DOI: 10.5061/dryad.zgmsbccg3
title: Floristic composition, structure and diversity of riparian forests in southwestern Nigeria: Conservation is inevitable

description: The Nigerian riparian forest ecosystems had declined in extent and
 distribution and this had been attributed mainly to land use change. This
 study intended to provide an understanding of the links between plant
 diversity, composition, structures, and disturbances both anthropogenic
 and natural processes inducing the vegetation dynamics. Nine study sites
 were used for this study, within each site, five (5) plots (0.25 ha in
 size) were marked out and placed systematically at an interval of 10 m
 along the transect. A complete enumeration of plant species was carried
 out and identified at the species level. Diversity indices and structural
 parameters were determined and anthropogenic activities were ranked. A
 total number of 233 plant species were identified, belonging to 80
 families; out of which, Euphorbiaceae and Apocynaceae were dominant
 families The density and basal area ranged from 2,200-6,000 ha-1 and
 2.59-17.58 m2 ha-1 respectively across the study sites. Pterocarpus
 santalinoides, Alchornea cordiflora, Chassalia kolly, Tetracera spp,
 Fimbristylis, Bambusa vulgaris and Cyrtosperma senegalense were the
 dominant species. The Shannon diversity index ranged from (1.38-3.49),
 Simpson (0.66-0.97), and Evenness diversity (0.43-0.84). Fisher alpha
 (10.03-30.21) and Whittaker beta diversity (0.36-0.89) values were highest
 in Ipetumodu (site VIII) and lowest in Ilesha (site II). Seventy-three
 (73%) of the species in this study had a low important value index (IVI).
 The dominance of some lianas and herbaceous species in the riparian forest
 sites showed disturbances, stages of ecological succession, and
 regeneration of the vegetation. Conservation is inevitable towards
 maintaining and protecting species diversity, ecosystem roles, and
 services of these forests in Nigeria.
description: <strong>Description of the study area</strong>: The
 study was carried out in Osun State located in Southwestern Nigeria. The
 state lies within latitude 7° 30′ N and longitudes 4° 30′ E. Ilesha West,
 Atakumosa West, Ife North, Ife North East, Ife South, and Ayedade were the
 Local Government Areas, where the sites are situated within Osun State,
 Southwestern Nigeria. Nine riparian forest sites (Ifetedo, Ilesha, Osu,
 Famia, Ibodi, Tonkere Gbongan, Edunabon, and Ipetumodu) were selected from
 these Local Government areas and designated as site I-IX respectively.
 These areas represented riparian zones with flooding patterns of
 Southwestern Nigeria and have high plant species diversity, despite
 varying ongoing anthropogenic activities (Figure 1, Table 1). The detailed
 climate, soil, and vegetation of the study area in Southwestern, Nigeria
 have been described (Borisade 2020, Borisade and Odiwe2021).
 <strong>Data
 Collection</strong> Vegetation
 Sampling and Data Collection for Plant Species
 <strong>Woody species assessment</strong>: In each
 community, forest stands within the selected sites were sampled on a
 transect, 150 m long extended perpendicular to and across the
 streams/river. Nine sites were used for this study, within each site, five
 plots (0.25 ha in size) were marked out and each plot was placed
 systematically at an interval of 10 m along the transect, thereby giving a
 total of 45 plots across the study sites. Each plot was demarcated by
 narrow-cut thin lines to mark the boundary of the plot. Stem diameters
 were measured using a diameter tape at breast height (1.3 m). For
 individuals with buttresses or other stem irregularities at breast height,
 the diameter was measured above the buttresses, following usual forestry
 procedures. For shrub and lianas encountered, stems were counted, the
 diameter of three “average” stems from individual species was measured and
 then the composite (stem diameter (dbh)) of the shrub and lianas were
 calculated to enable the computation of its basal area in the manner used
 for the trees (Burton et al. 2005). For density calculations, each
 individual was considered as one, independent of its number of
 branches. <strong>Herbaceous species
 assessment</strong>: To assess the species composition, density, and
 cover of herbaceous species mainly forbs, and other understory forms such
 as grasses, sedges, and ferns. In each established sampling plot (0.25
 ha), five 1m x 1m quadrats were randomly laid. In each quadrat, all rooted
 plant species were identified and counted. Furthermore, five 10 m line
 transects were randomly laid using a measuring tape. A single cover pin
 was perpendicularly dropped at every meter point along the transect. Any
 plant species part or base touched was noted to estimate the aerial and
 basal cover of the species in the plot. The percentage cover was
 calculated as the number of ‘hits’ per species divided by the total number
 of pins dropped multiplied by 100. Plants were
 identified to species level and unknown species were cross-checked using
 IFE herbarium, Flora of West Tropical Africa by Hutchinson and Dalziel
 (1954), Handbook of West African Weeds (Akobundu and Agyakwa1998), Trees
 of Nigeria (Keay1989), Plant lists and PROTA. Voucher specimens of the
 different species were collected, dried, and deposited in the IFE
 herbarium, Department of Botany, Obafemi Awolowo University, Ile-Ife,
 Nigeria. <strong>Disturbance
 ranking</strong>: Indication of anthropogenic activities in the
 riparian forest was made within the study sites and disturbance scores
 were obtained for each site using the method described by Mani and
 Parthasarathy (2006) which was modified. Disturbance scores were given to
 each site by qualitatively evaluating various disturbances (tree logging,
 mining, grazing, flooding occurrence, agriculture, and settlement) ranked
 into low (1), occasional (2), and frequent (3) levels of disturbances. The
 summation of all the scores within each site was considered and sites with
 high values were ranked as high disturbance sites while those with low
 values were as low disturbance sites.
 <strong>Data and Statistical Analysis: </strong>The
 data from the complete enumeration of woody species were used to determine
 stem densities per hectare for different life forms (trees, shrubs, and
 climbers) and this was calculated using the number of individuals divided
 by sample areas. The data were used to establish floristic composition in
 terms of species, genera, and families. The important
 quantitative analysis such as density, frequency, and abundance of tree
 species, shrubs, climbers, and forbs species was determined as described
 by Curtis and McIntosh (1950). The basal area for woody species was also
 determined. Density: The
 density of each species, D<sub>i</sub>, was calculated as the
 number of individuals in a unit area: D<sub>i</sub> =
 n<sub>i</sub> / A where D<sub>i</sub> is the
 density for species i; n is the total number of individuals counted for
 species i, and A is the total area sampled. Frequency:
 The frequency for each species was calculated as:
 f<sub>i</sub> = j<sub>i</sub> / K where
 f<sub>i</sub> is the frequency of species i,
 j<sub>i</sub> is the number of samples taken and k is the
 number of occurrences. Basal area:
 Basal area which indicated the importance (dominance) of tree and shrub
 cover (Carratti et al.2004), was determined as: Basal area
 (m<sup>2</sup>ha<sup>-1</sup>) =
 C<sup>2</sup> / 4π where: C = girth size (diameter at breast
 height), π = 22/7 = 3.14. The basal area of each
 species was determined by adding the basal area of individuals of the
 species and the plot basal area was calculated by adding basal areas of
 all species in each plot while the site basal area was calculated as the
 mean woody species basal of all the sample plots.
 Importance Value
 Index: This index was used to determine the overall
 importance of each species in the community structure.
 Relative Dominance = Basal area per family/ Total Basal
 area×100 Relative Frequency = Frequency of each
 species/ Sum of frequency values of all species×100
  Relative Density = Density of each species/ Density of all
 species ×100 The three relative values were added
 together to obtain Importance Values (IV) for each species.
 Diversity
 Indices: The total number of species recorded in the sampling
 plots (species richness), rarefaction measure as well as Shannon-Wiener
 diversity Index (H’), dominance and beta diversity, and species evenness
 were employed to quantify and characterise species diversity and
 species-abundance distributions of the plant communities.
 Alpha species diversity was calculated for each site as
 Shannon-Weiner diversity, described by (Magurran2004), Dominance was
 evaluated using Simpson’s index of dominance (1949); Beta diversity was
 determined using Whittaker’s (Whittaker 1972); Rarefaction diversity
 (E(Sn)) was computed from the floristic data in order to compare species
 numbers from samples of different sizes among the community types (Hsieh
 and Li 1998). The Species evenness was calculated using Pielou Index (J)
 (1977). The confidence interval of structural
 parameters such as density and basal area was set at 95%. The disturbance
 scores, species density, and diversity indices (Shannon-weiner diversity,
 species evenness, and dominance) were subjected to Correspondence Analysis
 (CA) and the overall influence of these parameters on the riparian
 vegetation sites to Detrended Correspondence Analysis (DCA) using
 PAleontological STatistics (PAST) version 3.17 software (Hammer et al.
 2001). 
description: PAleontological STatistics (PAST) version 3.17 software (Hammer
 et al. 2001).