Biovolume and Diveristy of Tree Species in Bitumen Producing Areas of Ondo State, Nigeria

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O R I G I N A L P A P E R

Diversity and biovolume of tree species in natural forest

ecosystem in the bitumen-producing area of ondo state,Nigeria: a baseline study

V. A. J. Adekunle A. O. Olagoke

Received: 16 May 2007 / Accepted: 23 October 2007 / Published online: 17 November 2007 Springer Science+Business Media B.V. 2007

Abstract Tree species diversity, biovolume and forest stand structure were investigated

in natural forest ecosystem located around some selected communities in the bitumen-

producing area of Ondo state, Nigeria. Two forest reserves and four free areas distributed

in 4 Local Government Areas (LGA) of the state were selected for data collection, out of

the total 6 LGAs in the bitumen belt of the state. The two forest reserves are Oluwa FR at

Legge in Odigbo LGA and Eba Island FR in Ese-Odo LGA while the four free areas are

located close to each of Omotoso community (Odigbo LGA), Ode-Aye community(Okitipupa), Igbo-Egunrin community (Ilaje) and Igbotako community (Okitipupa). Eight

plots of equal size (20 9 20 m) were located in each of the selected location, using

systematic line transect sampling design. In each plot, all living trees with dbh C 10 cm

were identified with their botanical names and their dbhs were also measured. The results

of the study reveal that there were ninety nine (99) tropical hardwood timber species

(range: 21 to 48 species per selected forest). These species were distributed among twenty

nine (29) families. While Funtumia elastica has the highest population distribution across

the selected communities forest, Euphorbiaceae was the dominant family in the entire

area. Although there was a moderate variation in the biodiversity indices among the

selected communities forest, the Shannon-Weiner diversity index of H1 = 4.02 and spe-

cies evenness of E = 0.88 were obtained for the entire study area. Tree density summing up

to 2,740 trees/6 ha varied moderately, with a range of 361609 tree/ha, among the com-

munities. Though most of the trees encountered belonged to the lowest diameter size class,

the mean basal area and biovolume were 26.69 m2 /ha and 262.36 m3 /ha respectively.

Recommendations guiding the decision on the allocation of the communities forest to the

bitumen exploratory industries are made and the need for good forest management of

the prospective area of bitumen exploration of Ondo state is emphasized. This is to prevent

the imminent loss of biological diversity that would eventually accompany the exploration.

V. A. J. Adekunle (&) A. O. OlagokeDepartment of Forestry and Wood Technology, Federal University of Technology, Akure,

Ondo State, Nigeria

e-mail: [email protected]

A. O. Olagoke

e-mail: [email protected]

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Biodivers Conserv (2008) 17:27352755

DOI 10.1007/s10531-007-9279-y


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environmental conditions that are required for optimal agricultural productivity (ENS

2003).

Biodiversity assessment has been recognized by international policy processes such as

the Convention on Biological Diversity, as inevitable tool guiding biodiversity conserva-

tion (Margules and Pressey 2000; Phillips et al. 2003; Royal Society 2003). According toSlik et al. (2003), floristic analyses are very useful for identifying spatial patterns in plant

diversity and composition. Quantitative floristic inventories have been used in recent years

to characterize forest vegetation throughout the tropics, though many of the investigators

were interested in documenting the structure and floristic composition of forest commu-

nities (Smith and Killeen 1995). Adekunle (2005) reported that tree species composition,

size and age distributions and their horizontal and vertical order were found to be the key

values to determine diversity in a forest stand.

The massive vegetal clearance expected to accompany the commencement of bitumen

exploration and dearth of recent information on the diversity and biovolume of tree species

in this area brought the impetus for the floristic, structural and quantitative description of

the selected communities forest. This paper, aimed at characterizing and comparing this

communities forest, reports the patterns of diversity and biovolume of tree species in the

bitumen-producing area of Ondo state, Nigeria and recommend appropriate measure to

ensure biodiversity conservation during mineral exploration. So this study provides

baseline information prior to exploration of bitumen in these communities forest.

Methodology

The study area

This study was conducted in forests around each of six bitumen-producing communities in

Ondo State within the tropical lowland rainforest ecological zone of southwest Nigeria.

Two forest reserves and four free areas distributed in 4 Local Government Areas (LGA)

were selected for data collection, out of the total 6 LGAs in the bitumen belt of the state.

The two forest reserves are Oluwa FR at Legge in Odigbo LGA and Eba Island FR in

Ese-Odo LGA while the four free areas are located close to each of Omotoso community

(Odigbo LGA), Ode-Aye community (Okitipupa), Igbo-Egunrin community (Ilaje) and

Igbotako community (Okitipupa) (Table 1).

Table 1 Communities, type of forest assessed and their location coordinates

Community name and the type of forest assessed. Location mid-point coordinates

Latitude Longitude Easting Northing

Atijere (Eba Island forest reserve) 60 251N 40 311E 667739 709447

Igbo-Egunrin (Free area forest) 60

271

3011

N 40

371

E 678789 713167Igbotako (Free area forest) 60 341 3011N 40 381 E 680588 726995

Legge (Oluwa forest reserve) 60 531 3011N 40 441 E 691524 762051

Ode-Aye (Free area forest) 60

351

3011

N 40

451

E 693485 728881

Omotoso (Free area forest) 60

441

3011

N 40

391

E 682370 745431

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The climate and site conditions

Characteristic of all the study sites is two distinct seasons (rainy and dry seasons), with

frequent rainfall that normally starts in March and ends in November. The annual rainfall

ranges from 1,700 to 2,200 mm. The dry season is experienced from December to

February. Mean annual temperature falls between 26C and 28C while the average daily

humidity is 80%.

The soils are predominantly ferruginous tropical soils and are typical of the variety

found in the intensively weathered areas of basement complex formations in the rainforest

zone of Southwest in Nigeria (Onyekwelu et al. 2005).

Data collection

Systematic sampling design (systematic line transect) was employed for the laying of plots

(Fig. 1). Two transects of 1000 m in length with a distance of at least 500 m between them

were laid in each of the study sites. Sample plot of 20 X 20 m in size was laid in alternate

along each transect at 250 m interval and thus summing up to 4 sample plots per 1000 m

transect and a total of 8 sample plots per study site. All living trees with dbh C 10 cm were

identified and their dbh were measured in every plot. All computed values were extrap-

olated for a hectare.

Tree species identification

The botanical name of every living tree that was encountered in each sample plot was

recorded for each of the study sites. In cases trees botanical name was not known

##

# #

#

#

100000 100000 Meters

N

EW

S

Scale: 1:2000000

12

3456

1. LEGGE (OLUWA FOREST RESERVE)

2. OMOTOSO FOREST3. ODE-AYE FOREST4. IGBOTAKO FOREST5. IGBO-EGUNRIN FOREST6. ATIJERE (EBA FOREST RESERVE)

OR 1CM TO 20 KILOMETERS

Map of Nigeria

Map of Africa

Ondo State

Nigeria

LEGEND

0

Fig. 1 Map of Ondo State showing the location of the forests with maps of Africa and Nigeria in inset

2738 Biodivers Conserv (2008) 17:27352755

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immediately, such a tree was identified by its commercial or local name. Such commercial

or local names were translated to correct botanical names using Gbile (1984) and Keay

(1989). Trees that could not be identified were tagged unknown. Specimens of such

unknown trees were collected and preserved for the identification in the herbarium. Each

tree was recorded individually in the field forms and possible effort was made not to omitany eligible stem in a sample plot.

Computational and statistical procedures

Basal area calculation

The basal area of all trees in the sample plots in the six study sites was calculated using the

formula:

BA pD2

41

Where BA = Basal area (m2), D = Diameter at breast height (cm) and p = Pie (3.142).

The total basal area for each of the sample plots was obtained by the sum of the BA of

all trees in the plot while mean BA for the plot (BAp) was obtained by dividing the total BA

by the number of sample plots (i.e. 8 plots).

Basal area per hectare was obtained by multiplying mean basal per plot with the number

of 20 9 20 m plots in a hectare (25).

BAha BAP 25 2

Where BAha = basal area per hectare.

Volume calculation

The volume of individual trees was estimated using the equation developed for trees

volume estimation in the lowland rainforests by FORMECU (1999). This equation is

expressed as follows:

V e8:4332:331 Ln D 3

Where V = Volume of tree (m3) and D = dbh (cm).

Total plot volume was obtained by adding the volume of individual trees encountered in

the plots. Mean volume for sample plots was calculated by dividing the total plot by the

number of sampling plots (8).

Volume per hectare was obtained by multiplying mean volume per plot VP with the

number of 20 9 20 m plots in a hectare (25).

Vha VP 25 4

Tree species classification and diversity indices

All trees were assigned to families and number of species in each family was obtained for

tree species diversity classification. Frequency of occurrence was obtained for species


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abundance/richness. This was repeated for all plants encountered in the sample plots for the

six sites. The following biodiversity indices were used to obtain tree species richness and

evenness within the forest in each community. They were also used as indices for com-

paring biodiversity among the communities forest.

I. Species relative density (RD) was obtained using the formula given by Oduwaiyeet al. (2002):

RD ni

N

h i 100 5

Where RD = relative density, ni = number of individuals of species i and N = total

number of individuals in the entire population.

II. Relative dominance was obtained using the formula given by Brashears et al. (2004):

RDo PBAi 100

PBAn

6

Where RDo = relative dominance, BAi = basal area of all individual trees belonging to a

particular species i and BAn = stand basal area.

III. Community diversity was obtained using a mathematical formula that takes into

account the species richness and abundance of each species in the ecological community.

The equation for the Shannon-Wiener diversity index given by Price (1997) that was used

is:

H1 XS

i1

piLnpi 7

H1 is the Shannon diversity index, S is the total number of species in the community, pi is

the proportion of a species to the total number of plants in the community and Ln is the

natural logarithm.

IV. Species evenness (E) in each community was determined using Shannons equi-

tability (EH) as stated by Kent and Coker (1992):

E H1

Ln S 8

S is the total number of species in each community.V. Sorensens species similarity index (SI) of Nath et al. (2005) was used to compare

diversity across the different communities selected for the study.

SI 2C

a b c d e f

100 9

C is the total number of species in six communities (i.e. aggregate of all species

encountered in the entire study area); while a, b, c, d, e and f are number of species at

communities 1, 2, 3, 4, 5 and 6, respectively.

Test of significance

One-way analysis of variance (ANOVA) was used for test for significant difference

between tree growth variables obtained in each study site. These variables are trees per


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hectare, tree species and family richness, mean dbh of trees, basal area and volume. The

analysis was performed using SPSS for windows 13.0 (SPSS 2003).

Results

Species diversity, evenness and similarity indices in the bitumen-producing area of

Ondo state

The total number of tree species (C10 cm dbh) encountered when data from the selected

communities forest were pooled was 99 species in 82 genera and 29 families. Number of

species (species richness) encountered varied significantly (Table 2). The overall Shannon-

Wiener diversity index was H1 = 4.02 and species evenness was E = 0.88. Shannon

diversity index and equitability varied moderately. Oluwa FR at Legge (3.42) had the

highest species diversity index. This is followed by free area forests at Igbotako (3.26),

Omotoso (3.21) and Ode-Aye (3.16), Eba Island FR at Atijere (2.76) and the least, Igbo-

Egunrin free area forest (2.46). Species evenness result were similar in all the selected

communities forest, expect for Igbo-Egunrin which had the least value of 0.77. The

highest E value of 0.91 was recorded for Atijere. Sorensens species similarity index (SI)

obtained for the entire communities forest was 97. Of the 82 genera, Ficus with 4 species

had the highest number of species. Other genera with more than one species include Cola

(3 species), Sterculia (3), Alstonia (2), Dialium (2), Diospyros (2), Uapaca (2), Antho-

cleista (2), Guarea (2), Trichilia (2), Albizia (2), Pterocarpus (2) and Celtis (2).

Family composition

A total of 29 families were encountered (Table 5). The families, Euphorbiaceae and

Moraceae with 10 species (10%) each dominated the forest canopy, followed by Meliaceae

and Sterculiaceae (9 species each). Apocynaceae, Caesalpinoidae, Mimosoidae, Papilio-

noidae and Rubiaceae had 5 species each, Anacardiaceae, Annonaceae, Guttiferae,

Sapotaceae and Ulmaceae had 3 species each, Bombacaceae, Ebenaceae, Loganiaceae,

Table 2 Summary of the tree growth variables obtained for some selected forests in the bitumen-producing

communities of Ondo State

Variables Communities All sites

Omotoso Legge Ode-Aye Igbo-Egunrin Atijere Igbotako

Number of trees/ha 361 609 380 423 541 426 2740

Basal area/ha (m2

) 12.13 54.36 16.73 19.19 28.66 28.30 26.46

Volume/ha (m3

) 117.47 572.90 164.93 172.36 259.30 287.19 262.36

Number of families 21 22 19 19 15 17 29Number of species 35 48 36 25 21 39 99

Mean Dbh (cm) 16.58 27.16 18.84 20.92 22.82 23.33 21.60

Dominant Dbh (cm) 130 130 132.5 99.6 88.1 120 132.5

Shannon diversity index 3.21 3.42 3.16 2.46 2.76 3.26 4.02

Species evenness 0.90 0.88 0.88 0.77 0.91 0.89 0.88


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Myristicaceae, Sapindaceae and Verbenaceae had 2 species each while Agavaceae,

Bignonaceae, Burseraceae, Combretaceae, Dichapetalaceae, Ochnaceae, Palmae, Rutaceae

and Simaroubaceae were only represented by 1 species each.

The result of the relative dominance indicated that Apocynaceae (21%) was the dom-

inant family. Other co-dominant families are Euphorbiaceae (15.11%), Sterculiaceae(12.02%), Papilionoidae (5.84%), Ochnaceae (5.76%) and Annonaceae (5.53%). Palmae

(0.02%) was the least of all the families encountered (Table 5).

Forest stand density, basal area and biovolume

An average of 457 stems/ha was obtained in the entire study area. Oluwa FR at Legge

(609) had the highest tree density, followed Eba Island at Atijere (541), Igbotako forest

(426), Igbo-Egunrin forest (423) and Ode-Aye forest (380) while the least was recorded in

Omotoso forest (361). There was no significant difference (P B 0.05) in the number of tree

encountered per hectare (tree density) among the selected communities (Table 3). In

contrast, the tree basal area differed significantly (P B 0.05) among the communities

forest. Except for Oluwa FR, which had the highest BA/ha (54.36 m2 /ha), the values

obtained for all other forests were similar. The mean basal area was 26.69 m2/ha while the

least was obtained for Omotoso forest (12.13 m2 /ha). The volume of trees followed a

similar pattern with basal area. It varied significantly (P B 0.05) among the forests selected

for this study, with a mean of 262.36 m3/ha (Table 2). There was no significant difference

(P B 0.05) in the volume of trees in the forests located at Igbotako, Atijere, Igbo-Egunrin,

Ode-Aye and Omotoso communities while significant difference was discovered to existbetween Oluwa FR at Legge when compared with others, as this forest had the highest

volume/ha (572.90 m3/ha).

Population structure of the communities forest vegetation

The population structure of the forest vegetation in each of the selected communities is

represented in Fig. 3af.

The structure of all the selected communities was typical inverted J-shaped curve. Basal

area distribution in various diameter size class was similar in four communities while Igbo-Egunrin and Atijere, which had no[100 cm dbh tree, were different from others.

Stem diameter distribution and density

Tree density consistently deceased with increasing stem diameter size from 10 cm to

100 cm (Fig. 3). The lowest size class (1020 cm) captured 57.65% of the total forest

stand density and about 8% were above the recommended dbh value of 48 cm for trees to

be merchantable. Mean dbh of trees in the selected communities differed significantly(P B 0.05). There were no significant difference in mean dbh obtained at Omotoso forest,

Ode-aye forest, Igbo-Egunrin forest, Eba Island FR at Atijere and Igbotako forest and that

of Eba Island forest, Igbotako forest and Oluwa FR at Legge (P B 0.05) (Table 3). Large

dimensioned trees whose dbh are 100 cm were not encountered in Eba island FR at Atijere

and Igbo-Egunrin forest. Dominant dbh had greatest value in Ode-Aye forest (132.5 cm

dbh), followed by Omotoso forest and Oluwa FR at Legge (130 cm dbh each), Igbotako


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forest (120), Igbo-Egunrin forest (99.6) and the least (88.1) was recorded in Eba Island at

Atijere.

Distinction in species composition and abundance among the selected communities

There was a considerable variation in the population density of the most abundant tree

species among the selected communities (Table 3). Funtumia elastica was the most-

abundant in Omotoso and Igbotako with 34 and 66 trees/ha respectively. Ricinodendron

heudelotii (75), Spondias mombin (66), Millettia thonningii (141) and Lophira alata (94)

had the highest population in Oluwa FR at Legge, Ode-Aye forest, Igbo-Egunrin forest and

Eba Island FR at Atijere respectively. The abundant species were well represented,

occasional or rare in all the selected communities. Funtumia elastica (most abundant) was

followed by Dialium guineensis (31), Macaranga barteri (28), Alstonia boonei, Aningeria

robusta and Pycnanthus angolensis (each with 25 trees/ha), and Gmelina arborea (22);these together formed 52.63% of the stand density in Omotoso forest. In Oluwa FR at

Legge, Ricinodendron heudelotii (most abundant), Celtis zenkeri (66), Cleistopholis patens

(35), Cola millenii (34), Funtumia elastica (28), Mitragyna ciliata and Staudtia stipitata

(each with 25 trees/ha) accounted for 47.29% of the forest stand density. Spondias mombin

(most abundant) which was followed by Albizia ferruginea (41), Sterculia tragacantha

(25), Funtumia elastica (22) and Pycnanthus angolensis (19) formed 45.53% of Ode-Aye

forest stand density. Millettia thonningii, which was the most abundant, formed with

Funtumia elastica (56), Anthocleista vogelli, Cleistopholis patens and Pycnanthus

angolensis (each with 31 trees/ha) 68.56% of the forest stand density in Igbo-Egunrin

forest. While Lophira alata (most abundant) coupled with Uapaca heudelotii (56), Millettia thonningii (44), Malacantha alnifolia (39), Baphia nitida and Diospyros mes-

piliformis (38 trees/ha each), Canthium subcordatum and Hannoa klaineana (31 trees/ha

each), Piptadeniastrum africana and Rothmania hispida (25 trees/ ha each) captured

67.47% of the forest stand density in Eba Island FR at Atijere, Funtumia elastica which

was the most abundant, in addition with Anthostemma aubryanum (38), Trema orientalis

Table 3 One-way ANOVA test of significance in some growth variables among the selected communities

forest

Variables Source of variation Sum of squares df Mean square F Sig.

Basal area Between groups 9069.75 5 1813.95 4.84 0.002Within groups 14241.16 38 374.767

Total 23310.91 43

Volume Between groups 1085584 5 217116.9 3.822 0.007

Within groups 2158626 38 56805.96

Total 3244211 43

No. of trees/ha Between groups 344161.9 5 68832.39 1.688 0.161

Within groups 1549531 38 40777.14

Total 1893693 43

Mean tree dbh Between groups 503.646 5 100.729 2.880 .027

Within groups 1329.046 38 34.975

Total 1832.692 43


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(25), Pausinystalia talbotii and Pycnanthus angolensis (with 22 trees/ha each) accountedfor 40.61% of Igbotako forest stand density (Table 4).

Discussion

The richness of tree species in forests located in the bitumen-producing area of Ondo state

is noteworthy. For tree species diversity in the area as a whole, high value of species

richness (99), distributed among 89 genera and 29 families were present in this study area.

The breakdown of species richness in each of the forests selected for this study revealedthat the number of species present are 21, 25, 35, 36 and 39 for Eba Island FR at Atijere,

Igbo-Egunrin, Omotoso, Ode-Aye and Igbotako free area forests in that order. These

values were lower in comparison with the values reported by Lowe (1997), Onyekwelu

et al. (2005) and Adekunle (2006) for other rainforests in Nigeria. This could indicate the

extent to which man has previously and presently degraded the forest vegetation in these

communities, since species diversity of a given forest area besides climate and geographic

location, would largely depend on the extent of human interaction in the past and present

(Parthasarathy 2001). In contrast, species richness of 48 obtained for Oluwa FR at Legge is

higher than that of 45 reported for other forests by these workers. Eba Island Forest

reserve, because of its environmental conditionsa riparian forest (surrounded bywater)that could only permit the survival of few species adapted to such conditions,

possessed the least number of species. Other places with lower species richness were in the

free area where logging, farming and other form of encroachment were not controlled.

Although, forest reserves are under the control of the state government and thus logging

and entrance are protected under the state forest laws, the fate of this ecosystem when the

exploration of bitumen commences is uncertain as the exploration would also be done with

the permission from the same government that owns the forest reserves (Table 5).

Parthasarathy (2001), Guo et al. (2003), Onyekwelu et al. (2005) and Adekunle (2006)

have in the recent time considered the use of Shannon diversity index for characterizing

community diversity in the tropics. The overall Shannon diversity index obtained for the

entire bitumenproducing area (4.02) is higher than the general limit of 1.53.5 reported by

Kent and Coker (1992), but compares well with 3.898 reported by Parthasarathy (2001).

Though all the values fall within the general limit, there is still a considerable variation in

this index among the selected communities forest. The overall Shannons equitability of

0.86 obtained in this study is higher than 0.66 reported by Onyekwelu et al. (2005) for

Table 4 Mean separation for growth variables with DMRT

Community Variable mean

Basal area Volume Mean tree dbh No. of trees/ha

Omotoso 12.13a 117.47a 16.58a 361a

Ode-Aye 16.73a 164.93a 18.84a 380a

Igbo-Egunrin 19.19a

172.36a

20.92a

423a

Igbotako 28.30a

259.30a

23.33b

426a

Atijere 28.66a 288.34a 22.82b 541a

Legge 54.36b 572.90b 27.16b 609a

Mean with the same alphabets in columns are not significant (P B 0.05)


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Table5

Diversityandrelativedominanceoftreefam

iliesencounteredinselectedcommunitiesforestinthebitumen-pr

oducingareaofOndostate

S/N

Family

Omotoso

Legge

Ode-Aye

Igbo-Egunrin

Atijere

Igbotako

No.of

Species

RDO

N

o.of

Species

RDO

No.of

Species

RDO

No.of

Species

RDO

No.of

Species

RDO

No.o

f

Species

RDO

1

Agavaceae

1

1.8

865

1

0.3

043

2

Anacardiaceae

1

1.6

641

1

1.7

537

2

6.3

411

1

3.7

927

1

0.1

715

3

Annonaceae

1

1.0

797

1

13.3

06

1

9.1

135

1

5.1

238

1

0.4

403

2

4.1

156

4

Apocynaceae

4

48.5

993

3

19.3

3

3

9.9

886

1

13.9

954

1

0.6

401

3

33.5

457

5

Bignonaceae

1

0.9

977

1

0.7

385

6

Bombacaceae

1

0.2

307

1

4.0

232

7

Burseraceae

1

1.6

074

1

1.9

659

8

Caesalpinoidae

3

5.2

856

2

2.0

574

2

0.7

49

1

1.6

527

9

Combretacea

e

1

0.3

214

1

2.1

67

1

0.2

319

10

Dichapetalaceae

1

0.1

788

11

Ebenaceae

1

2.1

405

2

1.2

286

1

1.4

477

12

Euphorbiaceae

3

8.8

668

2

18.3

106

4

6.4

248

1

0.4

884

3

40.6

421

8

15.9

521

13

Guttiferae

2

1.4

58

2

2.7

086

1

1.2

117

1

2.8

294

14

Loganiaceae

1

1.1

275

2

3.7

271

1

4.0

428

1

4.7

435

15

Meliaceae

2

1.2

511

5

5.6

527

1

0.1

59

3

1.7

564

1

1.1

586

5

1.6

007

16

Mimosoidae

1

0.8

39

1

1.0

062

3

7.6

568

2

5.4

996

2

8.7

952

2

0.5

702

17

Moraceae

3

1.5

215

5

5.4

242

5

5.2

047

2

4.0

903

1

1.6

734

3

5.8

794

18

Myristicaceae

1

3.8

078

2

3.6

885

1

3.2

118

1

6.2

918

1

5.3

25

19

Ochnaceae

1

0.2

234

1

0.1

586

1

19.5

841

1

14.3

522

1

0.2

67

20

Palmae

1

0.1

494

21

Papilionoidae

1

0.2

061

4

1.3

434

2

1.2

517

1

21.7

428

3

10.4

944

22

Rubiaceae

1

6.1

651

4

4.6

309

1

0.1

907

2

3.9

047

2

6.5

865

2

1.8

502

23

Rutaceae

1

2.6

927

1

0.5

764

1

0.1

329

1

4.3

173


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Table5

continued

S/N

Family

Omotoso

Legge

Ode-Aye

Igbo-Egunrin

Atijere

Igbotako

No.of

Species

RDO

N

o.of

Species

RDO

No.of

Species

RDO

No.of

Species

RDO

No.of

Species

RDO

No.o

f

Species

RDO

24

Sapindaceae

1

0.7

204

1

0.2

178

1

0.8

004

25

Sapotaceae

2

3.9

1

1

1.0

448

1

4.3

954

26

Simaroubaceae

1

2.2

604

27

Sterculiaceae

3

3.3

331

7

12.0

296

2

40.3

437

4

16.4

071

28

Ulmaceae

1

3.7

678

1

1.2

463

1

1.1

695

1

1.1

586

3

2.6

091

29

Verbenaceae

1

4.7

87

1

0.1

494

1

1.3

181


123


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Queen Forest an inviolate biosphere reserve in Ondo state. The closeness in species

evenness obtained for each of the selected communities forest, however revealed the

similarity in the pattern of tree species distribution in the area (Table 6).

A total of 29 families of trees encountered in this area shows that forests in the bitumen

belt are very rich in tree species, which are of immense value to rural livelihood. Isichei(1995), Were (2001), Akinyemi et al. (2002), Onyekwelu et al. (2005) and Adekunle

(2006) are few among research workers that have reported on the dominance of members

of the Sterculiaceae, Moraceae, Ulmaceae, Meliaceae, Euphorbiaceae families in the

Nigerian tropical forests. The data presented here is in conformity with their unanimous

assertion; the dominant families in the study area are Euphorbiaceae, Moraceae, Meliaceae

and Sterculiaceae. The number of trees (tree density) of 457 trees/ha obtained in the study

area indicates that the area has a dense vegetation and also reflects the recovery status of

preciously disturbed parts by man as a result of his daily indispensable activities in the

forest. Legge (Oluwa Forest) and Atijere (Eba forest), which are forest reserves and

reservations under protection (gazette) greatly demonstrate higher values of tree density

609 and 541 trees/ha respectively. The lesser value in other communities forest can be

attributed to the significant disturbances done to the forest vegetation there, as consequent

to human activities, since they were not protected by any law (free areas) (Table 7).

The mean tree basal area of 26.69 m2 /ha obtained in this study favourably compares

with the 25.5 m2 /ha also obtained for tropical humid forest in Rio Xingu, Brazil (Par-

thasarathy 2001) and higher than 22.68 m2 /ha recorded in a similar forest in Onigambari,

Nigeria (Akinyemi et al. 2002). This value is also higher than the recommended 25 m2/ha

for a fully stocked forest (Alder and Abayomi 1994). The variation in the tree basal area

among the selected communities forest is pronounced. Except for Omotoso free areaforest that has a value of 12.13 m2 /ha and Ode-Aye forest with a value of 16.73, other

communities forest are higher than the 16.84 m2/ha reported by Adekunle et al. (2002) for

Omo forest reserve in the tropical rainforest ecosystem of southwest Nigeria.

Although, the mean tree volume obtained in this study could be seen as relatively high

(262.36 m3/ha), but value is relatively lower that for some other forest reserves in the same

ecosystem in Nigeria. This, in support of the diameter distribution result, gives the indi-

cation that most of the trees are not yet mature for sawn-timber harvest.

Nath et al. (2005) asserted that the inverted J-curve, where the abundance decreases

with increasing diameter, is an indication of good regeneration of the constituent species.

The conformity of the population structure of trees in all the selected communities forestwith this reverse J-shaped structure as shown in Fig. 2(af) clearly reflects the potential of

these communities forest to regenerate over a space of time. Larger population of the trees

falls in the lowest diameter size class; the amount of merchantable trees with dbh C48 cm

dbh is very meager. Non-availability of large dimensioned trees with dbh that is above

100 cm in Eba Island FR at Atijere and Igbo-Egunrin forest could be due to the fact that

these forests are very close to the river or as a result of previous human activities in the

area.

The presence of Funtumia elastica in all the forests, Albizia ferruginea, Alstonia boonei,

Baphia nitida, Celtis zenkeri, Cleistopholis patens, Lophira alata, Macaranga barteri,Pycnanthus angolensis, Ricinodendron heudelotii and Sterculia tragacantha to more than

half of the selected communities forest reveal a moderate floristic overlap in their forest

vegetation. In contrast, each of Aningeria robusta, Anthostemma aubryanum, Cola

millenii, Gmelina arborea, Hannoa klaineana, Piptadeniastrum Africana, Trema orientalis

and Uapaca heudelotii is exclusively found in a community. The occurrence of Gmelina

arborea can be attributed to the dispersal of its seeds from the existing plantation in


123


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Table 6 Tree species diversity, population density, Shannon index and species evenness in selected

communities forest in the bitumenproducing area of Ondo state

Familiesspecies Omotoso Legge Ode-Aye Igbo-Egunrin Atijere Igbotako piLNpi

Agavaceae Dracaena mannii 12 7 -0.034

Anacardiaceae

Lannea welwitschii 7 9 -0.030

Mangifera indica 6 6 -0.024

Spondias mombin* 66 3 0.093

Annonaceae

Cleistopholis patens* 3 35 12 31 13 -0.116

Enantia chlorantha 7 -0.015

Xylopia aethiopica 4 -0.010

Apocynaceae

Alstonia boonei* 25 17 6 13 -0.085

Alstonia congensis 6 -0.013

Funtumia elastica* 34 28 22 56 13 66 -0.202

Holarrhena floribunda 4 -0.010

Rauvolfia vomitoria 6 3 -0.019

Bignonaceae

Newbouldia laevis 6 10 -0.030

Bombacaceae

Bombax buonopozense 3 -0.007

Ceiba pentandra 3 -0.007

Burseraceae

Canarium schweinfurthii 3 8 -0.022

Caesalpinoidae

Amphimas pterocarpoides 3 3 16 -0.039

Daniellia ogea 3 -0.007

Dialium dinklagei 7 -0.015

Dialium guineense* 31 9 -0.062

Distemonanthus

benthamianus

3 -0.007

Combretaceae

Terminalia superba 3 12 3 -0.033

Dichapetalaceae

Dichapetalum

madagascariense

3 -0.007

Ebenaceae

Diospyros dendo 9 -0.019

Diospyros mespiliformis* 6 17 38 -0.085Euphorbiaceae

Alchornea cordifolia 3 -0.007

Anthostemma aubryanum* 38 -0.059

Drypetes spp 12 -0.024

Macaranga barteri* 28 3 9 7 6 -0.076


123


15/21

Table 6 continued


Maesobotrya barteri 3 -0.007

Phyllanthus discoideus 13 12 19 -0.066

Ricinodendron heudelotii* 6 75 7 3 -0.113

Uapaca guineensis 13 -0.025

Uapaca heudelotii 56 -0.080

Unknown 1 6 -0.013

Guttiferae

Allanblackia floribunda 9 19 -0.047

Garcinia kola 9 -0.019

Harungana madagascariensis 3 3 19 -0.043

LoganiaceaeAnthocleista djalonensis 9 19 -0.047

Anthocleista vogelli* 6 31 13 -0.073

Meliaceae

Carapa procera 3 6 3 6 -0.033

Ekebergia senegalensis 3 -0.007

Entandrophragma angolensis 9 -0.019

Guarea cedrata 3 6 -0.019

Guarea thompsonii 6 -0.013

Khaya ivorensis 3 4 -0.015Trichilia monadelpha 19 9 3 -0.051

Trichilia prieureana 12 19 3 -0.054

Unknown 2 3 -0.007

Mimosoidea

Albizia ferruginea* 10 41 4 10 -0.089

Albizia zygia 3 -0.007

Brachystegia eurycoma 9 7 6 3 -0.043

Pentaclethra macrophylla 16 -0.030

Piptadeniastrum africana* 25 -

0.043Moraceae

Antiaris africana 12 -0.024

Bosqueia angolensis 3 4 6 -0.025

Ficus capensis 3 -0.007

Ficus exasperata 4 6 6 -0.030

Ficus mucuso 3 6 3 -0.024

Ficus sur 13 -0.025

Melicia excelsa 3 -0.007

Mussanga cecropiodes 6 3 6 6 -0.037

Myrianthus arboreus 9 -0.019

Treculia africana 12 -0.024

Myristicaceae

Pycnanthus angolensis* 25 9 19 31 22 -0.126


123


16/21

Table 6 continued


Staudtia stipitata* 25 6 -0.051

Ochnaceae

Lophira alata* 3 7 6 94 3 -0.131

Palmae

Elaeis guineensis 3 -0.007

Papilionoidae

Baphia nitida* 3 6 6 38 -0.076

Lonchocarpus sericeus 16 -0.030

Millettia thonningii* 7 141 44 -0.186

Pterocarpus osun 3 -0.007

Pterocarpus soyauxii 3 13 -0.030Rubiaceae

Canthium subcordatum* 13 3 31 -0.070

Mitragyna ciliata* 25 3 -0.047

Nauclea diderrichii 19 17 -0.057

Pausinystalia talbotii* 3 22 -0.043

Rothmannia hispida* 3 9 25 -0.058

Rutaceae

Zanthoxylum zanthoxyloides 10 9 3 19 -0.063

Sapindaceae Blighia sapida 6 6 -0.024

Lecaniodiscus cupanioides 12 -0.024

Sapotaceae

Aningeria robusta* 25 -0.043

Chrysophyllum albidum 3 -0.007

Malacantha alnifolia* 3 38 -0.063

Simaroubaceae

Hannoa klaineana* 31 -0.051

SterculiaceaeCola gigantea 3 9 6 -0.033

Cola millenii* 35 -0.056

Cola nitida 3 -0.007

Mansonia altissima 19 -0.034

Pterygota macrocarpa 6 9 -0.029

Sterculia oblonga 4 -0.010

Sterculia rhinopetala 3 6 16 -0.043

Sterculia tragacantha* 7 9 6 6 -0.047

Triplochiton scleroxylon 6 -0.013

Ulmaceae

Celtis integrifolia 3 -0.007

Celtis zenkeri* 66 9 3 6 9 -0.115

Trema orientalis* 25 -0.043


123


17/21

Omotoso. Likewise, the presence of Elaeis guineensis in Ode-Aye forest is typical of

degraded natural forest (Fig. 3).

The demonstration of high level of tree species diversity in this area gives the impetus

for the placement of an economic value on the forest vegetation and many biologicalresources there. Failure to recognize this would be tantamount to failure of these resources

to compete on a level playing field with the forces driving their decline (OECD 2002). The

need for paying cognizance to the practical valuation of these communities forest vege-

tation and their constituent biodiversity therefore becomes conspicuous. The inevitability

of this deeply seated upon the premises that the clearance of forest vegetation to pave way

Table 6 continued


Verbenaceae

Gmelina arborea* 22 -0.039

Vitex doniana 3 -0.007

Total 361 609 380 423 541 426 -4.02

* Most common species in one or more of the selected communities forest

Table 7 A comparison of recent forest inventories in the tropical rainforest ecosystem

Locality Source Number

of trees/ha

Basal

Area (m

2

)/ha

Number of

families/ha

Number of

species/ha

H1

Nigeria

Omotoso This study 361 12.13 21 35 3.21

Oluwa FR This study 609 54.36 22 48 3.42

Ode-Aye This study 380 16.73 19 36 3.16

Igbo-Egunrin This study 423 19.19 19 25 2.46

Eba Island FR This study 541 28.66 15 21 2.76

Igbo-Egunrin This study 426 28.3 17 39 3.26

Ala FR Adekunle (2006) 148 n.a 24 54 3.62

Omo FR Adekunle (2006) 115 n.a 21 41 3.34

Shasha FR Adekunle (2006) 159 n.a 20 55 3.66

Queen FR* Onyekwelu et al.

(2005)

742 85.4 26 51 3.31

Onigambari FR Akinyemi et al.

(2002)

308 22.68 22 50 n.a.

Selected other localities

Sengaltheri* (India) Parthasarathy

(2006)

965 55.34 n.a 82 3.69

Cumbre Pilon (Bolivia) Smith and Killeen

(1995)

647 30.62 37 146 n.a

Rio Colorado (Bolivia) Smith and Killeen

(1995)

588 26.4 31 78 n.a.

* Undisturbed natural forest; n.a. not available


123


18/21


19/21

different combination of species. Most of the tree species are distributed in the lowest

diameter size class and the percentage of the merchantable trees ready for sawn-timber

harvest is very meager. The area, besides being distinct in its biodiversity constituents, alsoharbours a vast number of genetic resources. Allocation of the area for bitumen exploration

would therefore demands for good management of the area for the conservation and

sustainability of the constituent resources.

The variation in the species composition and abundance among the selected commu-

nities forest necessitates the preservation of the vast acreage of the forest in this area, as a

single large reserve. The forest should also be managed to provide other services like

tourism, wildlife sanctuary, watershed management and climatic amelioration. Meanwhile,

the authority in control of these forests should eschew from subsequent conversion of

natural forest to industrial plantation of exotic and indigenous species as the case of Oluwa

forest reserve- one of the richest ecosystem of Nigeria whose greater part had been clear-felled for plantation species, which defy biodiversity conservation in this area.

In view of the proposed allocation of this area to bitumen exploratory industries, it is

suggested that Legge (Oluwa forest) and Atijere (Eba forest) which are zones of high

protection and high conservation value should be tagged no-go area for bitumen

exploration while exploration can only commence in Igbotako with intense biodiversity

(a) Omotoso Forest

0

1

2

3

4

5

10--20

20--30

30--40

40--50

50--60

60--70

70--80

80--90

90--10

0

>10

0

Diameter size class (cm)

10--20

20--30

30--40

40--50

50--60

60--70

70--80

80--90

90--10

0

>10

0


10--20

20--30

30--40

40--50

50--60

60--70

70--80

80--90

90--10

0

>10

0


10--20

20--30

30--40

40--50

50--60

60--70

70--80

80--90

90--10

0

>10

0

Diameter size class (cm)10

--20

20--30

30--40

40--50

50--60

60--70

70--80

80--90

90--10

0

>10

0


10--20

20--30

30--40

40--50

50--60

60--70

70--80

80--90

90--10

0

>10

0


B

asalarea(m

2)

0

50

100

150

200

250

300

350

No.oftrees

No.oftrees

No.oftrees

No.oftrees

No.oftrees

No.oftrees

(b) Oluwa FR

0

24

68

10

12

14

16

0

50

100

150

200

250

300

(c) Ode-Aye Forest

0

1

2

3

4

5

Basalarea(m

2)

0

50

100

150

200

250

300

(d) Igbo-Egunrin Forest

0

1

2

3

4

5

6

0

50

100

150

200

250

300

(e) Eba Island FR

012

3456789

Basalarea(m

2)

Basalarea(m

2)

Basalarea(m

2)

Basalarea(m

2)

0

50

100

150

200

250

300

0

1

2

3

4

5

6

7

0

50

100

150

200

250

300(f) Igbotako Forest

Basal Area Adundance

Fig. 3 (af) Population structure of tree species in selected communities forest in the bitumen-producing

area of Ondo state based on diameter-frequency and basal area


123


20/21

conservation in view. Also, the industries can proceed into Omotoso, Ode-Aye and Igbo-

Egunrin under good forest management strategies. The prospective industries should also

be made to agree for compensation measures, including habitat creation to maintain

constant natural assets. Therefore, the decision to allow these bitumen exploratory

industries should be based on their readiness to sign a contract for good management of thebiological resources, with little deleterious effect and low environmental risk, should they

be allowed. Another study should be carried out a few years after the commencement of

the exploration such that the results in term of tree species diversity and yield could be

compared with the result of this baseline study. This will help allow for the effect of

bitumen exploration on the habitat to be assessed.

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Documents

Biovolume and Diveristy of Tree Species in Bitumen Producing Areas of Ondo State, Nigeria