Floristic Composition and Structural Diversity of Shasha Forest Reserve in Ile-Ife, Southwestern Nigeria

The floristic composition and structural diversity were studied in two plots, 25 m x 25 m each, in three different sites of varying vegetation physiognomy: Taungya system, Regrowth forest and Gmelina arborea plantation (TS, RF and GA respectively) of Shasha forest reserve in Ile- Ife southwestern Nigeria. A total of 119 plant species belonging to 51 families and 100 genera were identified in the forest reserve. Woody species represented the most diverse life form. Plant species diversity was higher in the GA (H ' =3.5) compared to the RF (H ' =3.4) and TS (H ' =2.9). Woody plant species density also differed significantly (p < 0.05) among the different physiognomy. Mean basal area and mean girth size were higher in RF compared to TS and GA. Species evenness was also quantitatively higher in the TS (E=0.12) compared to RF (E=0.09) and GA (E=0.08). Sorensen index of similarity were 12.12% (TS and RF), 19.71% (TS and GA) and 20.20% (RF and GA), which is an indication of the heterogeneity of the three different sites, as a result of different management systems of the sites. The knowledge about species composition in the forest reserve will go a long way in identifying important elements of plant diversity, protecting and preserving threatened plant species, monitoring and providing effective management of the forest reserve.


Introduction
Worldwide the degradation, fragmentation and conversion of forest ecosystems is progressing rapidly (Abramovitz, 1998). Globally, concerns are raised over the rapid loss of biodiversity in all its forms and at all levels. Habitat destruction is the main cause of the biodiversity loss. Habitats can either disappear completely or they may be degraded and/or fragmented, both causing serious impacts on species development, as well as lack of balance between ecosystems' processes (Raghubanshi and Tripathi, 2009). Presently, many forests persist as forest fragments and there is a growing interest in quantifying habitat characteristics such as forest structure, floristic composition and plant species richness in intact and degraded forest fragments and forest landscapes (Bierregaard et al., 1992;Myers et al., 2000). Knowledge of the floristic composition and structure of forest reserves is critical to understanding the greater dynamics of forest ecosystems and for identifying important elements of plant diversity, protecting threatened or economic species and monitoring the state of reserves, and it is with this data that management practices can be applied. The effective management of such ecosystems requires the understanding of their functioning, not only for their improvement, but also to arrest their further degradation (Oke and Isichei, 1997). Thus, the study of floristic composition and structure of tropical forest becomes more imperative in the face of ever increasing threat to the forest ecosystem.
Studies have shown that composition and structure of forests are influenced by a number of factors (Klinge et al., 1995;Haugaasen et al., 2003;Wittmann and Junk, 2003). One of these factors are disturbances which cause local species variation within forests based on their intensity, scale and frequency (Hill and Curran, 2003;Laidlaw et al., 2007). Disturbance regimes dominated by natural and anthropogenic factors may alter composition, diversity and structure of the forest. There have been massive deforestation and forest degradation of forest reserves in Nigeria as a result of human activities and inadequate or lack of effective management of the reserves. This endangers the forest reserves and the services (socio-economic and ecological) they render.
The Shasha forest reserve was one of the forest reserves established in Nigeria, generally believed to be ecologically rich and biologically abundant with forest tree species. Shasha forest reserve was originally created in 1925 and has witnessed a rapid rate of destruction from excessive logging, conversion to plantations and farming (Field Trip Earth, 2008). Deforestation is widespread, leaving no section untouched (Salami et al., 2007). For the conservation status of the Shasha forest reserve to be known and to allow effective management of the forest reserve there is a need for proper documentation of its plant species. Knowledge of the current floristic composition and structure of the Shasha forest reserve is thus invaluable. Floristic data obtained in this regard would be useful for the application of sound management practices in the forest. This study was therefore carried out to determine the composition, structure and assess the soil status of the forest reserve.

Study area
The study was carried out in the Shasha forest reserve in Ife south Local government Area of Osun state, southwestern Nigeria (Fig. 1). The forest reserve lies between latitude 7°8' and 7°10' N and longitude 4°20' and 4°40' E. The study site has a land area of 310798 km 2 /31079.85 ha (Salami et al., 2007). The vegetation is part of the tropical rainforest ecosystem in Southwest Nigeria. The altitude of the forest is 122 m, with a mean annual rainfall of 1421 mm (Adekunle, 2006). It is an area with high relative humidity. There are two prominent seasons: dry and rainy season. The dry season lasts from November to March, while the rainy season is from April to October. The soil of the site is ferruginous tropical soil on crystalline acid rock, the topography gently undulating plain. The study was specifically carried out in three distinct study sites, namely a Taungya system (TS), cultivated site consisting of arable crops and woody species-a natural Regrowth forest (RF) and a mono-culture plantation of Gmelina arborea (GA), which were all selected from the forest reserve based on their physiognomy. The reserve has witnessed some form of disturbance such as logging and farming activities.

Data collection and analysis
The data was collected in March (dry season month). Two sample plots of 25 m x 25 m in each being identified vegetation physiognomy (sites TS, RF and GA). The sites were laid out using a measuring tape and demarcated with wooden pegs for the study. In all three sites, all woody plants that were greater than (or about) one meter in height were enumerated and identified to species level. The diameter at breast height (dbh) of all the identified woody species was measured. The identification followed the Flora of West Africa (Hutchinson andDalziel, 1954-1972). The species whose identities were in doubt were collected and taken to IFE herbarium where proper identification was carried out. Floristic composition, densities, diversity and distribution of the plant species were determined using the following parameters: species richness, diversity indices, Shannon -Wiener index, Sorenson's index and species evenness (E).
The diversity index H' was calculated using the method prescribed by Shannon and Wiener index (1963) as: The Sorenson index of similarity (ISs) was calculated for each pair wise plot comparison (Blanc et al., 2000) and this was used to generate a dendrogram showing floristic similarities.

Results and discussion
Floristic composition A total of 119 plant species were identified in the three selected sites in the Shasha forest reserve. These belonged to 51 families and 100 genera (Tab. 1). Euphorbiaceae, Moraceae, Papilonaceae and Rubiaceae were the overall diverse families (in terms of species richness) of the adult species, contributing 28.5% of all the species in the study (Fig.  2). Trees (39.5%) were the most dominant life forms, followed by herbs (22.7%), shrubs (21.8%) climbers (12.6%) and grasses (3.4%) (Fig. 3). Euphorbiaceae and Moraceae were the most diverse families in the TS, whereas Euphorbiaceae and Rubiaceae constituted the most diverse families in RF. The most important family in GA was Rubiaceae (Fig. 4). Generally, 26 woody species were encountered in TS, 41 woody species in RF and 42 woody species in GA. Woody species common to the three sites include Deinbollia pinnata, Ficus exasperata, Milicia excelsa, Rauvolfia vomitora, Rinorea dentata and Terminalia superba. More herbaceous species were recorded in TS (14) than in the GA (12) and RF (7) sites, whereas climber species richness was greater in the GA (8) in relation to the other vegetation physiognomy TS (7) and RF (4). The herbaceous species common to the three sites is Chromolaena odorata. Some of the climber species observed in the three sites includes Acacia atazacantha, Cissus spp Combretum spp. Grass species were absent in RF, but present in both TS and GS and these include gayanus, Eragrostis spp, Oplismenus burmanii barbata. Similarity levels were depicted in a dendrogram (Fig. 5). Some plots from different forest types were floristically different. TS1 was floristically diff other plots. TS2 and GA2 were more similar to each other, more than GA1 and RF2, while RF1 was more similar to TS2 and GA2, more than RF2 and GA.

Structure
There were a total of 2628/ha individuals of woody species (excluding other life forms) identified in the three different physiognomies. Woody species density was highest in GA (1192/ha) followed by RF (1092/ha) and least in TS (344/ha) (Tab. 2). Shannon-Wiener index was higher in the GA (Hʹ=3.5) compared to the RF (H (Hʹ=2.9) (Tab. 3). Density of woody species differed significantly between the forest types (p < 0.05). In all, Gmelina arborea and Terminalia. superba  and Oke O.S./ Not Sci Biol, 2014, 6(4):433-440 Family dominance of plant species in the three study Fig. 3 In terms of size, majority of the trees were of diameter class (0-20 cm) (Fig. 4). The number of individual trees in the categories decreased with increasing size of the trees. Larger diameter trees (< 100 cm) were very low in TS and GA, but not found in RF. Mean basal area recorded in the RF (5.8±2.1 m²/ha) was higher compared with that of GA (2.5±0.2 m²/ha) and TS (0.1±0.05 m²/ha). The result of species evenness showed that evenness was quantitatively higher in TS (E =0.1) compared with RF (E =0.07) and GA (E = 0.08) (Tab. 3).
More herbaceous species were recorded in TS (14) than in the GA (12) and RF (7) sites, whereas climber species richness was greater in the GA (8) in relation to the other and RF (4). The herbaceous species common to the three sites is . Some of the climber species observed Acacia atazacantha, Cissus spp and . Grass species were absent in RF, but and GS and these include Andropogon gayanus, Eragrostis spp, Oplismenus burmanii and Setaria Similarity levels were depicted in a dendrogram (Fig. 5). Some plots from different forest types were floristically different. TS1 was floristically different from other plots. TS2 and GA2 were more similar to each other, more than GA1 and RF2, while RF1 was more similar to There were a total of 2628/ha individuals of woody life forms) identified in the three different physiognomies. Woody species density was highest in GA (1192/ha) followed by RF (1092/ha) and least in TS Wiener index was higher in =3.5) compared to the RF (Hʹ=3.4) and TS =2.9) (Tab. 3). Density of woody species differed significantly between the forest types (p < 0.05). In all, were the most In terms of size, majority of the trees were of the smaller 20 cm) (Fig. 4). The number of individual trees in the categories decreased with increasing size of the trees. Larger diameter trees (< 100 cm) were very low in TS and GA, but not found in RF. Mean basal area recorded in RF (5.8±2.1 m²/ha) was higher compared with that of GA (2.5±0.2 m²/ha) and TS (0.1±0.05 m²/ha). The result of species evenness showed that evenness was quantitatively higher in TS (E =0.1) compared with RF (E =0.07) and GA Dendrogram constructed from similarity Sorenson Index matrix based on species composition in the various plots

Discussion
Studies on floristic composition and structure in forests are instrumental in the sustainability of forest since they play a major role in the conservation of plant species and the management of forest ecosystems as a whole (Mohandass al., 2009;Tilman, 1988;Ssegawa and Nkuutu, 2006). The results of this study show that the floristic composition of Shasha forest reserve (119 species/ha) were higher than many other tropical forests. For instance, Addo-(2009) recorded a much lower species species/ha) in a moist semi-deciduous forest in Ghana, Mohandas and Priya (2009) recorded 86 species in a tropical montane evergreen forest in India. On the other hand, Gustav et al. (2001) recorded a much higher species richness of 208 species in the tropical wet lowland of Amazon forest, Parthasarathy (2001) recorded 125 species in a tropical wet evergreen forest in Sengaltheri of the Western Ghats in India. The species composition of the forest reserve might have been affected of the various human disturbances, such as slash and burn agriculture, felling of mature trees for timber, collection of fuel wood and other non-timber forest products, which has led to species attrition, thus leaving behind a less balanced structure.
The density of the woody and herbaceous species varied considerably in the different sites under consideration. There were more herbaceous species in TS than all other sites; this might be an indicator of the type of anthropogenic disturbances (slash and burn agriculture) in TS, facts which might favour the growth of herbaceous species. Mishra (2008) observed greater diversity of herbs in disturbed forest than in undisturbed ones. There were more woody species in GA than RF; this observation could be as presence of woody species and tree saplings in the under storey of the plantation. Studies from around the world have shown that single-species plantations are at the stand scale, often less diverse than natural or semi-natural forests wit respect to plants (Aubin et al., 2008). Nevertheless, it has been shown that forest plantations can contribute to restoring some of the floristic diversity on abandoned agriculture land (Newmaster et al., 2006; Aubin and some plantations may have a surprisingly diverse understory (Allen et al., 1995;Keenan Oberhauser, 1997) as observed in this study. This is in agreement with Tripathi and Bajrang (2009)  Number of climbers 7 Density of trees ha -1 8 Density of woody species ha -1 9 Basal area m 2 ha -1 10 Mean girth size (cm) 11 Shannon wiener Hʹ 12 Species evenness TS -Taungya system RF -Regrowth forest GA -Gmelina arborea plantation Tab. 3. Summary characteristics of floristic composition and structure of the Shasha forest reserve D.S. and Oke O.S./ Not Sci Biol, 2014, 6(4):433-440 tudies on floristic composition and structure in forests are instrumental in the sustainability of forest since they play a major role in the conservation of plant species and the management of forest ecosystems as a whole (Mohandass et 1988;Ssegawa and Nkuutu, 2006). The results of this study show that the floristic composition of Shasha forest reserve (119 species/ha) were higher than many Fordjour et al. The species composition of the forest reserve might have been human disturbances, such as slash and burn agriculture, felling of mature trees for timber, collection timber forest products, which has led to species attrition, thus leaving behind a less balanced density of the woody and herbaceous species varied considerably in the different sites under consideration. There were more herbaceous species in TS than all other sites; this might be an indicator of the type of anthropogenic agriculture) in TS, facts which might favour the growth of herbaceous species. Mishra et al. (2008) observed greater diversity of herbs in disturbed forest than in undisturbed ones. There were more woody species in GA than RF; this observation could be as a result of the presence of woody species and tree saplings in the under storey of the plantation. Studies from around the world have species plantations are at the stand scale, natural forests with ., 2008). Nevertheless, it has been shown that forest plantations can contribute to restoring some of the floristic diversity on abandoned ., 2006; Aubin et al., 2008) y have a surprisingly diverse ., 1995; Keenan et al., 1997;Oberhauser, 1997) as observed in this study. This is in agreement with Tripathi and Bajrang (2009) who observed that species richness at ground layer was higher in forest plantation than in natural re-growth forest. It has been observed that pure stands (i.e. deciduous or coniferous) could support, in some cases, a richer understory vegetation than mixed-species stands and that species richness was generally greater in deciduous stands than in coniferous stands (Barbier et al., 2008). Furthermore, some conifer plantations may have greater plant species richness than broad forests (Nagaike, 2002 Grass species were present in both TS and GA, but not in RF. This observation could be as a result of open canopy in the plantations versus closed canopy in Regrowth forest. The observations in this study regarding the re agreement with the results of White (1985) who noted that in a forest the ground layer is often sparse or absent, as grasses are absent or if present are localized or inconspicuous. The low similarity index observed among the three study sites is an indication of the heterogeneity in species composition in the standing vegetation of the three sites. Species similarity indices were 12.12 % (TS and RF), 19.71% (TS and GA) and 20.20% (RF and GA). Similarity index showed that similarity is highest between Regrowth forest and plantation and lowest between Taungya Regrowth forest. This may be due to the different system of management in the three sites. TS is a vegetation of tree species and arable crop species, while RF is a natural re growth forest and GA is a mono -culture plantation of Gmelina arborea. Shannon-Wiener diversity index (H followed the order Gmelina arborea > Regrowth forest > Fig. 5. Girth size distribution of trees that species richness at ground layer was higher in forest growth forest. It has been observed that pure stands (i.e. deciduous or coniferous) could support, in some cases, a richer understory vegetation than species stands and that species richness was generally uous stands than in coniferous stands (Barbier ., 2008 Grass species were present in both TS and GA, but not in This observation could be as a result of open canopy in the plantations versus closed canopy in Regrowth forest. The observations in this study regarding the re-growth forest is in agreement with the results of White (1985) who noted that round layer is often sparse or absent, as grasses are absent or if present are localized or inconspicuous. The low similarity index observed among the three study sites is an indication of the heterogeneity in species composition in the of the three sites. Species similarity indices were 12.12 % (TS and RF), 19.71% (TS and GA) and 20.20% (RF and GA). Similarity index showed that similarity is highest between Regrowth forest and Gmelina arborea Taungya system and Regrowth forest. This may be due to the different system of management in the three sites. TS is a vegetation of tree species and arable crop species, while RF is a natural reculture plantation of Wiener diversity index (Hʹ) > Regrowth forest > Taungya system. The Hʹ of 3.34 -3.66 for some rainforest sites in Nigeria (Adekunle, 2006) is similar to that of RF and GA, but higher than that of TS. The high species diversity of RF (3.47) and GA (3.49) is a reflection of the presence of high number of species found in these sites. Our results revealed that species evenness was low in all the three study sites. This might be due to the different disturbance in form of human activities such as slash and burn agriculture and gathering of wood for fuel in TS, logging in RF and GA where certain species are preferred than others. In rainforests from India, tree species richness decreased with the increase in intensity of forest disturbance (Nath et al., 2005). Tree size class distribution can be used as indicators of changes in population structure and species composition (Newbery and Gartlan, 1996). The distribution of the girth size class has shown that TS, RF and GA were characterized by small and young tree species whose girths were mostly 0-20 cm, 21-40 cm and 41-60 cm. Most species in the study plots followed reverse J-shaped distribution with greater number of individuals in small size classes. Such a trend has also been reported in the forests of Great Andaman groups (Padalia et al., 2004). This might be as a result of selective felling and logging of higher girth size classes' trees, hence most of the tree species are secondary forest re-growth species. It also indicates that the forest sites are disturbed and they are in their early successional stages. Moreover, the basal area of the woody species (8×10 -6 -4.5524 m 2 ) further shows that the sites were characterized by recovery from disturbance. The general small basal area of most species, respectively small girth size, is an evidence of disturbance and degradation in the three study sites of Shasha forest reserve.

Conclusion
The study revealed that Shasha forest reserve has a reasonably good tree and shrub species composition and richness in the face of logging and slash and burn agriculture in the forest. Anthropogenic disturbances have affected the floristic composition of the forest reserve to an extent. Logging affected the structural composition of the forest reserve through the removal of large and tall trees. Thus, there is need to control human activities in the forest reserve so as to protect the plant species for effective management and utilization.