Floristic Structure of Fire Experimental Plots of Olokemeji Forest Reserve, Ogun State, Nigeria

Total enumeration of tree species was carried out fire experimental plot of Olokemeji forest reserve for relative importance value (RIV) mean heights, mean diameter at breast height and mean basal area. Three investigative plot (Plot A, B and C) were established. Plot A which represents Early Burn is burnt annually during the dry season while Plot B was burnt annually during the rainy season when the trees are still wet and Plot C serves as the protected area. Gmelina arborea had the highest relative importance value (28.31), this is followed by Dalbergia sissoo which had RIV of 10.94. Plot C recorded the highest frequency of trees with the highest tree heights, this is closely followed by Plot A and Plot B being the plot with the lowest mean height. Plot C has the highest mean diameter at breast height (DBH) of trees and it is followed by Plot B and Plot A with very close mean DBH. The highest mean basal area was also recorded in Plot C, the highest total basal area and tree volume was also recorded at Plot C as 32.2 m ha and 188.8 m. The size distribution of tree species depicts that Plot C has been protected just as expected. DOI: https://dx.doi.org/10.4314/jasem.v24i11.3 Copyright: Copyright © 2020 Taiwo et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 22 September 2020; Revised: 29 October 2020; Accepted: 04 November 2020

The floristic structure of a forest depicts the dynamics of its biological diversity. Burning has an impact on the structure of the forest. Prescribed burning is carried out in fire experimental plots for the purpose of fire tracing. This process helps to determine the response of the forest to forest fires and in addition, plants which are fire tolerant are discovered. Fire experimental plots have three investigative plots named as Plot A where early burning takes place, Plot C where late burning is carried out and Plot C which serves as the control plot and is protected over time. In addition to natural causes of ignition, prescribed burning was one of the management tools commonly used by man in the forest (Lloret and Marí 2001;Naveh, 1975). Fire thus acted as a fundamental factor on ecosystem functioning and as a major ecological driver of vegetation changes (Attiwill, 1994;Moreno and Oechel, 1994;Lavorel et al., 1998), by modeling landscapes (Gillson, 2009) and contributing to maintain habitat heterogeneity and biological diversity (Blondel et al., 2010;Moreira et al., 2001). Given the very strong effect of fire on vegetation, plant distribution and ecosystem function could likely be more heavily affected by the on-going modifications in fire regime consequent to global changes than by the direct effect of climate changes (Pausas and Fernández-Muñoz, 2012). Moreover, the combined impact of these fire regime changes and enlarged environmental limitations for post-fire tree recruitment could deeply affect the species composition of Mediterranean forests (Lloret et al., 2004(Lloret et al., , 2009Peñuelas and Filella, 2001). Natural fires cannot be controlled but anthropogenic fires can be subjected to control. Such controlled fires canbe used as a forest management technique. Prescribed burning has also been used to reduce potentially hazardous natural fuels and remove accumulated dead plant materials. The fire investigation plots at Olokemeji Forest Reserve, Nigeria were set up by Forestry Research Institute of Nigeria in 1929 to monitor the number of trees on each plot when fire is introduced at different times during the dry season and to be able to recommend appropriate time of burning of fields for shifting cultivators and to monitor the effects of prescribed periodic burning on long term vegetation development (Akinsoji and Sowemimo, 2005). The plots were clear-felled in 1929 and left to grow naturally and the prescribed burning was applied annually. Charter and Keay (1960) observed that late burn promoted the development of Savanna vegetation on Plot A while early dry season burn was changing the vegetation to forest type but at a rate slower than Plot C (the fire-protected plot). Forest dynamics can be enumerated through the understanding of its floristic composition, relative importance value of species, diameter class distribution, tree basal area and volume. The manipulation of stand basal area to achieve forest management goals can be as important as the use of prescribed fire or other vegetation treatments. The manipulation of stand basal area to achieve forest management goals can be as important as the use of prescribed fire or other vegetation treatments (Chen et al., 2007). The fire experimental plot of Olokemeji has gone through series of human interference since its establishment, most especially the prescribed burning as well as other anthropogenic impacts, hence, the purpose of this study is to evaluate the structure of the forest in order to understand the dynamics of the fire experimental plot.

MATERIALS AND METHODS
Study Site: Olokemeji forest reserve occupies a total land area of 58.88 km 2 .The reserve, which was established in 1899 is the second forest reserve in Nigeria. The forest reserve is situated between latitude 70 " 25 0 N 30 0 to 70 " 39 0 N and Longitude 30" 32 0 E to 30" 44 0 E. The site lies approximately 32km west of Ibadan, and 35km north-east of Abeokuta.
Olokemeji fire experimental plot was established in the year 1929 for the purpose of fire tracing. Three plots were established within the fire experimental plot and were named as Plot A (Early burn), Plot B (Late burn) and Plot C (Protected). The fire investigation plots are bordered by plantations of Gmelina arborea, Dalbergia sisso and Senna siamea. The physical features, climate and vegetation of the reserve have been described by Hopkins (1972). Sampling technique: Total tree standswith diameter at breast height (DBH) greater than or equal to 10 cm girth were enumerated for this study. Tree Species Identification: The botanical names of every stands encountered in each plot was recorded. Each tree was recorded fully in the field with extra effort made not to omit any eligible tree in a plot, this is to reduce the possibilities of any species omitted within the plot.
Data Analysis: The Relative Importance Values (RIV) of all species were determined following Olubode et al., 2011. It was computed as: Where: Frequency: It is the number of occurrence of a species in a set of quadrats or area; Relative Frequency = It is a relative value of occurrence of a species in a set of quadrats to total species in the quadrats.

Relative frequency = × 100
Density is the quantity of individual species to abundance of species per unit area.

Density =
Relative density was obtained using the formula; Relative Density = × 100 Where ni is the number of individual species; N is the total number of different species in the entire population Basal area Basal area (m 2 ha -1 ) = (Onyekachi and Johnson, 2018) Where π is 3.142 and d' is the diameter at breast height (cm) The total basal area for each of the sample plots was then obtained by summing the basal area of all trees in the plot while mean basal area for the plots was determined by dividing the total basal area by the number of trees in the sample plot.

Mean Height Mean Diameter at breast height and Mean Basal Area Of Fire Experimental Plots of
Olokemeji, Forest Reserve: Out of the three investigation plots in the fire experimental, Plot C has the highest mean height at 13.11 m, this is followed by Plot A at 12.32 m and Plot B has the lowest mean height at 11.69 m. Plot C has the highest mean diameter at breast height at 71.32 cm, this is followed by Plot B at 51.91 cm which is closely followed by Plot A 50.52 cm. Plot C has the highest mean basal area at 0.4m 2 ha -1 , followed by Plot B which has 0.21 m 2 ha -1 and Plot A has 0.2 m 2 ha -1 mean basal area.

Basal Area and Volume of Tree Stands in the Fire Experimental Plot of Olokemeji Forest Reserve, Ogun
State, Nigeria: The total basal area of each plots examined at the fire experimental plot revealed that Plot A had the least which is 11.2 m 2 ha -1 , this is closely followed by Plot B which has 13.68 m 2 ha -1 and Plot C having the largest basal area at 32.2 m 2 ha -1 .The total tree volume followed a similar order with Plot C having the largest tree volume at 188.8 m³ ha -1 , Plot B has 74.43 m³ ha -1 and Plot A has the least total tree volume at 55.84 m³ ha -1 The fire experimental plot of Olokemeji forest reserve comprise of diverse tree species, sixteen species of trees belonging to eight families, with family Fabaceae being the largest amongst all.   Most of the species found on the plots are fire resistant plants, this is in agreement with the findings of Akinsoji (2013). Plot B which represents late burning has the lowest mean height (11.69 m) among the three divisions. Plot A which represents early burning has a mean height (12.32 m) which is lower than the average height of trees expected to be found in a tropical forest. Plot C has the highest mean height at 13.11m which is also lower than the height of trees expected in a tropical rainforest. This could be attributed to the regeneration and regrowth of the tree species after a major loss of the forest (Ogunwale, 2015). The horizontal and vertical structures of the forest as revealed by the mean diameter at breast height and mean height show a forest that is growing, ensuring it's stability. The highest mean diameter at breast height was found in the protected plot, Plot C at 71.32 cm, this is followed by Plot B at 51.91 cm which is closely followed by Plot A 50.52 cm, this reveals the occurrence of matured trees in the plots with DBH greater than 40 cm. This is also in agreement with the findings of Akinsoji (2013). In addition, the diameter distribution curve, which depicted a reverse J-shape indicate an healthy and stable ecological structure and population. Plot C has the highest mean basal area at 0.4 m 2 ha -1 , followed by Plot B which has 0.21 m 2 ha -1 and Plot A has 0.2 m 2 ha -1 mean basal area. The total basal area for the three plots followed the same order of increase with Plot C having the highest total basal area at 32.2 m 2 ha -1 , Plot B has 13.68 m 2 ha -1 while Plot A had the lowest total basal area at 11.2 m 2 ha -1 . The total volume of tree stands in each plots followed a similar order with Plot C having the highest tree volume at 188 m³ ha -1 , this is followed by Plot B with 74.43 m³ ha -1 and Plot A with the least tree volume of 55.84 m³ ha -1 . These revealed that Plot C has actually been protected just as expected and it is almost attaining the status of a mature forest, Plot A is gradually becoming a savannah forest as a result of the annual early burning and Plot B has changed into a transitional woodland, this is in agreement with the findings of Akinsoji (2013) and Addo-Fordjour et al., 2009;Anning et al., 2009.

Conclusion:
The floristic structure of Olokemeji fire experimental plot has proofed without doubt that prescribed burning can serve as a management tool for maintaining stability in the forest ecosystem, however, the fire experimental plot needs to be further protected to prevent the loss of the tree stands which could invariably hinder the plots from serving their individual purposes.