees was calculated
using Eqn. 1:
4
2
D
BA
(1)
Where D = Diameter at breast height (m)
The total basal area for trees in each sample plot
was obtained and used to determine the per
hectare equivalents.
The volume of individual trees was estimated
using the Eqn. 2:
V = BAHƒ (2)
Where V = Volume of tree (m
3
), H = Total Height
of tree (m) and ƒ = Form factor. Total plot
volumes were obtained by adding the volume of
individual trees encountered in each plot and
then mean plot volume was calculated. This was
also scaled up to per hectare basis.
2.3.2 Tree species classification and diversity
indices
All trees were identified insitu by an experienced
forest taxonomist where possible and samples
were compared with voucher specimens in
Forest Herbarium Ibadan (FHI), Forestry
Research Institute of Nigeria. Tree species were
classified into taxonomic families and number of
individuals in each family was used for species
diversity classification. The frequency of
occurrence was used to determine tree species
abundance/richness. The diversity indices were
determined using paleontological statistics
software (PAST 2.14) [11] and some of them
were listed as follows:
(i.) Shannon-Wiener diversity index (H
I
): This
determines both the richness and abundance of
each tree species in the vegetation types using
Eqn. 3 [12]:
=∑
(3)
Where S = Number of tree species in each
vegetation type; p
i
= proportion of each tree
species to the total number of trees in each
vegetation type; Ln = the natural logarithm.
(ii.) Relative Density (RD): This determines the
number of individual per hectare in the forest
types using Eqn. 4 [10]:
=
100% (4)
Where n
i
= number of individuals of each tree
species i and N = total number of individuals in
the entire tree population
(iii.) Relative Dominance (RD
o
): This determines
the level of abundance of individual species over
other species in the forest types using Eqn. 5
[10]:
=
(
%)
∑
(5)
Where BA
i
= Basal Area of individual trees
belonging to a particular species i and BA
n
= Total
Basal Area.
(iv.) Margalef’s index of species richness (M) was
determined using Eqn. 6 [13]:
=
()
(6)
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
5
3. RESULTS AND DISCUSSION
3.1 Results
3.1.1 Tree species composition and
distribution in Eda forest reserve
A total of 60 tree species were encountered
during the study and 41 of them were absent in
encroached farmland. The primary forest had the
highest number of tree species (39), followed by
the secondary forest (38) while encroached
farmland had the least (19). There were 380
trees/ha in the primary forest while secondary
forest and encroached farmlands had 280
trees/ha and 137 trees/ha, respectively (Table 1).
Khaya ivorensis had the highest number of trees
in each vegetation type with 75, 65 and 38
trees/ha in the primary forest, secondary forest
and encroached farmland respectively. Similarly,
Milicia excelsa, Sterculia rhinopetala and
Triplochiton scleroxylon had high representations
in all vegetation types (Table 1).
Table 1. Tree species composition and distribution in Eda Forest Reserve
S/N
Species
Family
Primary
forest
(No./ha)
Secondary
forest
(No./ha)
Encroached
farmland
(No./ha)
1 Afzelia bipindensis Harms Caesalpiniaceae --- 3 ---
2 Albizia adianthifolia (Schumach) W.
Wight
Mimosaceae 5 3 3
3 Alstonia congensis Engl. Apocynaceae 10 --- ---
4 Alstonia boonei De Wild. Apocynaceae --- 8 3
5 Aningeria robusta
(A. Chev.) Aubrev.&
Pellegr
Sapotaceae 5 --- 5
6 Antiaris toxicaria Lesch. Moraceae 10 10 8
7 Blighia sapida K. Konig. Sapindaceae --- 13 15
8 Bombax buonopozense P. Beauv. Bombacaceae 15 --- 3
9 Brachystegia eurycoma Harms Caesalpiniaceae 5 --- ---
10 Brachystegia kennedyi Hoyle Caesalpiniaceae --- --- 3
11 Bridelia atroviridi Wild. Euphorbiaceae --- 3 ---
12 Ceiba pentandra (L.) Gaertn. Bombacaceae 15 8 5
13 Celtis zenkeri Engl. Ulmaceae 10 8 3
14 Chrysophyllum albidum Linn. Sapotaceae 5 8 ---
15 Cola gigantea A. Chev. Sterculiaceae --- 3 ---
16 Cordea millenii Baker Bignoniaceae 5 --- ---
17
Cynometra megalophylla
Harms Caesalpiniaceae 10 --- ---
18 Dialium guineense Willd Caesalpiniaceae 5 3 ---
19 Daniella ogea (Harms) Rolfe ex.
Holland
Caesalpiniaceae 5 --- ---
20 Diospyros mespiliformis Hoshst. Ebenaceae 5 --- 3
21 Distemona bentamianus Baill. Caesalpiniaceae 5 --- ---
22 Enantia chlorantha Oliv. Annonaceae --- 3 ---
23 Entandrophragma angolensis (Welw.)
C. DC.
Meliaceae --- 5 3
24 Etandrophragma cylindricum Sprague
Meliaceae --- 5 ---
25 Erythrophylum suaveolens (Guill. &
Perr.) Brenan
Caesalpiniaceae --- 3 ---
26 Ficus exasperata Vahl Moraceae --- 8 ---
27
Ficus mucuso
Welw. Ex. Ficalho Moraceae 5 --- ---
28 Funtumia elastica (Preuss) Stapf. Apocynaceae 5 3 ---
29 Gossweilodendron balsamiferum J. Caesalpiniaceae --- 3 ---
30
Hildergardia barteri
(Mast) Kosterm. Sterculiaceae 5 --- ---
31 Hollarrhena floribunda (G. Don) Dur &
Schinz
Apocynaceae --- 8 ---
32 Khaya ivorensis A. Chev. Meliaceae 75 65 38
33 Kigelia africana (Lam) Benth Bignoniaceae 5 3 ---
34 Lophira alata Banks ex. Ochnaceae 10 --- ---
35 Lovoa trichilioides Harms Meliaceae --- 3 ---
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
6
S/N
Species
Family
Primary
forest
(No./ha)
Secondary
forest
(No./ha)
Encroached
farmland
(No./ha)
36 Mansonia altissima A. Chev Sterculiaceae 5 3 ---
37 Milicia excelsa (Welw.) C.C. Berg. Moraceae 25 15 15
38 Milletia aboensis (Hook. F.) Baker Papilionaceae 5 --- ---
39 Mitragyna ciliate Aubrev & Pellegr. Rubiaceae --- 3 ---
40 Monodora myristica (Gaertn) Dunal Annonaceae --- --- 3
41 Musanga cecropioides R. Br. Moraceae 5 8 ---
42 Nesogordonia papaverifera (A. Chev.)
R. Capuron
Sterculiaceae 5 3 ---
43 Newbouldia laevis (P. Beauv.) Seem Bignoniaceae --- 3 ---
44 Parinari excelsa Sabine Chrysobalanaceae
5 --- ---
45 Pentaclethra macrophyla Benth Mimosaceae --- 3 ---
46 Piptadeniastrum africanum (Hook F.)
Brenan
Mimosaceae 5 --- ---
47 Pterocarpus erinaceus Poir Papilionaceae --- 3 ---
48 Pterygota macrocarpa K. Schum Sterculiaceae 5 --- ---
49 Pycnantus angolensis (Welw) Warb. Myristicaceae --- 3 3
50 Ricinodendron heudelotii (Baill) Pierre
Euphorbiaceae 5 8 ---
51 Sterculia rhinopetala K. Schum Sterculiaceae 45 25 5
52 Sterculia tragacantha Lindi Sterculiaceae 5 --- ---
53 Strombosia pustulata Oliv. Olacaceae 5 --- ---
54 Terminalia ivorensis A. Chev. Combretaceae 5 5 3
55 Terminalia superba Engl. & Diels Combretaceae 10 --- ---
56 Pterocarpus osun Craib. Papilionaceae 5 3 3
57 Tetrapleura tetraptera Taub. Mimosaceae 5 --- ---
58 Triplochiton scleroxylon K. Schum. Sterculiaceae 10 10 13
59 Xylopia aethiopica (Dunal) A. Rich Annonaceae --- 3 ---
60 Zanthoxylum zanthoxyloides (Lam)
Zepern
Rutaceae 5 3 ---
Total 380 280 137
3.1.2 Family distribution of trees in Eda
forest reserve
There were 22 families represented by tree
species enumerated in Eda forest reserve (Table
2). Sterculiaceae family (53 trees/ha) had the
highest population, followed by Caesalpiniaceae
family with 38 trees/ha, while Sapindaceae family
had the least of 3 trees/ha. The species from
Chrysobalanaceae and Rutaceae families were
found only in the primary forest. However, tree
species from the Annonaceae, Myristicaceae,
Rubiaceae and Sapindaceae families were
absent in the primary forest.
3.1.3 Relative abundance and diversity
indices of tree species in the primary
forest of Eda forest reserve
Khaya ivorensis had the highest relative density
(19.74%), relative dominance (2.42%) and
Species Importance Value Index (IVI) (22.16%)
in the primary forest (Table 3). This was followed
by Sterculia rhinopetala with a relative density of
11.84%, relative dominance of 2.30% and IVI of
14.14%. Twenty seven different tree species had
the lowest relative density (1.32%). These
included Albizia adianthifolia, Brachystegia
eurycoma, Aningeria robusta, Cordea millenii to
mention a few. Strombosia pustulata had the
least relative dominance (0.23%) and species
importance value index (1.55%), along with
Ricinodendron heudelotii which also had the
least species IVI (1.55%).
3.1.4 Alpha and beta diversity indices of tree
species in Eda forest reserve
Simpson index revealed that the primary forest
was the most diverse (0.93), while secondary
forest and encroached farmland had indices of
0.92 and 0.87, respectively (Table 4). Similarly,
the Shannon Wiener index had the highest value
for primary forest (3.22) when compared with
secondary forest (3.14) and encroached
farmland (2.51). The species evenness revealed
that primary forest contained more species (0.88)
than the other vegetation types (Table 4).
Species richness (Margalef’s index) revealed that
primary forest was more endowed than other
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
7
vegetation types with 39 species/ha, followed by
secondary forest with 38 species/ha, while 19
species/ha occurred in encroached farmland.
However, the fisher alpha index revealed that
secondary forest (11.86) was slightly diverse in
species composition than other vegetation
types because the values for the primary forest
(10.89) and encroached farmland (5.99) were
lower.
3.1.5 Growth variables of tree species in Eda
forest reserve
The encroached farmland had the highest mean
dbh (83.35 ± 9.04 cm), while secondary forest
had the least (34.60 ± 3.22 cm) in the forest
reserve (Table 5). On the other hand, mean
basal area was 3.18 m
2
/ha, 0.36 m
2
/ha and 1.68
m
2
/ha for primary forest, secondary forest and
encroached farmland, respectively. The tree
volume followed a similar trend with primary
forest being the highest (122.44 m
3
/ha), followed
by encroached farmland (53.02 m
3
/ha) while
secondary forest had the lowest (13.20 m
3
/ha).
The mean height varied from 23.87 m - 27.93 m
across the vegetation types (Table 5). Trees with
dbh < 20.99 cm and 41 - 50.99 cm were only
present in secondary forest, while all other
diameter class distributions were represented in
primary forest and encroached farmland (Fig. 3).
The highest frequency was observed for trees in
the > 60 cm diameter class which dominated the
primary forest.
3.2 Discussion
Tree species composition was highest in the
primary forest which had a richer and more
diverse tree population than other vegetation
types. The primary forest was dominated by the
light demanding species, characteristic of the
emergent layer in a tropical forest. This tall
species provide cover for shade tolerant
understorey species [10,14]. The primary forest
was characterized by an abundance of lianas
which entangled the branches and crowns of
larger trees. On the other hand, the secondary
forest was in the recovery mode with medium
size trees, most of which were < 60cm in
diameter. This distribution of diameter across the
dbh range is an indication of the high level of
exploitation that the forest had experienced [4].
In addition, the large trees scattered in the
encroached farmland were economic species
retained to provide shade and protection for farm
crops [5]. The 60 tree species from 22 families in
the forest reserve, represent the high level of
complexity in terms of structure and function in
rainforest ecosystems. Fabaceae, Moraceae,
and Sterculiaceae have been consistently
reported as dominant plant families in Nigerian
tropical forests [10,4].
Table 2. Family composition and distribution of tree species in Eda forest reserve
S/N
Family
Primary
forest
Secondary
forest
Encroached
farmland
No of tree species
/ha in each family
1 Annonaceae --- 5 3 8
2 Apocynaceae 10 8 3 21
3 Bignoniaceae 10 5 --- 15
4 Bombacaceae 10 3 5 18
5 Caesalpiniaceae 25 10 3 38
6 Chrysobalanaceae 5 --- --- 5
7 Combretaceae 10 3 3 16
8 Ebenaceae 5 --- 3 8
9 Euphorbiaceae 5 5 --- 10
10 Meliaceae 5 10 5 20
11 Mimosaceae 15 5 3 23
12 Moraceae 20 10 5 35
13 Myristicaceae --- 3 3 6
14 Ochnaceae 5 --- --- 5
15 Olacaceae 5 --- --- 5
16 Papilionaceae 10 5 3 18
17 Rubiaceae --- 5 --- 5
18 Rutaceae 5 --- --- 5
19 Sapotaceae 10 5 5 20
20 Sterculiaceae 35 13 5 53
21 Ulmaceae 5 3 3 11
22 Sapindaceae --- 1 2 3
Total
195
99
54
348
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8
Table 3. Diversity indices of tree species in the primary forest in Eda forest reserve
S/N Species name Family Mean
height
(m)
Mean
DBH
(cm)
Number
of trees
(/ha)
Relative
density
(%)
Relative
dominance
(%)
Species
impotance
value (%)
Volume
(m
3
/ ha)
Basal
area
(m
2
/ha)
Shannon
wiener (H
I
)
1 Albizia adiantifolia Mimosaceae 16.7 115.8 5 1.32 0.48 1.80 10.03 0.60 0.057
2 Alstonia congensis Apocynaceae 26.7 56 10 2.63 3.07 5.70 114.45 3.80 0.096
3 Aningerea robusta Sapotaceae 21.8 31 5 1.32 1.26 2.57 33.98 1.56 0.057
4 Antiaris toxicaria Moraceae 49 134.6 10 2.63 10.17 12.80 616.90 12.60 0.096
5 Bombax buonopozense Bombacaceae 46.77 101.2 15 3.95 6.14 10.08 350.25 7.60 0.128
6 Brachystegia eurycoma Caesalpiniaceae 32.3 40.1 5 1.32 7.28 8.59 291.17 9.01 0.057
7 Ceiba pentandra Bombacaceae 53.87 38.5 15 3.95 13.89 17.83 936.85 17.20 0.128
8 Celtis zenkerii Ulmaceae 25.25 27.4 10 2.63 1.98 4.61 61.50 2.45 0.096
9 Chysophylum albidum Sapotaceae 32 71 5 1.32 1.60 2.91 63.36 1.98 0.057
10 Cordea millenii Bignoniaceae 23.4 201.7 5 1.32 0.99 2.31 28.82 1.23 0.057
11 Cynometra megalophylla Caesalpiniaceae 16.2 77 10 2.63 0.61 3.24 12.05 0.75 0.096
12 Dalium guinensis Caesalpiniaceae 16.8 64.6 5 1.32 0.37 1.68 7.63 0.45 0.057
13 Daniella ogea Caesalpiniaceae 38.5 65.2 5 1.32 3.13 4.44 149.10 3.87 0.057
14 Diospyros mespiliformis Ebenaceae 18.8 34.7 5 1.32 1.20 2.51 27.84 1.48 0.057
15 Distemona bentamianus Caesalpiniaceae 16.1 32.5 5 1.32 0.33 1.65 6.68 0.41 0.057
16 Ficus mucuso Moraceae 18.4 168 5 1.32 0.38 1.70 8.70 0.47 0.057
17 Funtumia elastic Apocynaceae 28.6 101.2 5 1.32 1.88 3.20 66.60 2.33 0.057
18 Hildergadia baterii Sterculiaceae 19.6 64.9 5 1.32 1.06 2.38 25.81 1.32 0.057
19 Khaya ivorensis Meliaceae 34.03 132.7 75 19.74 2.42 22.16 114.70 3.00 0.320
20 Kigelia Africana Bignoniaceae 21.3 96.4 5 1.32 1.15 2.46 30.22 1.42 0.057
21 Lophira alata Ochnaceae 37.35 61.1 10 2.63 3.67 6.31 171.15 4.55 0.096
22 Mansonia altissima Sterculiaceae 17.6 227.8 5 1.32 0.89 2.21 19.49 1.11 0.057
23 Melicia excels Moraceae 45.68 57.9 25 6.58 5.69 12.27 331.70 7.05 0.179
24 Milletia aboensis Papilionaceae 14.4 39.1 5 1.32 0.36 1.68 6.42 0.45 0.057
25 Musanga cecropioides Moraceae 9.2 80.5 5 1.32 0.38 1.69 4.28 0.46 0.057
26 Nesogodonia papaverifera Sterculiaceae 27 101.2 5 1.32 3.20 4.52 107.11 3.97 0.057
27 Parinari excels Chrysobalanaceae
16.1 71 5 1.32 1.51 2.83 30.11 1.87 0.057
28 Piptadeniastrum africanum Mimosaceae 28.4 34 5 1.32 1.93 3.24 67.86 2.39 0.057
Olajuyigbe and Jeminiwa;
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9
S/N Species name Family Mean
height
(m)
Mean
DBH
(cm)
Number
of trees
(/ha)
Relative
density
(%)
Relative
dominance
(%)
Species
impotance
value (%)
Volume
(m
3
/ ha)
Basal
area
(m
2
/ha)
Shannon
wiener (H
I
)
29 Pterocarpus osun Papilionaceae 21.4 196.9 5 1.32 0.51 1.83 13.52 0.63 0.057
30 Pterygota macrocarpa Sterculiaceae 29.3 134.6 5 1.32 1.71 3.03 62.17 2.12 0.057
31 Ricinodendron heudelotii Euphorbiaceae 16.7 121.9 5 1.32 0.24 1.55 4.92 0.29 0.057
32 Steculia rhinopetala Sterculiaceae 29.88 172.1 45 11.84 2.30 14.14 94.35 2.85 0.253
33 Steculia tragacanta Sterculiaceae 46.8 108.2 5 1.32 2.95 4.26 170.81 3.65 0.057
34 Strombosia pustulata Olacaceae 14 92.6 5 1.32 0.23 1.55 4.01 0.29 0.057
35 Terminalia ivorensis Combretaceae 47.8 33.7 5 1.32 5.53 6.84 327.11 6.84 0.057
36 Terminalia superb Combretaceae 29 34.4 10 2.63 3.47 6.10 128.80 4.30 0.096
37 Tetrapleura tetraptera Mimosaceae 17 155.6 5 1.32 1.39 2.71 29.26 1.72 0.057
38 Triplochyton scleroxylon Sterculiaceae 43.35 103.7 10 2.63 4.36 6.99 240.10 5.40 0.096
39 Zanthoxylum zanthoxyloides Rutaceae 14.7 132 5 1.32 0.30 1.62 5.55 0.38 0.057
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Table 4. Diversity indices of tree species in Eda forest reserve, Ekiti state, Nigeria
Diversity indices
Primary forest
Secondary forest
Encroached farmland
Simpson index (D) 0.93 0.93 0.87
Shannon Wiener index (HI) 3.22 3.14 2.51
Mergalef’s index (d) 6.39 6.57 3.66
Evenness index (E) 0.88 0.86 0.85
Menhinck index 2.00 2.27 1.62
Fisher alpha index
10.89 11.86 5.99
Dominance index (C) 0.07 0.08 0.12
Table 5. Growth characteristics of trees in Eda forest reserve, Ekiti state, Nigeria
Growth variable
Primary forest
Secondary forest
Encroached farmland
Mean dbh (cm) 78.58 ± 6.93 34.61±3.22 83.35±9.04
Dominant dbh (cm) 82.00 30.00 140.00
Mean height (m) 27.23 ± 1.90 23.87±1.54 27.93±1.59
Dominant Height (m) 16.70 17.40 28.00
Mean Basal Area (m
2
/ha) 3.18±0.57 0.36±0.08 1.68±0.39
Total Basal Area (m
2
/ha) 123.86 13.68 31.87
Mean Volume (m
3
/ha) 122.44±29.92 13.20±4.06 53.02±14.67
Total Vol./ha (m
3
/ha) 4775.32 501.49 1007.31
Fig. 3. Diameter (Dbh) distribution pattern of Eda forest reserve
Khaya ivorensis had the highest relative density
of 19.74% and could be regarded as the most
abundant species in the forest reserve. The
dominance of emergent layer species (such as
Khaya ivorensis, Millicia excelsa) highlights the
fact that the forest was a climax old growth forest
Olajuyigbe and Jeminiwa;
AJRAF, 2(1): 1-12, 2018; Article no.AJRAF.42848
11
before exploitation and opening of the forest
canopy [15]. The importance value index (IVI),
which combines the attributes of relative density,
relative frequency and relative dominance;
measures the relative importance of a species in
a forest [16]. This study revealed that Khaya
ivorensis had the highest IVI (22.16%) indicating
that this species was the most abundant in the
forest reserve and was closely followed by Ceiba
pentandra (IVI of 17.83%). This species also had
the highest relative dominance value of 13.89%
which also presented the species as the indicator
species in the reserve. This was followed by
Antiaris africana with 10.17% and the least
relative dominance value of 0.23% was
contributed by Strombosia pustulata.
The highest mean height (53.87 m) was recorded
for Ceiba pentandra which is an indicator species
in tropical rainforest ecosystems. On the other
hand, the least height (9.2 m) was recorded for
Musanga cecropioides which is a pioneer
species that colonizes clearings and abandoned
farmlands [4]. The study revealed that despite
the high level of exploitation, Eda forest reserve
was a repository of many indigenous tropical
hardwood species and had high potential for
germplasm conservation.
The Shannon diversity index (H
I
) which
characterizes the level of diversity in tropical
forests ecosystems has a general limit of 1.5 –
3.5 [9]. Hawthorne et al. [15] opined that the H
I
index was an indication of the high species
diversity and reflected the dominance of a few
tree species in the forest. The H
I
value for the
primary forest was slightly lower than other
tropical rainforests. For instance, Parthasarathy
[2] reported H
I
= 3.89, while Adekunle and
Olagoke [17] reported H
I
= 4.02, for rainforests in
India and Nigeria, respectively. Nevertheless,
Alpha diversity index was highest in the primary
forests (Simpson index = 0.93 and H
I
= 3.22).
The Sorensen’s index indicated the species
similarities among vegetation types [18]. Primary
forest had a lower Sorensen’s index (0.19),
indicating it was more similar to
secondary forest (0.23) than encroached
farmland (0.28). This is evidenced by higher tree
population (380 trees/ha) in primary forest when
compared to encroached farmland (137
trees/ha). This finding agrees with similar studies
such as Sanwo et al. [12] who reported 335
trees/ha from 63 species and belonging to 25
families in a tropical rainforest in southern
Nigeria. Also, Aigbe et al. [13] documented 323
trees/ha from 68 species in Afi River forest
reserve, Nigeria. However, the stand density of
Eda forest reserve was lower than that of tropical
Amazonia forests with approximately 1720
trees/ha [19].
The dbh class distribution revealed the structure
of a degraded forest (encroached farmland), a
secondary and old growth forest. The presence
of more trees in the lower dbh classes (Fig. 3),
highlighted the process of recovery of the tree
vegetation in the secondary forest [20,14]. This
implies that the secondary forest has
relatively good regeneration and recruitment
potential which are indications of forest health
and vigour.
4. CONCLUSION
This study revealed the level of exploitation that
had influenced the tree species composition in
different vegetation types in Eda forest reserve.
Human disturbances had influenced the tree
species composition and structural complexity of
the forest reserve. Hence, the removal of large
trees resulted in tree density and volume
fluctuations in secondary forest and encroached
farmland. Notwithstanding, comparably high
floristic composition and diversity were observed
in the secondary forest. Thus, the degraded
areas have potential for recovery if
encroachment and uncontrolled exploitation are
curbed. Hence, there is a need for a
reconciliation of the demands for conservation
with social and economic expectations from Eda
forest reserve. Furthermore, interventions such
as enrichment planting, and regulated resource
utilization could aid the restoration of encroached
farmlands.
COMPETING INTERESTS
Authors have declared that no competing
interests exist.
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