Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation

Abstract

Watershed ecosystems are unique ecologies that provide services and functions that are critical for the sustenance of natural water bodies as well as people that depend on them. However, there is limited information on plant species composition and distribution in most watershed forests in Nigeria. The watershed forest surrounding Arinta waterfall in Ipole Iloro, is an area protected by local communities, but facing anthropogenic threats associated with increased patronage from tourists from within and outside Nigeria. This study determined the floristic composition and vegetation structure in the Arinta watershed forest, using a stratified sampling technique. Nine sample plots, each 20 m by 20 m, were laid along three horizontal strata from the river valley upwards at 456 m, 470 m and 489 m above sea level. Within each sample plot, all plant forms were identified and enumerated, while growth variables like diameter and total height were measured for tree species. The species diversity indices were determined for all tree species encountered. Ninety two plant species from 51 families were found in the watershed ecosystem. These included 48 trees, 12 shrubs, 12 lianas, 16 herbs, 1 grass and 3 fern species. Ceiba pentandra was the most dominant species with an Importance Value Index (IVI) of 43.54% while Chrysophyllum albidum was the least (IVI = 0.78%). Fabaceae family had the highest species representation (14.13%) in the plant population. Shannon Wiener diversity index was 3.28, while the species evenness was 0.85. Approximately 64% of the tree species population had heights <10 m, while only 0.2% had heights >30 m. Similarly, tree species in diameter class 1-10 cm had the highest population of 630 wildlings/saplings ha-1.. The high floristic composition indicated the need for the conservation of the watershed which provides both protective and productive functions for the environment as well as the local community.

Full text

79
Environtropica, March 2019, Vol. 15, 79 -94 ISSN 1597-815X
Floristic composition and stand structure in a tropical watershed forest:
implications for biodiversity conservation.
Olajuyigbe Samuel Olalekan1 and Akwarandu Karachi Eyeribe2
1Department of Forest Production and Products, University of Ibadan, Ibadan, Oyo State, Nigeria.
2Forestry Technology Department, Yobe State College of Agriculture, Damaturu, Yobe State,
Nigeria. Corresponding author: lekito2001@yahoo.com, so.olajuyigbe@mail1.ui.edu.ng
Accepted on February 08, 2019
Abstract
Watershed ecosystems are unique ecologies that provide services and functions that are critical for
the sustenance of natural water bodies as well as people that depend on them. However, there is
limited information on plant species composition and distribution in most watershed forests in
Nigeria. The watershed forest surrounding Arinta waterfall in Ipole Iloro, is an area protected by
local communities, but facing anthropogenic threats associated with increased patronage from
tourists from within and outside Nigeria. This study determined the floristic composition and
vegetation structure in the Arinta watershed forest, using a stratified sampling technique. Nine
sample plots, each 20 m by 20 m, were laid along three horizontal strata from the river valley
upwards at 456 m, 470 m and 489 m above sea level. Within each sample plot, all plant forms were
identified and enumerated, while growth variables like diameter and total height were measured for
tree species. The species diversity indices were determined for all tree species encountered. Ninety
two plant species from 51 families were found in the watershed ecosystem. These included 48 trees,
12 shrubs, 12 lianas, 16 herbs, 1 grass and 3 fern species. Ceiba pentandra was the most dominant
species with an Importance Value Index (IVI) of 43.54% while Chrysophyllum albidum was the
least (IVI = 0.78%). Fabaceae family had the highest species representation (14.13%) in the plant
population. Shannon Wiener diversity index was 3.28, while the species evenness was 0.85.
Approximately 64% of the tree species population had heights <10 m, while only 0.2% had heights
>30 m. Similarly, tree species in diameter class 1 – 10 cm had the highest population of 630
wildlings/saplings ha-1.. The high floristic composition indicated the need for the conservation of
the watershed which provides both protective and productive functions for the environment as well
as the local community.
Keywords: Nigeria, Ipole Iloro, Arinta waterfall, ecotourism, watershed forest
Introduction
A watershed is an area of land that drains water, sediments and dissolved materials to a common
receiving body or outlet. This vegetation area forms the drainage of streams or rivers and reduces
flooding and erosion damages, while providing clean water and opportunities for recreation.
Watersheds improve the structure and function of wetlands and wildlife habitats associated with
them. Invariably, watersheds help to enhance the quality of life and the environment of adjoining
communities (Aju, 2017; Asinwa et al., 2018). These forests are unique vegetative features used to
define the boundaries of ecosystems and they play a vital role in the biogeochemical cycles that
occur on both the terrestrial and aquatic parts of the ecosystem. The woody plant species found in

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
Akwarandu.
80
watershed forests influence soil and hydrological processes such as erosion control, soil and water
quality enhancement, increment of water volume, stream flow stabilization, sediment distribution as
well as control (Hans et al., 2007; Aju, 2017; Manik and Sidle, 2018).
Arinta watershed forest is a naturally occurring forest growing on steep, weathered, sedimentary
rock in the lowland rain forest zone of Nigeria. This dense, humid rainforest is rich in biological
diversity and serves as a natural seed bank for important tree species. The waterfall is a tourist site
in Ekiti state, Nigeria. The government and community have been making efforts towards the
development of this natural monument into an Ecotourism Centre, which could boost revenue
generation. However, a direct impact of urbanization and infrastructural development is the
destruction of the watershed vegetation. Although, recreation and tourism infrastructure within
protected areas may be limited, the creation of tracks, trails, roads, lookouts, fixed campsites and
other types of accommodation affect biodiversity. It becomes pertinent that the plant diversity in
the Arinta watershed vegetation be evaluated to assist in future conservation and sustainable
management of the site, particularly with the impending pressure that may result from several
disturbances due to the commercialization of the site for ecotourism activities (Ijasan and Izobo-
Martins, 2012; Olaniyi et al., 2015). Hence, this study determined plant species diversity,
composition and distribution along the Arinta waterfall topography in order to provide information
on the current status of the watershed forest.
Materials and Methods
Site description
Arinta watershed forest is in Ekiti West Local Government Area, Efon Alaaye, Ekiti state, Nigeria.
It lies along latitudes 7° 33.02' N and 7° 33.50' N; longitudes 4°55.1' E and 4°55.58' E. It has a
rhythmically undulating hilly terrain with elevation that ranges from 455 m – 495 m above sea level
(Figure 1). The 4.5 ha watershed forest experiences two distinct seasons: rainy (from April –
October), and dry (from November - March). The forest grows on a long steep valley formed out of
a weathered mountain and it is dominated by broadleaf hardwood trees which form dense layered
stands. The waterfall that emanates from the rocky outcrop is the source of a stream which supplies
most of the water requirements of residents of Ipole Iloro town in Ekiti state, Nigeria. Farming and
logging are prohibited within the watershed forest based on customary laws.
Sampling procedure
Nine sample plots, each measuring 20 m by 20 m, were laid along the slope of the watershed forest
in three horizontal strata from the river valley upwards: lower stratum (456 m) middle stratum (470
m), and upper stratum (489 m). The stratification was based on the elevation, which was determined
using a Hand-held Gemini Sotrex GPS. The sample plots were laid at 30 m interval within each
stratum, and a distance of 40 m was used to separate the strata from one another. All trees, shrubs,
lianas and herbs within each sample plot were identified and enumerated with the help of a plant
taxonomist as well as with reference to relevant local plant manuals and Flora of West Tropical
Africa (Hutchinson and Dalziel, 1994). Voucher specimens of plants that could not be identified
were collected, preserved and taken to the Herbarium of the Forestry Research Institute of Nigeria,
Ibadan, Nigeria for identification. The diameter at breast height (dbh) ≥ 10 cm and total height of
tree species were measured. A 2 m-wide strip or belt transect (1 m on either side of the centre line)
was laid at the middle of each sample plot, and used to determine shrubs, wildlings, herbs and
lianas with collar diameter ≥ 2 cm (Olajuyigbe and Adaja, 2014).

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81
Figure 1. Arinta waterfall Ipole Iloro, Ekiti State, Nigeria (Inset: Maps of Nigeria and Ekiti
state)
The Basal Area BA (m2), of all the trees in the sample plots were calculated using Equation 1
following the method of Adekunle et al. (2013):
BA = 2
4 ………………………………………. (1)
Where D = diameter at breast height (m) and  = (3.142). The plot basal area for each of the sample
plots was obtained by summing all BA of trees in the plot while the mean plot basal area (BAp) was
calculated as the sum of basal areas of all sampled plots divided by the number of plots. Basal area
per hectare was obtained by multiplying mean plot basal area with the number of 20 m ×20 m plots
in 1 hectare.
The tree volume was estimated by multiplying the individual tree BA with total height. The plot
volume was estimated by summing up individual tree volumes and then the mean volume per plot
and per hectare was determined.
All tree species were assigned to families and number of species in each family was obtained and
used to determine tree species diversity indices. The following diversity indices were used to obtain
tree species abundance/richness and evenness within the watershed forest.
I. Relative frequency of tree species was obtained using Equation 2:
 = 
 × 100% …………………………….(2)
Where RF= Relative Frequency, Fi = number of plots where species i was encountered, and Fn =
total frequency of all species.
II. Species Relative Density (RD) was obtained using Equation 3:
RD= [ ′
] x 100% ………………..………. (3)

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
Akwarandu.
82
Where RD = Relative Density, ni = number of individuals of species i and N = total number of
individuals in the entire population.
III. Relative dominance was obtained using Equation 4:
RDO =( ∑¡ ×100 )
∑…………………….. (4)
Where RDo = relative dominance, BAi = basal area of all individual trees belonging to a particular
species i and BAn = Total basal area of all species encountered.
IV. Shannon Wiener diversity index was estimated using equation 5:
1=�  

=1
… … … … … … …(5)
Where; H1 = the Shannon Wiener diversity index, S = total number of species in the watershed
forest,  = the proportion of a species to the total number of plants in the watershed forest and Ln =
the natural logarithm.
V. Species evenness (E) was determined using Shannon’s Equitability index (EH) (Equation 6):
EH = 1
()……………………………….. (6)
Where; S = the total number of species in the watershed forest.
VI. Importance Value index (IVI): was calculated by summing the Relative density, Relative
dominance and Relative frequency.
Results
A total of 92 plant species belonging to 82 genera from 51 families were identified in the watershed
forest ecosystem (Table 1). These comprised six plant forms (48 trees, 12 shrubs, 12 lianas, 16
herbs, 1 grass and 3 ferns) with trees being the most dominant species. The mean number of trees
per plot was 34 ± 3.61, while the mean plot basal area and mean tree volume per plot were 1.24 ±
0.25 m2 and 16.82 ± 4.33 m3, respectively (Table 2). It was estimated that density of trees > 10 cm
dbh were 850 ± 90.14 trees ha-1, with a basal area and total volume of 31.06 ± 6.30 m2 ha-1 and
420.61 ± 108.36 m3 ha-1, respectively in the forest.
Table 1: Floristic composition and plant form at Arinta Watershed forest, Ipole Iloro, Ekiti
state, Nigeria.
Species
Family
Common name
Ferns
Cyclosorus afer (Christ.) Ching
Thelypterideae
Cyclosorus fern
Nephrolepis biserrata (Sw.) Schott
Polypodiaceae
Giant sword fern
Nephrolepis undulata (Afzel. ex Sw.) J.Sm
Oleandraceae
Annual sword fern
Grass
Axonopus compressus (Sw.) P. Beauv.
Poaceae
Blanket grass
Herbs
Acanthus montanus (Nees) T. Anders.
Acanthaceae
False Thistle

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83
Ageratum conyzoides L.
Asteraceae
Billy Goat Weed
Alternanthera sessilis (L) DC.
Amaranthaceae
Sessile joyweed
Aspilia africana (Pers.) C. D. Adams
Compositae
Haemorrhage plant
Colocasia esculenta (L.) Schott.
Araceae
Elephant ears
Commelina lagosensis C.B. Clarke
Commelinaceae
Day flower
Ipomoea reticulata O’Donell
Convolvulaceae
Morning glory weed
Ipomoea involucrata P. Beauv.
Convolvulaceae
Morning glory weed
Mucuna pruriens (L.) DC.
Fabaceae
Velvet beans
Plumbago zeylanica L.
Plumbaginaceae
Ceylon leadwort
Phyllanthus amarus Schum. & Thonn.
Euphorbiaceae
Hurricane weed
Physallis angulata L.
Solanaceae
Wild goose berry
Scoparia dulcis L.
Scrophulariaceae
Sweet broom weed
Smilax kraussiana Meisn.
Smilacaceae
Smilax
Thaumatococcus daniellii (Benn.) Benth.
Marantaceae
Miraculous berry
Tragia vogelii Keay
Euphorbiaceae
Nose burn
Lianas
Alafia barteri Oliv.
Apocynaceae
Guinea fowl’s crest
Asparagus africanus Lam.
Asparagaceae
Climbing African asparagus
Canthium venosum (Oliv.) Hiern
Rubiaceae
Raisin-fruit Keetia
Chasmanthera dependens Hochst.
Menispermaceae
Chasmanthera
Dioclea reflexa Hook F.
Fabaceae
Marble’s vine
Dioscorea dumetorum (Kunth) Pax.
Dioscoreaceae
African bitter yam
Gnetum africanum Welw.
Gnetaceae
Wild spinach
Momordica foetida Schumach.
Cucurbitaceae
Ejirin (Yoruba)
Parquetina nigrescens (Afzel.) Bullock
Apocynaceae
Ewe Ogbo (Yoruba)
Piper guineense Schum. & Thonn.
Piperaceae
West African black pepper
Quisqualis indica Linn.
Combretaceae
Rangoon creeper
Saba thompsonii (A.Chev.) Pichon
Apocynaceae
Komero-akowa
Shrubs
Alchornea cordifolia (Schumach. & Thonn.)
Müll Arg.
Euphorbiaceae
Christmas bush
Alchornea laxiflora (Benth.) Pax & K
Hoffm.
Euphorbiaceae
Three-veined bead string
Annona reticulata Linn.
Annonaceae
Bullock's heart
Bridelia atroviridis Müll. Arg.
Euphorbiaceae
Rare forest bridelia
Bridelia micrantha (Hochst.) Baill.
Euphorbiaceae
Bridelia
Cnestis ferruginea Vahl. ex. DC.
Connaraceae
Short pod
Combretum micranthum G. Don
Combretaceae
Bush tea
Icacina trichantha Oliv.
Icacinaceae
Gbegbe (Yoruba)
Rauvolfia vomitoria Afzel.
Apocynaceae
Swizzle stick
Rinorea elliotii Engl.
Violaceae
Iparoko
Sphenocentrum jollyanum Pierre
Menispermaceae
Akerejupon
Xylopia aethiopica (Dunal) A. Rich
Annonaceae
West African pepper tree

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
Akwarandu.
84
Trees
Antiaris toxicaria Lesch.
Moraceae
Antiaris
Albizia adianthifolia (Schumach.) W. F.
Wight
Fabaceae
Flat crown
Albizia gummifera (J. F. Gmel.) C. A. Sm.
Fabaceae
Peacock flower
Alstonia boonei De Wild.
Apocynaceae
Alstonia
Anthocleista djalonensis A. Chev.
Loganiaceae
Cabbage tree
Artocarpus communis J. R. Forst. & G.
Forst.
Moraceae
Breadfruit
Bombax buonopozense P. Beauv.
Bombacaceae
Wild kapok
Brachystegia eurycoma Harms
Fabaceae
Achi
Brachystegia kennedyi Hoyle
Fabaceae
Okwen
Ceiba pentandra (L.) Gaertn.
Malvaceae
White silk cotton tree
Chrysophyllum albidum G. Don.
Sapotaceae
White star apple
Cola afzelii (R. Br.) Mast.
Sterculiaceae
Kola nut
Cola gigantea A. Chev.
Sterculiaceae
Witch's bread
Cordia millenii Bak.
Boraginaceae
Drum tree
Dialium guineense Willd.
Fabaceae
Velvet tamarind
Diospyros mespiliformis Hochst.
Ebenaceae
West African Ebony
Distemonanthus benthamianus Baill.
Fabaceae
African satinwood
Dracaena arborea (Willd.) Link
Dracaenaceae
Dragon tree
Dracaena mannii Baker
Dracaenaceae
Small-leaved dragon tree
Elaeis guineensis Jacq.
Arecaceae
Oil palm
Entandrophragma angolense (Welw.) C.
DC.
Meliaceae
Tiama mahogany
Erythrophleum suaveolens (Guill. & Perr.)
Brenan
Fabaceae
Sasswood tree
Ficus exasperata Vahl.
Moraceae
Forest sandpaper fig
Funtumia elastica (Preuss.) Stapf.
Apocynaceae
Rubber tree
Garcinia kola Heckel.
Clusiaceae
Bitter kola
Hollarrhena floribunda (G. Don) Dur. &
Schinz.
Apocynaceae
False rubber
Khaya ivorensis A Chev.
Meliaceae
African Mahogany
Lecaniodiscus cupanioides Planch.
Sapindaceae
Limba
Massularia acuminata (G. Don) Bullock
Rubiaceae
Maiden’s breasts tree
Milicia excelsa (Welw.) C. C. Berg
Moraceae
Iroko (Yoruba)
Musanga cecropioides R. Br.
Cecropiaceae
Umbrella tree
Myrianthus arboreus P. Beauv.
Cecropiaceae
Giant yellow mulberry
Napoleonaea vogelii Hook. & Planch.
Lecythidaceae
Wallia
Newbouldia laevis Seem.
Bignoniaceae
Akoko tree
Pentaclethra macrophylla Benth.
Fabaceae
Oil of bean tree
Piliostigma thonningii (Schumach.) Milne-
Redh
Fabaceae
Camel’s foot tree

Environtropica - An International Journal of the Tropical Environment
85
Piptadeniastrum africanum (Hook. F.)
Brenan
Fabaceae
African greenheart
Pterygota macrocarpa K Schum.
Sterculiaceae
African pterygota
Pycnanthus angolensis (Welw.) Warb.
Myristicaceae
African nutmeg
Ricinodendron heudelotii (Baill.) Pierre
Euphorbiaceae
African wood-oil nut tree
Senna siamea (Lam.) Irwin & Barneby
Fabaceae
Siamese tree
Spondias mombin Linn.
Anacardiaceae
Hog plum
Sterculia rhinopetala K Schum.
Sterculiaceae
Brown sterculia
Sterculia tragacantha Lindl.
Sterculiaceae
African tragacanth
Terminalia superba Engl. et Diels
Combretaceae
White afara
Trema micrantha (L.) Blume
Ulmaceae
Nettle tree
Trema orientalis (Linn.). Blume
Ulmaceae
Charcoal tree
Zanthoxylum zanthoxyloides (Lam.) Zepern.
& Timler
Rutaceae
Toothache bark
There was no tree species in the emergent layer (i.e. trees >50 m in height) with the tallest trees
being Ceiba pentandra (32 m) and Cordia millenii (30 m). The understory represented 34.8% of the
tree population, while 1.1% of the tree population were in the lower canopy (canopy height = 20 to
30 m). Only 4 individual tree species (0.2%) were in the upper canopy (Figure 2).
Table 2: Strata and plot distribution of tree species (>10 cm dbh) in Arinta watershed forest
at Ipole Iloro Ekiti, Ekiti state, Nigeria. (Stratification levels were at the lower (456 m), middle
(470 m) and upper (489 m) sections of the watershed forest, each plot was 20 m by 20 m in
size)
Stratum
Plot number
Number of trees
(per plot)
Basal area (m2/
plot)
Tree volume
(m
3
/plot)
1
1
35
1.02
12.84
1
2
26
3.09
49.45
1
3
37
1.54
19.95
2
1
34
0.72
8.33
2
2
23
0.67
7.91
2
3
19
0.70
9.18
3
1
50
0.92
9.59
3
2
32
1.16
16.85
3
3
50
1.36
17.31
Tree species in the lowest diameter class (1 – 10 cm) had the highest population (630
wildlings/saplings ha-1) while those in the highest diameter class (>50 cm) had the lowest (25 trees
ha-1). Nevertheless, trees with diameter class >50 cm, had the highest basal area and dominated the
watershed vegetation. The increase in area sampled had a positive effect on the number of species
with additional taxa being encountered with increased sampling intensity. However, the species-
area curve (Figure 4) did not level off to a horizontal asymptote after the maximum number of
species (77) had been encountered. Hence, the slope of the curve did not approach zero (R2 = 0.98,
P < 0.0001).
The herb population was dominated by species such as Alternanthera sessilis, Ageratum
conyzoides, Phyllanthus amarus, while climbers were dominated by Alafia barteri, Chasmanthera

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
Akwarandu.
86
dependens, Quisqualis indica, and Saba thompsonii. In addition, the dominant shrubs included
Bridelia atroviridis, Rauvolfia vomitoria and Combretum micranthum.
Figure 2: Population of trees and canopy structure of Arinta watershed forest in Ipole Iloro,
Ekiti state, Nigeria .
Figure 3: Tree basal area and diameter class distribution of tree species at Arinta watershed
forest, Ipole Iloro, Ekiti State, Nigeria (bars represent number of trees; line represent basal
area)
63.9
34.8
1.1
0.2
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
<10m
10-20m
>20-30m
>30m
Proportion of tree population (%)
Tree canopy structure
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
0
100
200
300
400
500
600
700
1 - <10
10 - <20
20 - <30
30 - <40
40 - <50
>50
Basal Area (m
2
ha
-1
)
Number of trees (ha-1)
Dbh class distribution (cm)
Number of trees
Basal area

Environtropica - An International Journal of the Tropical Environment
87
Y = 105.1X
0.35
R
2
= 0.98, P< 0.0001
Area sampled (ha)
0.0 0.1 0.2 0.3 0.4 0.5
Number of species
0
20
40
60
80
100
Figure 4: Plant species-area curve at Arinta watershed forest, Ipole Iloro, Ekiti State, Nigeria
(taxa presence increased with sampling intensity).
Table 3: Tree growth variables and species diversity indices at Arinta watershed forest, Ipole
Iloro, Ekiti State, Nigeria.
Species
Basal
area
(m2)
Volume
(m3)
Relative
Frequency
(%)
Relative
density
(%)
Relative
dominance
(%)
IVI
H1
Albizia adianthifolia
1.58
23.23
4.12
5.57
6.05
15.74
0.16
Albizia gummifera
0.06
0.66
1.18
0.54
0.22
1.94
0.03
Alstonia boonei
0.25
3.70
2.94
0.90
0.94
4.78
0.04
Anthocleista djalonensis
0.09
0.90
2.94
1.62
0.33
4.88
0.07
Antiaris toxicaria
0.43
5.64
4.12
3.05
1.66
8.83
0.11
Artocarpus communis
0.60
8.08
1.18
1.44
2.28
4.90
0.06
Bombax buonopozense
0.19
2.59
1.76
1.26
0.74
3.76
0.06
Brachystegia eurycoma
0.01
0.07
0.59
0.54
0.06
1.18
0.03
Brachystegia kennedyi
0.02
0.21
0.59
0.18
0.07
0.84
0.01
Ceiba pentandra
11.16
357.26
0.59
0.18
42.77
43.54
0.01
Chrysophyllum albidum
0.003
0.02
0.59
0.18
0.01
0.78
0.01
Cola afzelii
0.01
0.08
1.18
0.90
0.06
2.13
0.04
Cola gigantea
0.11
1.10
1.18
1.08
0.41
2.66
0.05
Cordia millenii
1.02
24.55
5.88
4.49
3.92
14.29
0.14
Dialium guineense
0.85
8.27
4.71
9.52
3.25
17.48
0.22

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
Akwarandu.
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Diospyros mespiliformis
0.30
4.38
1.18
0.54
1.15
2.87
0.03
Distemonanthus
benthamianus
0.02
0.20
0.59
0.18
0.09
0.86
0.01
Dracaena arborea
0.07
0.79
0.59
0.72
0.26
1.57
0.04
Dracaena mannii
0.01
0.05
0.59
0.54
0.05
1.17
0.03
Elaeis guineensis
0.03
0.06
0.59
0.18
0.12
0.89
0.01
Entandrophragma
angolense
0.01
0.05
0.59
0.18
0.02
0.79
0.01
Erythrophleum suaveolens
0.04
0.47
0.59
0.36
0.14
1.09
0.02
Ficus exasperata
0.09
1.21
2.35
1.80
0.34
4.48
0.07
Funtumia elastica
0.48
5.80
3.53
3.59
1.84
8.96
0.12
Garcinia kola
0.06
0.50
0.59
0.18
0.21
0.98
0.01
Hollarrhena floribunda
1.25
15.46
5.88
8.26
4.77
18.91
0.21
Khaya ivorensis
0.24
4.05
0.59
0.54
0.92
2.04
0.03
Lecaniodiscus cupanioides
0.01
2.84
4.71
6.10
0.02
10.83
0.17
Massularia acuminata
0.06
0.38
0.59
2.87
0.23
3.69
0.10
Milicia excelsa
1.49
36.42
2.35
0.90
5.72
8.97
0.04
Musanga cecropioides
0.05
0.77
0.59
0.36
0.20
1.14
0.02
Myrianthus arboreus
0.20
3.11
2.35
0.90
0.78
4.03
0.04
Napoleonaea vogelii
0.06
0.40
3.53
3.41
0.23
7.17
0.12
Newbouldia laevis
0.02
0.13
1.18
0.72
0.06
1.96
0.04
Pentaclethra macrophylla
0.61
17.22
1.18
0.54
2.32
4.03
0.03
Piliostigma thonningii
0.37
4.34
4.71
3.23
1.41
9.34
0.11
Piptadeniastrum
africanum
1.02
16.70
4.71
3.77
3.90
12.38
0.12
Pterygota macrocarpa
1.01
11.04
4.12
10.41
3.87
18.40
0.24
Pycnanthus angolensis
0.15
1.71
2.35
2.51
0.58
5.45
0.09
Ricinodendron heudelotii
0.04
0.23
0.59
2.69
0.14
3.43
0.10
Senna siamea
0.02
0.15
0.59
0.18
0.07
0.84
0.01

Environtropica - An International Journal of the Tropical Environment
89
Spondias mombin
1.30
18.74
5.88
7.54
4.99
18.41
0.19
Sterculia rhinopetala
0.03
0.21
0.59
0.72
0.10
1.41
0.04
Sterculia tragacantha
0.01
0.11
0.59
0.18
0.05
0.82
0.01
Terminalia superba
0.04
0.56
0.59
0.18
0.14
0.91
0.01
Trema micrantha
0.30
6.65
2.94
1.44
1.16
5.53
0.06
Trema orientalis
0.18
2.88
1.76
0.90
0.70
3.36
0.04
Zanthoxylum
zanthoxyloides
0.17
2.55
2.94
1.97
0.65
5.57
0.08
Total
26.10
596.52
3.28
The tree species with the highest relative frequencies were Cordia millenii (5.88%), Hollarrhena
floribunda (5.88%) and Spondias mombin (5.88%) while the lowest were Brachystegia eurycoma
(0.59%), Brachystegia kennedyii (0.59%), Ceiba pentandra (0.59%), Chrysophyllum albidum
(0.59%), Distemonanthus bentamianus (0.59%), Dracaena arborea (0.59%), Dracaena mannii
(0.59%), Elaeis guineensis (0.59%), Entandrophragma angolensis (0.59%), Erythrophleum
suaveolens (0.59%), Garcinia kola (0.59%), Khaya ivorensis (0.59%), Massullaria acuminata
(0.59%), Musanga cecropioides (0.59%), Ricinodendron heudelotii (0.59%), Senna siamea
(0.59%), Sterculia rhinopetala (0.59%), Sterculia tragacantha (0.59%) and Terminalia superba
(0.59%). Ceiba pentandra had the highest basal area (11.16 m2), relative dominance (42.77%), total
volume (357.26 m3) and Importance Value Index (IVI: 43.54%). Hollarrhena floribunda was the
next species with a high IVI (18.91%), while Chrysophyllum albidum had the lowest IVI (0.78%).
The individual tree species with the highest number of stems were Pterygota macrocarpa (131 trees
ha-1), Dialium guineensis (120 trees ha-1) and Hollarrhena floribunda (104 trees ha-1). The
Shannon Wiener diversity Index (H1) was 3.28 while the species evenness (EH) was 0.85. Fabaceae
(14.13%), Apocynaceae (7.61%), Euphorbiaceae (7.61%) and Moraceae (4.35%) were the most
dominant families and had the highest species representation in the watershed forest (Figure 5).
Discussion
Plant species richness provides an insight to the structural and functional complexities in an
ecosystem. The species diversity gives an indication of the ecological gradients and the
environmental quality of forests (Gillespie et al., 2004; Olajuyigbe and Adaja, 2014). Hence, the
assessment of the floristic composition of sensitive ecosystems such as watersheds is essential for
the development of proper management, sustainability and conservation strategies (Cannon et al.,
1998; Addo-Fordjour et al., 2009). Unfortunately, very few studies have focused on the plant
species diversity and structure in Nigerian watersheds (Olaniyi et al., 2015; Asinwa et al., 2018).
The 92 plant species from 82 genera (Table 1) encountered in the Arinta watershed forest suggests a
relatively conserved ecosystem through the activities of the adjoining community (Ipole Iloro
town). This is because the people identify with the environmental, economic and social importance
of the watershed ecosystem.
The 48 tree species recorded indicate a high structural diversity in the watershed ecosystem
(Gillespie et al., 2004; Long and Shaw, 2009; Sambas and Siregar, 2017). Although, the tree
species richness in this study was higher, it was similar to the 36 tree species identified by Olaniyi

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
Akwarandu.
90
et al. (2015), in the same ecosystem. It was also similar to the tree species richness reported for the
Ogun river watershed forest (43) in Ogun state, Nigeria (Asinwa et al., 2018).
The high species richness observed is rare with many Nigerian watershed forests already degraded
through anthropogenic activities (Aju, 2017). For instance, Ogbemudia et al. (2013) reported only
43 plant species in a disturbed watershed forest in Akwa Ibom state, Nigeria. It has been suggested
that the vegetation at Arinta waterfall, protects the water source from erosion and landslide events.
Hence, the waterfall needs to be officially designated as a protected area despite the government’s
interest in ecotourism development (Ijasan and Izobo-Martins, 2012; Olaniyi et al., 2015).
The family dominance in the forest followed the pattern reported for other tropical rainforest
ecosystems in Nigeria. Previous studies on rainforests ecosystems in Nigeria have reported that
Fabaceae, Euphorbiaceae and Sterculiaceae are the most dominant families in such forests
(Adekunle et al., 2013; Olajuyigbe and Adaja, 2014; Olajuyigbe and Jeminiwa, 2018). Fabaceae is
the third largest plant family and also the most common in the tropical rainforest comprising trees,
shrubs, lianas and herb species (Burnham and Johnson, 2004; Mahbubur-Rahman and Ismot-Ara-
Parvin, 2014). The dominance and abundance of species from the Fabaceae family could also be
partly, attributed to the method of seed dispersal by species in this family. These plants use an
explosive mechanism for releasing seeds from their pods and these seeds are then wind dispersed to
great distances from their origin. To this end, seeds travel far from the mother tree and germinate
under favourable conditions in the rich luxuriant watershed ecosystem (Ihenyen et al., 2009).
The species-area curve gives an inference to ecosystem biological processes such as disturbances,
competition, and division of niches. In this study, the species-area curve which predicts extinctions
due to biotic collapse (loss of species due to reduction in habitat area), approached the asymptotic
level after 77 species had been encountered suggesting the maximum value for plant species
richness in the watershed forest (Palmer and White, 1994; Hambler and Canney, 2013; Olajuyigbe
and Adaja, 2014). Hence, the quantitative sampling of the vegetation was representative, with a
minimum area of 0.44 ha revealing the maximum representation of floristic similarity and species
diversity among sample plots. .
The diameter distribution of trees formed an upside down 'J' curve which is typical of tropical
rainforests undergoing dynamic changes. The abundance of trees in the lower diameter size
categories (630 wildings/saplings ha-1 in < 10cm and 500 trees ha-1 in >10 – 20 cm) indicates the
propensity for natural regeneration of the tree community (Olajuyigbe and Adaja, 2014; Sambas
and Siregar, 2017). On the other hand, trees in the >50 cm diameter size category dominated the
spatial distribution with basal area of 35.14 m2ha-1. The mean basal area (31.06 ± 6.30 m2 ha-1)
compared favourably with reported mean tree basal area (25 m2 ha-1) for fully stocked forests
(Alder and Abayomi, 1994). The emergent layer was absent in the vertical stratification of the
watershed forest with the tallest trees (>30 m) in the upper canopy representing only 0.2% of the
tree population while 1.1% of the trees were in the lower canopy (10 - 20 m) (Figure 2). This was
probably due to the steep topography which limits the ability of trees to grow without constraints in
watershed forests (Sambas and Siregar, 2017).
The Shannon Wiener diversity index (H1) has been used for characterizing community diversity in
tropical watershed forest ecosystems (Olaniyi et al., 2016; Asinwa et al., 2018). The H1 obtained
for Arinta watershed forest (3.28) was within the upper limit of 1.5–3.5 reported for Nigerian
rainforests (Adekunle and Olagoke, 2008; Olajuyigbe and Adaja, 2014). The species equitability
index (EH = 0.85) was also similar to values reported by Adekunle and Olagoke, (2008) in natural
forest (0.86) of Ondo state, Nigeria. This suggests that there is even distribution of species in the
watershed forest.

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Figure 5: Family dominance at Arinta watershed forest, Ipole Iloro, Ekiti state, Nigeria.

Floristic composition and stand structure in a tropical watershed forest: implications for biodiversity conservation. Olajuyigbe &
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The Importance Value Index (IVI) combines the attributes of relative density, frequency and
dominance (Table 3). It highlights the relative importance of a species to its community (Anning et
al., 2009). In this study, Ceiba pentandra, which is an indicator species in high humidity ecologies,
had the highest IVI (43.32%). This pioneer species has also been reported as a dominant species in
various types of forest including watershed and gallery ecosystems (Burnham and Johnson, 2004).
Although the local people were committing a lot of effort towards the protection and conservation
of Arinta watershed forest, there is need for government legislation that would ensure protection
and sustainable management of this sensitive ecosystem. Scientific studies such as the current
study would provide empirical information on vulnerable plant communities and the habitats that
require conservation (Wang et al., 2002; Sambas and Siregar, 2017). Arinta watershed forest can
serve as an in situ conservation area for many plant species that have already become locally rare or
scarce in degraded forests of Nigeria. Some of these species include; Afzelia africana, Antiaris
africana, Brachystegia spp., Cordia millenii, Dialium guineense, Entandrophragma spp., Khaya
ivorensis, Milicia excelsa, Pterygota macrocarpa, Sterculia setigera, Terminalia superba and
Triplochiton scleroxylon (Olajuyigbe and Adaja, 2014).
Watershed forests help to maintain hydrological cycles by absorbing excess precipitation and
reducing the speed of rainfall runoffs, while recharging aquifers, maintaining stream flows, filtering
and preventing pollutants from entering water bodies, which serve as habitat for aquatic life (Aju,
2017). They provide recreational and ecotourism opportunities for local communities while
ensuring that good quality water can be accessed by the people. Unfortunately, the ecosystem
services are usually undervalued and this beclouds the general understanding of people on these
essential life support services that are beneficial to man and the environment. Hence, there is need
for awareness creation on the invaluable quality of watershed ecosystems and why they need to be
conserved, especially with the increased impacts of the changing global climate.
Conclusion
Arinta watershed forest is a highly diverse ecosystem with high potential for biodiversity
conservation in Nigeria’s tropical rainforest. The conscious efforts of the Ipole Iloro community
coupled with the difficult terrain had ensured the preservation of this cultural heritage. There were
92 plant species with Fabaceae family dominating the population. The canopy structure lacked an
emergent layer, with Ceiba pentandra being the indicator species. Arinta watershed ecosystem had
created a refuge for many plant species that have become scarce in most Nigerian forests. However,
there is need for laws that would designate protection area status on the watershed vegetation
especially with increasing ecotourism activities in the area.
Acknowledgements
Authors wish to thank the traditional ruler and members of the ruling council as well as the
indigenes of Ipole Iloro, Ekiti state Nigeria, for their support during the field assessment. We are
also grateful to the Departments of Forestry and Tourism in the Ministry of Environment, Ekiti state
for granting access to the watershed forest. The comments and suggestions of the reviewers and
editorial team are deeply appreciated.
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