Tropical forests are the habitat of numerous species of living things that constitute biodiversity through webs of life (Shiferaw, Lemenih, & Gole, 2018). The natural cover of Sudan is generally poor, sparse, and scanty vegetation due to its location in such arid environment of the country, except along the Nile banks and watercourses where ephemeral herbs and grasses occur after the short rainy seasons (Elsiddig et al., 2007; Elsiddig et al., 2011). The reserved forests consist of around 837 forests distributed all over the country (Badi, 2004). They constitute a great potential for biodiversity conservation and play a vital role as an important component of natural resources and land use. These forests, including woodland savanna, are believed to be home to more than 500 unique species of trees and 185 species of shrubs based on expert opinions. Still, this information has not yet been verified by field surveys (Mukhtar & El Wakeel, 2002). This biodiversity is increasing the importance of conservation in the production of new, highly resistant clones and genotypes. The significance of the relationship between human beings and the forest reserves has been increasing as a result of the conflict of tremendous increase of population growth and crucial pressure on natural resources. Over the years, the forest has been a continuous source of wood, charcoal, and land for agricultural purposes that have led to present depletion (Asifat et al., 2019; Kunwar & Sharma, 2004). In buttressing this view, (Armenteras et al., 2009) reported that trees have provided many ecosystem services such as species conservation, prevention of soil erosion, and preservation of habitat for plants and animals. However, overexploitation of floristic composition has resulted in the rapid loss of tree diversity, which has been recognized as a major environmental and economic threat around the world (Mani & Parthasarathy, 2006). In forest management operations, inventories on biodiversity are used to determine the nature and distribution of biodiversity region in the region under management. The quantification of tree species diversity is an essential aspect as it provides resources for many species (Suratman, 2012). Being a dominant life form, trees are easy to locate precisely and to count (Condit et al., 1996; Adesina, 1997) and are also relatively better known taxonomically (Gentry, 1992). Therefore, information on the composition, diversity of tree species, and species-rich communities are of primary importance in the planning and implementation of biodiversity conservation efforts. This research aimed to evaluate tree diversity and abundance in Rashad Natural Reserved Forest, South Kordofan, Sudan, and to assess the distribution pattern of tree species.
2. Materials and Methods
2.1. Description of Study Area
This study was conducted in Rashad Natural Reserved Forest, South Kordofan, Sudan, which is located between (Latitudes 10˚, 13˚ North and longitudes 29˚, 33˚ East). It covers an area of about 7872 Km2, mostly useful for agriculture and grazing activities (Figure 1). The weather in the Rashad Natural Reserved Forest is characterized by continuous high temperatures ranging from 20˚C at night and 35˚C in the day. About 400 mm is the annual rainfall ranges in the northern parts and 676 mm in the southern parts of the locality (Daldoum & Massaud, 2018). The soils belong to the ferruginous tropical red soil associated with basement rocks.
The vegetation in the study area varies with pattern of rainfall. Mean annual rainfall is 372 mm and has poor to moderate vegetation, Tree species such as Acacia and short grasses and shrubs (El Ghazali, 1993). The mean annual rainfall is 712 mm, with the denser vegetation of Acacia and other trees (Pantuliano, 2005).
2.2. Data Collection
Eight plots of 20 × 20 m each were demarcated for identification and accurate physical measurement. The collected data in each sample included identification and counting of the number of trees, shrubs, trees, and shrubs diameter (cm) and height (m). All the woody species found in each plot were counted. The representative samples, such as leaves, fruits, and bark of trees that could not be identified in the field, were collected and taken to the herbarium for identification. Social data were collected through surveys and interviewing 150 individuals of the local inhabitants for information about focusing on species degradation, vegetal composition.
2.3. Data Analysis
The data were analyzed by using Microsoft excels sheet, and used the following equations:
1) Basal area: the basal area of all trees in the sample plots were calculated using the following formula:
where d = tree diameter, and π = 3.142 (constant).
2) Species relative dominance (RDo) was computed using:
Bai = basal area of individual trees belonging to species i, Ban = stand basal area of all species.
3) Relative density (%) of each species was computed using the equation of (Brashears et al., 2004).
Figure 1. Map of Study area, Source by Author.
This was used to express the share of each species in the tree community ni = number of individual species; N = the Total number of species in the entire community.
4) Importance Value Index (IVI): This is the sum of RD and RDo divided by 2 that gave importance value index for each species.
Species diversity index was calculated using the Shannon-Wiener diversity index (Kent & Coker, 1992).
H' = Shannon-Wiener diversity index; ∞ = total number of species in the community; Pn = proportion of S made up of the ith species; ln = natural logarithm.
Shannon’s maximum diversity index was calculated using the formula; Hmax = ln(s)
5) Species richness or variety index (d) was determined by the formula
where S = number of species, N = number of individuals of all species.
3. Results and Discussion
3.1. Species Diversity
A total of 237 tree stands were encountered within the eight randomly sampled plots. Fifty-six different tree species distributed within 22 families were found within the reserve. Family Fabaceae had the highest number of 13 species in 8 genera, followed by Combretaceae with 8 species belonging to 3 different genera, Family Malvaceae with 5 species belonging to 4 different genera, Family Apocynaceae with 3 species belonging to 3 different genera. Families Arecaceae, Burseraceae, Capparaceae, Euphorbiaceae, Meliaceae and Rubiaceae had 2 species each, and all the other 11 families had one species each as presented in Table 1.
Among the 56 different tree species found within the reserve, Table 2 showed the relative density (RD) and relative dominance (RDo). Each of the 56 different tree species found within the reserve. The results indicated that Tamarindus indica L and Ziziphus spina-christi (L.) Desf. had the highest relative density and dominance of 4.64% and 11%. Adansonia digitata L, Grewia villosa Willd, Vepris nobilis (Delile) Mziray. had density and dominance of 4.80% and 9% followed by Anogeissus leiocarpa (DC.) Guill. & Perr, Adansonia digitata L, Catunaregam nilotica (Stapf) Tirveng. (Syn: Xeromphis nilotica (Stapf) Keay, Vangueria madagascariensis J. F. Gmel. with 3.38% and 8%, respectively. Eleven species recorded the least relative dominance of 0.42%, which may be as a result of the over-exploitation of the woody genetic resources.
3.2. Diversity Index
The diversity of the sites is shown in Table 3; the Shannon-Weiner diversity index (H’) value stood at 3.82. As diversity indices varied with location depending on the species available within an ecological zone, Rashad forest reserve is blessed with a moderate diversity index, which lies within the general limits of 1.5 - 3.5.
Table 1. List of all tree species, family, local name and frequency in the Rashad Area.
Table 2. Comparison of relative density (RD), relative dominance (RDo), Basal area (BA), and important values (IVI) of the 56 species in the Rashad area.
Table 3. Summary of diversity measures for trees in the Rashad area.
The tree girths, tree height and species enumeration were carried out. Key Informant Interview was also part of this study in which both junior and senior forest officials were interviewed. Tree diversity indices such as Shannon Wiener index and Simpson concentration index were used. It was concluded that the forest is quite rich in terms of tree species and that some species are at risk of extinction, which may be a result of over-exploitation, climatic and/or edaphic factors. The Important Value Index revealed the most ecologically important tree species in the study area and those to be prioritized for conservation. There is a need to have inventory, information system, research and development network, and a framework of legal provisions for identification, assessment and monitoring of species diversity of trees to make the area ecologically viable as well as a source of recreation.
The authors are thankful to the college of forest and rangeland, University of East Kordofan. We are grateful to the local communities of Rashad Locality, Sudan, for participating in the survey and providing valuable information. We are also thankful to all those who helped us in this work.
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