Diversity, structure and effect of rare species and relative abundance of the over-represented species on arborescent layers’ diversity

The present study aims to analyze diversity, structure and effect of rare species rate and relative abundance of the over-represented species on arborescent layers’ diversity in Rubi Tele Domaine de Chasse (RTDC) mature forests. The RTDC (6227.74 km2) straddles the Tshopo and Bas-Uélé provinces in the North-Eastern part of the Democratic Republic of Congo (DRC). Inventories were realized and data were collected in 30 nested plots of 1 ha each where all trees with DBH ≥ 10 cm were taken into account. The floristic composition was evaluated thanks to taxonomic richness, Fisher’s Alpha index, Pielou equitability index and to the determination of rare species’ rate. Vegetation structure was determined using tree density, basal area and above ground biomass. The effect of rare species’ rate and relative abundance of the over-represented species on arborescent layers diversity was appreciated. The results of this study revealed a variability of diversity and structure parameters in this layer category and not in the other. These results revealed, moreover, an existence of effect of the rare species rate and the relative abundance of the over-represented species on diversity and structure of arborescent layers. *Corresponding Author: Katembo W. Eric  ericwasingya@gmail.com Journal of Biodiversity and Environmental Sciences (JBES) ISSN: 2220-6663 (Print) 2222-3045 (Online) Vol. 13, No. 2, p. 102-112, 2018 http://www.innspub.net J. Bio. Env. Sci. 2018 103 | Eric et al. Introduction Tropical forest ecosystems are one of the major reservoirs of biodiversity (Leigh et al., 2004). They shelter more than fifty per cent of terrestrial species (FAO, 2015). But unfortunately, they are the most threatened where profound changes have occurred in recent decades (Yenni et al., 2015). The vegetal formations of the Democratic Republic of Congo (DRC) are not spared from this drama of deforestation and considerable reduction or even extinction of wood resources diversity. According to the FAO, the deforestation of tropical forests, which annually represented about 7 million hectares in the 1970s attains currently (1990-2015) approximately 10.4 million hectares each year (FAO, 2015). Among various factors responsible for this evolution, the considerable increase in populations has carried away the needs for wood and agricultural land and has led to the clearing of considerable areas of natural forests. Consequently, the structure of the stands leads to the reduction of the number, the basal area and the above-ground biomass of ligneous plants. Through its effects on forest ecosystems, deforestation has been the basis for a new spatial redistribution of forests where individuals are unequally distributed among species (Pitman et al., 1999). This inequitable distribution is illustrated, on the one hand, by the over-representation of a species, the case of monodominance (Gross et al., 2000, Torti et al., 2001; Makana, 2004; Barbier et al., 2016, Kearsley et al., 2017) and secondly by innumerable relatively rare species (Hubbell and Foster, 1986; Ngo and Dirk, 2013). The understanding of this phenomenon of species dominance and rarity has prompted by several authors to attempt to formulate elements of response. Some thought this was due to specialization of layers (Terbogh et al., 1996), disturbance (Ter Steege and Hammon, 2001), niche diversification (Connell, 1978), compensatory mortality (Connell et al., 1984) and pathogen pressure (Gillett, 1962). Others, on the other hand, have assumed that environmental heterogeneity (Phillips et al., 2003, Pitman et al., 2001) and limited dispersal of diaspores (Condit et al., 2002) of some species have a part in this phenomenon. Several studies have focused on the phytosociology of tropical forests (Louis, 1947) to Yangambi, (Pierlot, 1966) on different altitudes, Mandango (1982) in the islands of Zaire River. Others, later, were also interested in the trade-offs put in place by the species of these forests (Gillet et al., 1991; Senterre, 2005; Nshimba, 2008; Amani, 2011; Amani et al., 2013). Kouob (2009) further found that in Cameroon the stand structure illustrated an over-representation of some ecologically dominant species or families, accumulating the largest number of individuals and most of the basal area. The RTDC is one of the DRC’s phytogeographic entities less known phytosociologically, floristically and structurally. The only works carried out there are those of Lisingo (2016) and Katembo et al. (2018). Unfortunately, they have not analyzed the turnover of species. With this in mind, this study aims to analyze diversity, structure and effect of rare species rate and relative abundance of the over-represented species on arborescent layers’ diversity in the RTDC’s mature forests because conservation and management of forest ecosystems depend on. Material and methods Study area Created through the 51th legislative Ordonnance/ Agri. since the 12th December 1930, the RTDC (Fig. 1) is located on south of the Buta city, spanning from 2°32’22.9’’ N to 2°43’50.04’’ N and 24°38’25.17” E to 25° 04’ 35.98” E. The RTDC straddles the Tshopo and Bas-Uélé provinces in the north-eastern part of DR Congo where it covers an area of 6227.74 km2 (ICCN, 2012). J. Bio. Env. Sci. 2018 104 | Eric et al. Fig. 1. Map illustrating the Rubi Tele Domaine de Chasse. The RTDC benefit is under an Am type climate (Koppen, 1936). The monthly mean temperature fluctuates around 25°C and the yearly mean rainfall range from 1500 and 1800 mm. Data collection The sampling sites were chosen after the field prospection. Data were collected in 30 homogeneous plots of 1 ha each placed in mature forests established on periodically flooded soil (PI: 10 ha); on the dry deep soil (TFPr: 10 ha) and on dry gravelly soil (TFGr: 10 ha). Each survey was conducted according to the Integrated Synusial Phytosociology approach (Gillet et al., 1991; Senterre, 2005; Nshimba, 2008; Amani, 2011; Amani et al., 2013) where all trees with DBH ≥ 10 cm were taken into account and categorized into upper (A) and lower (Ad) arborescent layers. These ligneous plants have, in addition, been identified and measured (DBH). Data analysis The floristic diversity of the A and the Ad arborescent layers was assessed by means of richness (families, genus, species), Fisher’s Alpha index and Pielou equitability index. The rate of rare species has also been evaluated. The following structural parameters were performed to characterize the woody stand: Tree density: is the number of trees per unit area (individuals/ha); Tree basal area (G): corresponds to the crosssectional area of these trees at diameter at breast height (DBH). It is expressed as follows: G (m2 / ha) = πD2 / 4 where D is the DBH. Estimated aboveground biomass (AGB): refers to the total mass of living organisms on a unit area. It is expressed in Million grams per hectare (Mg / ha): AGB = ρ x exp (-1,499 + 2,148 ln (D) + 0,207 (ln (D))2– 0,0281 (ln(D))3) (Chave et al., 2005) where ρ is the woody density. The relationship between the rate of rare species with alpha diversity and that of the relative abundance of the dominant species with the Pielou equitability of J. Bio. Env. Sci. 2018 105 | Eric et al. upper A and lower Ad arborescent layers were calculated. Stats were done thanks to Excel 2016 and R 3.1.3 softwares. Analysis of Variance (ANOVA) with Tukey's tests were used to assess the differences between the values of richness (families, genus, species), floristic diversity, rare species rates, density, basal area and aboveground biomass between the A and Ad arborescent layers of the targeted mature forests. The student's t test was used to assess the effect of the species rate and the relative abundance of the over-represented species on the diversity of A and Ad layers. Results and discussion Diversity and species rarity A total of 184 species divided into 155 genus and 44 families was recorded in the nested plots. The comparison of richness (at the family, genus and species levels), diversity (Fisher alpha and Pielou equitability index) and the rate of rare species between A and Ad arborescent layers of different forest types (PI, TFPr and TFGr) showed a difference in some cases and not in others (Table 1). Table 1. Diversity and proportion of rare species. Groups Family Genus Species Fisher α Pielou % rare species PI_A 12.2±2.90a 16.8±4,61a 17.9±5.92a 5.48±2.42a 0.47±0.21a 51.35±13.73 TFPr_A 7.6±3.24b 10.9±4.12b 11.2±4.34b 2.97±1.45b 0.31±0.07b 57.29±13.43 TFGr_A 9.7±3.06a,b 14.4±4.01a,b 15.1±4.56a,b 4.54±1.75a,b 0.51±0.04a,b 57.28±9.02 p < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 > 0.05 PI_Ad 16.9±3.21 24.3±7.02 28.9±8.14 8.06±3.33 0.6±0.09 43.22±9.79 TFPr_Ad 15.0±2.49 22.2±4.61 28.5±6.33 8.75±2.31 0.61±0.09 47.56±11.72 TFGr_Ad 17.9±3.25 28.7±6.60 15.1±4,56 4.54±1.75 0.51±0.04 43.25±13.09 p > 0.05 > 0.05 > 0.05 > 0.05 > 0.05 > 0.05 Within column means per taxon followed by the same letter were not significantly different from each other at P = 0.05 following Tukey’s comparisons. The species diversity differed from one compartment to another and from one forest type the another (Table 1). This difference was influenced by the comparison of means of the A layer’s diversity index between forest types. The A and Ad layers of the mature forests were dominated by the family Fabaceae, followed by the Rubiaceae and Sapotaceae (A), Ebenaceae and Clusiaceae (Ad). Table 2. Dispersion of density, basal area and aboveground biomass (AGB) values of nested plots. Density Basal area AGB Forests A Ad A Ad A Ad PI 158.2 ± 49.55 311.9 ± 73.42a 25.5 ± 4.64 6.74 ± 3.10a 401,22±70,36a 67,78±37,61 TFPr 137.5 ± 21.42 224.3 ± 39.38b 28.4 ± 2.61 4.08 ± 1.04b 494,17±55,12b 34,83±11,11 TFGr 134.7 ± 28.25 279.7 ± 48.84a,b 27.82 ± 3.17 5.21 ± 1.37a,b 472,33±61,97b,c 43,21±13,22 P > 0.05 < 0.05 > 0.05 < 0.05 < 0,05 > 0,05 Within column means per flayer followed by the same letter were not significantly different from each other at