Can Stand Density and Stem Stratification Be Indicators of Aboveground Biomass in Woody Plant Recruitment in Savannah

Stem density and size stratification of woody species are informative of vegetation conditions and its physiognomy in savannah whereas their variation influence woody population functioning. Current study endeavoured to eva-luate the stand density and size variability of woody species related to aboveground biomass in a Sudanian savannah. Total height, stem diameter at breast height (dbh) ≥ 5 cm were measured in 30 plots of 50 m × 20 m laid in respect to vegetation type as bowal, shrubland and woodland. Species diversity, stem density, height and basal area were calculated and compared across sites and variation in stem dbh classes evaluated. Total aboveground biomass was estimated and thereafter linear relationships were established between stand density and aboveground biomass, and basal area. Results revealed three different sites with an overall 58 species identified through vegetation type including liana species (4 stems in bowal) with 18 genera and 42 families. sity and stratification could be considered as indicators of aboveground biomass fluctuating in regeneration monitoring.


Introduction
Woody plant regeneration and conservation as well as restoration and suitable management of ecosystems are permanently major challenges for ecologists, researchers, other plant scientists and for environmental decision makers seeking continuous provision of multiple ecosystem goods and services (Lorenz & Lal, 2010;Manaye et al., 2021). The challenges are exacerbated today due to climate change, demographic growth, agricultural activities expansion and galloping urbanization that jeopardize the local biodiversity stability and resilience and vegetation productivity (Rakotomavo et al., 2018). In the light of challenges for the sake of pursuing sustainable management and suitable conservation of lands and biodiversity (ecosystem goods and services), a many management practices as controlled livestock grazing and/or prescribed fire (early or late), protected area establishment, agroforestry systems, silvicultural thinning have been implemented in tropical and subtropical ecosystems (Sawadogo et al., 2005;Higgins et al., 2007;Lorenz & Lal, 2010;Murphy et al., 2015;Adjahossou et al., 2017;Ganamé et al., 2019). For instance, in arid and sub-arid regions protected areas are promoted as reserve forests or state forests (Higgins et al., 2007) through marking the boundaries of some natural wooded spaces, parklands on smallholdings and fallows or cultivated lands in many parts of the country as national strategy. All these protected areas with or without management tools and, considered as favourable habitats endeavour to promote biodiversity conservation, threatened species regeneration and, degraded land rehabilitation (Sawadogo et al., 2005;Lorenz & Lal, 2010;Murphy et al., 2015). At the same time these protected spaces are experiencing heavy anthropogenic pressure worldwide such as the illegal harvest of woody species and forest degradation that induce and maintain spatial heterogeneity as a discontinuity in vegetation layer (canopy cover) in place and the distributions of size-classes. Yet, to be effective and efficient, any strategy of biodiversity conservation must solve as possible as, the problems of erosion which apparently occurs at a raised rate. Otherwise, all the operations undertaken become useless due to misfit practices and/tools. Previous researches have reported that variability in species diversity induce important variability in ecosystem functioning allocated to the density and spatial distribution of stem sizes (ontogenic stages), tree biomass productivity within or inter plant community (Hooper et al., 2005;Lorenz & Lal, 2010;Matula et al., 2015). Biomass production at any given time and in any vegetation is considered to be a key guide of plant species capability to incorporate carbon and nutrients to be later used (Weiber & Grulke, 1995). As such, it plays an important role in plant growth and establishment. By contrast, variability in plant species diversity may also result from the favoured establishment of some abundant individuals (complementarity actions or compensatory) or the impediment recruitment of others (antagonism) through the effects of vegetation types and land cover degree (Lorenz & Lal, 2010;Traoré et al., 2015).
While stand density and stem size distribution of woody species (conspecific) in a community can vary in growth rate due to unbalance disturbance intensity and other stress (Attua et al., 2017) and to the degree of physical injuries on aerial systems, the functioning of woody plant can also be influenced in terms of aboveground biomass production of each species during regeneration process. Importantly, AGB producing might be an attribute of good or weak growth performance but many species in turn exhibit different physiological and/or morphological responses to vegetation conditions. For instance, Coomes & Allen (2007) argued that small trees were less competitive in large canopy vicinity and grew slowly and weakly as a consequence, while larger trees were not affected by canopy cover. The obvious conditions include woodland, shrubland and bowal with different degree of canopy. Reports support that the coexistence of several species in abundance may significantly be complementary for area characteristics (Holmgren et al., 2004;Hooper et al., 2005) whereas some species are the mark of particular areas (e.g. desert, rain forest, mountain…). In consequence, the way and rate at which plant stem is extended depend firstly on each species properties and, secondly on species response to the vegetation conditions in which it grows under interaction between different plant strata and other factors (biotic and abiotic) (Sawadogo et al., 2005;Coomes & Allen, 2007). Each plant species elaborate new tissues as gained biomass during its (re)growth for its stem diameter and height (Weiber & Grulke, 1995).
Community structure and regeneration statu are frequently evaluated by the pattern of size classes (or horizontal distribution) measured as diameter at breast height (dbh) (Holl & Zahawi, 2014) as important features in ecosystem properties. Therefore, evaluating stand density and trunk size stratification relating to aboveground biomass can also help to understand the functioning of woody populations in savannah ecosystem in accordance with their physiognomy. The current work has been undertaken to highlight linkage between woody stand conditions and its functioning. It may positively contribute to understand stem size attributes on ecological processes and then help in designing and achieving future research leading to sustainable management and strategy development.
Recent studies at relative natural savanna have focused on particular woody species (biodiversity and biomass) with purpose to develop allometric equations (Mbow et al., 2014;Dayamba et al., 2016;Balima et al., 2019;Damena & Teshome, 2019) or quantify carbon budget and/or released included only individuals with dbh > or ≥10 cm (large trunks). In contrast, studies based on entire woody communities (combination of many populations) growing in the same environmental conditions considering stem stratification as clue of plant recruitment are scarce (Gebrewahid et al., 2018) in arid and sub-arid ecosystems.
This investigation generally aimed at assessing the stand density, stem size stratification as bio-estimators of living aboveground biomass (AGB) of woody plants during (re)growth process. As for the specific objectives, we explored 1) the species diversity of woody communities, 2) we assessed stem horizontal structure as the evaluation of recruitment degree of woody plant, and 3) we quantified AGB relating to stand density and stem diameter patterns as the magnitude of primary productivity. We assume that stem size be indicator of increase aboveground biomass as positive features of small sized tree recruitment to large sized trees.

Description of Study Site
The present study was carried out in the state forest of Dindéresso. This forest is  Soils are rocky and are identified as Lixisols according to the FAO classification system (Driessen et al., 2001). They are slightly deep (>85 cm) and poor (Nitrogen = 0.058%; phosphorus = 1.77 ppm and carbon = 0.33%) with low amounts of organic matter.
Local vegetation is a humid savannah of the Sudanian regional centre of endemism (White, 1983). This vegetation is a mosaic patch constituted of gallery forest, clear bush savannah, wooded and opened savannahs, dense wooded savannah, with annual and/or perennial grasses (Fontes & Guinko, 1995

Experimental Design and Woody Plant Sampling
During the midst of rainy season in 2018, the study area was explored using the official road delimited by Forest Department and the network of trails drawn during illegal livestock grazing. Survey was conducted from August to December 2018 using vegetation community characteristics and scientific criteria of Arbonnier (2000). According to this author wooded land (or opened savannah) is characterized by separated trees and the abundant individuals of species as Hubb., and Pennisetum pedicellatum. Bowal also refers to a shrubland but characterized by the combination of an annual grass layer, some scattered shrub species on bare soil surface and crusted soils that are practically inappropriate productively. Abundant mushroom-shaped mounds of termite Cubitermes spp (humivorous) were found at 99 ha −1 (Kéita, 2019). We randomly set 3 compact transect lines up in area based on a preliminary identification as old-growth (or relic) vegetation where no plantation and conservation techniques exist. This part of forest was not affected by wild fire disturbance. Distance between transect lines is over 1 km. The first transect was laid out in shrubland, the second in woodland and the third one in bowal. A total of 10 contiguous rectangular plots  (Krebs, 1972).
In each delimited plot, all the vascular plants of which the diameter measured at breast height, or 130 cm above soil surface was equal or greater than 5 cm (dbh ≥ 5 cm) and stem height greater than 130 cm, were systematically recorded per species. Liana species was recorded for complete species assessment only but not included in structural analysis as well as in biomass calculation because of the lack of wood specific-density for savanna vegetation. The samples of unidentified species were collected for being later identified. Scientific nomenclature was later updated following IPNI (

Data Handling and Statistical Analysis
Each transect with 10 plots was considered as study block (Krebs, 1972) and the formula of Bray-Curtis index defined in Equation (1) (Kindt & Coe, 2005) was performed to assess ecological similarity or dissimilarity between vegetation type. This index of similarity allows assessing species diversity and abundance as single statistic and its value ranges from 0 (similarity for all species) to 1 (no species in common). This is because habitat heterogeneity may lead to an increasing or scarcity in species diversity (Kadéba et al., 2014) and (Traoré et al., 2015).
where: i & j are vegetation type; S i & S j refer to total species number counted for vegetation type i & j respectively; C ij is the sum of only the minor counts for each species found in both vegetation type.
Species richness as the total number of plant species was determined based on observed and estimated species number. Estimated species richness was graphically obtained with the rarefaction curves of species-cumulative stem number using EstimateS 9.10 package (Colwel, 2013). Species richness was later compared between vegetation types during sampling period in 2018.
Community structure was described for each vegetation type as average stand density and basal area at breast height (dbh). Histogram was drawn to highlight the distribution of dbh classes as the function of stem density in spatial distribution (horizontal distribution). Beyond total height, the distribution of stems in dbh classes may be an indicator of recruitment constituents and a predicate of the different successional stages in seedling to saplings, and saplings to adult tree transition or degree of disturbance. For this purpose, we considered stems of 5 ≤ Open Journal of Forestry dbh ≤ 10 cm as small-sized and those of dbh > 10 cm as large-sized trees (Kuma & Shibru, 2015). Within large stems, other groups were discriminated as 10 < dbh ≤ 15 cm; 15 < dbh ≤ 20 cm and, dbh > 20 cm to explore the contribution of each class to density distribution and biomass group. Prior to assessing fresh biomass, the total AGB of living plants was indirectly estimated. Allometric equation (Equation (2) Relationship was established between successional groups referring to stem dbh classes as recruited categories. The values of wood density for each species were obtained from the global wood density database (Chave et al., , 2014Zanne et al., 2009) available from the Dryad data repository (http://datadryad.org/). The average value wood density was calculated and used when different values of wood density exist for the same species taken in the same climatic conditions. In case specific-species wood density was unavailable, we calculated the average value of wood density of plant genus or family and afterwards integrated this value. Finally, the values of wood density, stem dbh and total height H were computed. AGB was firstly calculated for each stem, summed for each species and then pooled for each plot. The estimated total AGB of tree community was obtained as the sum of AGB of pooled species per plot in tons per hectare as AGB density (t·ha −1 ) and compared through the three vegetation types. Total AGB of living plant was considered as a measure of the ability of each species population to elaborate new tissues and organs for regenerating or repairing after physical injuries.
The one-way analysis of variance (ANOVA) was performed to assess the influence of vegetation type on total stem density and basal area at breast height for the average values among vegetation patches. Total basal area was previously Log(x + 1) transformed to meet homogeneous variance. Multiple comparisons were then performed with Tukey's HSD test for homogeneous subsets for averages that exhibited significant difference (p-value < 0.05). Non-parametric Kruskall Wallis H test (Zar, 1999) was applied to data on AGB because of unequal variance distribution. In addition, relationship was established using linear regressions for assessing any variation in AGB as a function of stem density and basal area at breast height for small and large stem. Statistical analysis were carried out at p-value < 0.05 using IBM SPSS version 20 package for Windows.

Species Diversity and Composition of Plant Community
The calculated value of Bray-Curtis's index of similarity was under value 1. This   Table  1). Among recorded families, 13, 13 and 14 were noted on bowal, shrubland and woodland respectively. Fabaceae was the dominant family with 18 species followed by Combretaceae (15 species), Anacardiaceae (4 species) and Rubiaceae (3 species).
Twenty-six genera were present on bowal, 25 on shrubland and 36 genera on woodland. Bowal and woodland did not shelter any species of Combretaceae family.

Community Structure of Woody Plants
Stand structure was analysed by excluding liana species as Saba senegalensis (A. DC) Pichon (4 stems in bowal) leading to use 1553 as total stem number to follow up. All stems encountered and measured in 3 ha of area constituted a total density of 517.67 stems·ha −1 . Total stem density ranged from 436 in bowal to 620 stems·ha −1 in shrubland as showed in Table 2. The density of small and large stems follows the same trend.  In contrast, woodland exhibited taller individuals with the highest average value of stem height (5.43 ± 0.73 m) and maximum stem height value (6.58 m) ( Table 2).
The overall difference observed in stem density tended to be significant (F = 2.22, p-value = 0.048) as the influence of vegetation type (Table 2)  In return, the average value of stem basal area at breast height (dbh) ranged from 4.72 ± 2.69 m 2 ·ha −1 for woodland to 3.03 ± 2.06 m 2 ·ha −1 for bowal respectively ( Table 2).
The higher values of stem basal area (5.36 ± 0.97 m 2 ·ha −1 ) and maximum value of stem dbh (49.66 cm) were recorded in shrubland and lower values on bowal according to Table 2.

Relationship between Stand and Aboveground Biomass
The total net primary productivity of woody plants as living AGB linearly increased from bowal (AGB = 7.59 t·ha −1 ) to woodland with the highest value (AGB = 18.53 t·ha −1 ) ( Linear regression analysis were performed on the one hand between stem density and biomass, and on the other hand between stem basal area at breast height. Results revealed that variation in AGB was highly positive and significant (p-value < 0.05) for small sized stem density and basal area and for large sized stem basal area in woodland only (Table 3). More than 67% of variation observed in AGB in bowal (p-value < 0.0001) and woodland (p-value = 0.004) were due to the great density of small stem. Solely shrubland in where 91% and 76.3% variation in AGB were remarkably caused by higher density (p-value < 0.001) of small stem and by basal area of large stems (p-value = 0.001). In woodland, almost  Regression equation corresponded to AGB (t·ha −1 ) = b + ax (with x = stems·ha −1 or x = basal area (m 2 ·ha −1 ).
50% of variation observed in AGB were significantly induced by higher density of large sized-stem (p-value = 0.02).

Discussion
The present study, done in 3 ha identified 58 species as an evolution of species richness with the number of 1557 recorded plant individuals toward those (67 species) obtained by Nombré (2019). Species richness as well as density of woody plant differs from one vegetation type to another corresponding to the attitude of each species to colonize and establish in one or more vegetation conditions. Woodland was the richest area that exhibits higher species richness and flora (45 species, 36 genera and 14 families) while bowal was the poorest area with lower species richness and composition. But according to the calculated values of Bray-Curtis's index, only shrub-and woodland were different from each other in term of species richness and diversity whereas bowal, shrubland and woodland were similar and formed together homogeneous vegetation. This similarity seems to be linked with the fact that 17 species representing 29.31% of total species were simultaneously observed on these three vegetation types. Ac-cordingly, when we calculate the ratio of species number over stem number (bowal 30/440 = 0.07; shrubland 35/620 = 0.06 and woodland 36/497 = 0.09) for each area we obtain a value that tends toward value 0.1 proving that species are evenly distributed between areas. Thoroughly, increase in plant number did not conduct to increase in species richness indicating the (co)-existence of few species represented by many individuals (see Table 1) as their response to each vegetation singularity and spatial heterogeneity. It is obvious that vegetation type as canopy layer (with or without soil type) is preponderant in woody species assemblage, density and through the plasticity of some species to colonize and establish in savannah ecosystem (Williams et al., 1996;Oliveira et al., 2017). But the level at which variation occur differ from one to another vegetation type with respect to tree adaptive strategy and performance (Traoré and Jouquet, 2020).
Overall results showed pronounced decrease in large stem density and an increase in small trees density across vegetation types. Such habitats may affect the natural regeneration of tree species through the recruitment and the density of vascular undergrowth (dbh < 5 cm) species and consequently induce heterogeneity in the distribution at small scales. Decrease density of large tree may be the consequence of the absence of positive recruitment in upper sized class due to the negative influence of area that can be observed in many vegetation type (Novor & Abugre, 2020). In effect, bowal dominated by bare and encrusted soils as ultimate degree of area erosion can further impede the establishment of luxurious woody assemblage. Such, stem density can increase and being limited to the prevalence of small trees resulting from degraded or oppressed site in which plant community is developing strategies that allow them to survive hard conditions but also from a slow growth rate, common in African Sudanian zone (Solbrig, 1994;Zwarg et al., 2012;Padonou et al., 2014) and thereafter influence AGB production. Significant relationship was found between small tree density and AGB (R 2 ≥ 67%) in each vegetation type, between larger tree density and AGB (R 2 = 49.6%) in woodland and between the basal area of larger tree and AGB (R 2 = 76%) in shrubland. This suggests that distinction in tree dbh class pattern positively and significantly affects AGB as well as vegetation type as bowal, shrubland and woodland with greatest AGB for small trees in bowal. As possible explanation, small individuals related to great AGB may reflect a level of resilience of plant community after disturbance or a great degradation of plant community. In case of restoration post disturbance, plant actively grow through production and differentiations new aerial organs as multiple stems (Zida et al., 2009) in juvenile to tree transition and become established as adult trees. Our results agree with previous studies carried out in dry and humid environments (Oliveira et al., 2017;Astri et al., 2020). Oliveira et al. (2017) had indicated that in forest and non-forest environments, more individuals are distributed in the minor diameter classes while few individuals are found in the major diameter classes and Astri et al. (2020) reported that small sized trees were positively connected with great AGB in savannah system. But, according to these last authors, the contri- butions of small sized trees to basal area and stem density were significantly different between vegetation types. Therefore, attention may be paid to the horizontal structure in communities and bowal contribution for management and restoration monitoring since our analyses also pointed out a decline in vascular vegetation layer.

Conclusion
We have assessed woody community related to the functioning of ecosystems through stand density and size structure of woody species and AGB through three vegetation types as bowal, shrubland and woodland. Results revealed similarity between them for species diversity and composition while significant difference was underlined in stand abundance with small tree predominance. We found that the positive variation in plant community structure was generated by the positive relationship between stem size and produced AGB. Such, increase density in small sized stem in community result from increase accumulated aboveground biomass as positive regeneration features, but positive stem recruitment in upper diameter class was hindered as the higher degree of vegetation degradation. The results illustrate the importance of including stand structure in total AGB estimation for plant community. Therefore, we suggest considering stem size to be used to discriminate AGB level and vegetation conditions in regeneration monitoring regarding species composition. And then, measures could be taken to adapt co-management of natural vegetation and reinforce ecosystem services and goods production.