Characterization and Carbon Sequestration Potential of Sahelian Agroforestry Parklands of Chad

: Agroforestry parklands are agroforestry systems which farmers created by preserving useful species on their farms. These species are those whose flowers, fruits, seeds, barks or leaves can be processed into useful products. Parklands are thus the result of human intervention in natural vegetation dynamics and their diversity results from ecological factors affecting the neighborhood and the management of trees. The purpose of this work is to characterize and assess the carbon potential of Chad sahelian agroforestry parklands. A total survey area of 90 hectares of parklands of which 30 ha per site were explorated. Compass and GPS were used to establish transects of 50 m in length over 20 m in width and the community sampling units were established (20 sampling plots/ha). In each plot, plants were identified and enumerated. Diameter of plants was measured at 1.30 m aboveground for trees and at 0.30 m for shrublets. The dendrometric parameters measures effectuated throughout transects follow standard forestry methods. The result revealed that three types of agroforestry parklands exist in sahelian zone of Chad among which Hyphaene thebaica , Balanites aegyptiaca and mixed Acacia senegal - A. nilotica parklands. Their structure varies according to the specific flora and dominant species. Floristic survey showed a total of 4801 trees recorded and distributed in 22 species, 17 genera and 14 families. The ecological services of the Chad sahelian parklands fluctuate from 20.25 tCO 2 /ha in Balanites aegyptiaca parkland to 77.36tCO 2 /ha in Hyphaene thebaica parkland, confirming the heterogenous carbon sequestration potential of the parklands. These informations will help to develop the REDD+ mechanisms in the area.


Introduction
The biodiversity of natural ecosystems, already fragilized by anthropic pressures, are accelerated by climate change [1].These degradation factors provoke plant cover reduction leading to loss of plant diversity with consequences in reducing carbon sequestration potential and increasing temperature [2].In the semi-arid and sub-humid zones of West Africa, farmers have for generations implemented a traditional land use system where trees are deliberately associated with crops in a dispersed spatial arrangement dominated by one or more useful species whose flowers, fruits, seeds or leaves can be processed into useful products [3][4][5].This system, known as agroforestry parklands, serves several needs and functions ranging from agricultural production to the production of wood and non-wood forest products [6,7].In sahelian zone of Chad, parklands are overexploited by local populations through wood usage (fuelwood or charcoal, service wood and agriculture).However, these agrosystems contribute effectively to the conservation of biodiversity, the improvement of the living conditions of the populations and the maintenance of the carbon sequestration capacity.
The accumulation of biomass by agroforestry parklands depends on their floristic composition and structure.Therefore, the effects of climate changes and anthropogenic activities influence the structure of these woody formations and consequently their carbon sequestration capacity [8].Despite the great ecological, social and economic importance of these production systems, which have been traditionally practiced for many generations, scientific knowledge is certainly huge but scattered and especially very little renewed.In sahelian zone, a comprehensive compilation of scientific works on agroforestry parklands was made [9,10].Nevertheless, its inspection revealed nothing concerning Chad.Recently, a few works were carried out on this problematic.Traditional agroforestry systems and biodiversity conservation of the Tandjile East were characterized in Chad [11].In addition, the spatial dynamics of Borassus aethiopum parklands in the Loug -Chari Department was studied [12] while the ecological and ethnobotanical potential of Borassus aethiopum parklands in Mayo Kebbi East were treated [13].Knowledges on the structure and carbon sequestration potential of agroforestry parklands in Chad is still very limited.In the REDD+ perspective, the ecosystem services provided by carbon markets on the international level, agroforestry parklands deserve special attention.
The study aims to identify, characterize and assess plant diversity and carbon sequestration potential of the Sahelian agroforestry parklands of Chad.

Methodology
For the floristic data, a total survey area of 90 hectares of parklands of which 30 ha per site were explorated.Compass and GPS were used to establish transects of 50 m in length over 20 m in width and the community sampling units were established (20 sampling plots/ha).In each plot, plants were identified and enumerated.Diameter of plants was measured at 1.30 m aboveground for trees and at 0.30 m for shrublets.
The dendrometric parameters measures effectuated throughout transects follow standard forestry methods [15].Likewise, for measuring diameter at breast height (dbh), diameter of canopy and height of trees, instruments like dbh meter and Finnish Caliper and measuring tape were employed for all woody species.

Data processing and analysis
The aboveground biomass (BA) of tree species is calculated using standard allometric equations applicable to the study area [16].This model takes into account the trees of large and small diameters also the rainfall and the temperature of the environment.Aboveground biomass (BA) = exp [−1.803-0.976E+ 0.976ln(Ω) + 2.673ln(Dbh)−0.0299[ln(Dbh)] 2 ], where, Dbh is a diameter of the tree at breast height (m); is density of wood (t/m 3 dried at 0.65).Exp = exponential; ln is natural logarithm.In Arecaceae (Hyphaene thebaica, Borassus aethiopium), aboveground biomass (BA) specific equation was used [17]: BA = 6.666 + 12.826 * (HT) 0.5 * ln(HT).HT is total height.The underground biomass was determined using the following model [18].
2) The carbon of a tree is estimated by multiplying its biomass by the factor 50% [19].The total stock of carbon estimation in t/ha was converted into equivalent amount of CO 2 eq absorbed using the ration CO 2 /C corresponding to 44/12.The ecosystem service was subsequently converted into monetary value.In the carbon marketplace, due to the unsteadiness of prices, the mean cost of 1 ton of CO 2 for five years (2015-2020) is 10 USD.For all the quantitative data, analyse of variance was performed using the program Statgraphic plus 5. 0.

Typology of Sahelian Parklands
In the sahelian zone of Chad, farmers practice three kinds of agroforestry parklands according to the truncation line of the dendrogram fixed at the similarity index 0.1145066 (Figure 2).The three well known types of agroforestry parklands characterized are Hyphaene thebaica parkland (A), Balanites aegyptiaca parkland (B) and mixed Acacia senegal -Acacia nilotica parkland (C).The dominant plant species (Acacia senegal, A. nilotica, Balanites aegyptiaca, Hyphaene thebaica) of parklands are of socio-economic interests.They are retained in farms by peasants during farming clearing.Their fruits, seeds, flowers, leaves, barks, gums, etc. are processed into useful products which are eaten and/or sold, contribute to the welfare of the households [7].The above main dendrogramm can be summarised in three categories (C) as presented below.Each category or type (C) of parkland in the table below is caracterized by Ri parklands where i is the number from 1 to 180.

Hyphaene Thebaica Parkland
The physiognomy of this type is dominated by Hyphaene thebaica species.This Arecaceae species represents 47.33% of trees of the woody population (Figure 3).The population density is 59.84 trees /ha, all species overcome of which 28.88 stems /ha for H. thebaica.This Arecaceae is followed by Balanites aegyptiaca (7.94 individuals/ha), representing 13.01% of species.The specific richness of this parkland is 22 species.The density aforementioned is differend than the one reported in the Niger's H. thebaca parkland which is 27 trees /ha [9].This parkland has an equilibrated structure because farmers cut palms for fuel, also to manufacture ropes and for house construction.In addition they collect fruits for consumption and selling.

Balanites Aegyptiaca Parkland
The Balanites aegyptiaca parkland is a monospecific one with a density of 41.78 trees/ha.This Zygophyllaceae represents 37.94% of woody species of the population.The density of the dominant species is 16.53 trees /ha.It is followed by Acacia senegal (6.62 feet/ha) and Hyphaene thebaica (5.84) (Figure 4).The specific richness of the parkland is 21 species.

Mixed Acacia Senegal -A. Nilotica Parkland
The physiognomy of the parkland is dominated by two species of which their numbers is the highest in the woody population.Species characterizing this agrosystem flora are Acacia nilotica -Acacia senegal (Figure 5). A. senegal is the most representative (34.32%) followed by A. nilotica (23.46%).Their densities are 21.05 trees/ha and 14.38 stems/ha respectively.The specific richness of this parkland is 20 woody species with a density of 57.84 trees /ha.The companion species diversity related to each population physiognomy varied with parkland type and crops production characteristics as well as the objectives of the farmers.Some of these multipurpose tree specices are regular while others are scarce.Representative species met in these parklands are composed by Acacia nilotica, Acacia senegal, Balanites aegyptiaca, Faidherbia albida, Hyphaene thebaica, Ziziphus mauritiana.The scarce one are represented by Azadirachta indica, Mangifera indica, Psidium guajava, Musa sp., Moringa oleifera, Ziziphus mauritiana, Bombax costatum, Piliostigma thonningii and Citrus lemon.Exotic species in these parklands include: Azadirachta indica, Mangifera indica, Psidium guajava and Moringa oleifera while the rest is natural.The fruits, leaves, flowers, gums, barks, etc. of these trees species of socio-economic importance can be transformed into useful products which are eaten and/or sold in local and regional markets.

Specific Richness and Floristic Diversity of Agroforestry Parklands
Floristic survey revealed a total of 4801 trees recorded and distributed in 22 species, 17 genera and 14 families.The shannon index varies from 1.2 in mixed Acacia senegal -A.nilotica parkland to 2.91 bits in Hyphaene thebaica.This trend indicates a low floristic diversity of the sahelian parklands (Table 2).The Fabaceae and Arecaceae families were the most representative.The value of the Simpson index confirms the low diversity aforementioned.

Structure of Sahelian Parklands
The global parkland structure varies according to tree parameters considered.

Diameter of the Trees
The diameter distribution of the species in sahelian parklands presents a belt shape apart from the type of parkland.The maximum of individuals is observed in the interval 10.1-40 cm independently of the parkland selected (Figure 6).This suggests that in these parklands, regeneration of seedlings and maintenance of old trees are difficult.Generally, these agroforestry parklands present a homogenous diametric structure.The belt shape exhibited is similar with those reported in the Niger's parklands [9].

Height of the Trees
Like in diameter at the breast height, vertical distribution of trees in sahelian parklands, shows analogous shape (Figure 7).These parklands present a dynamic trend according to relative dominancy of young individuals.The density of trees is high in lower woody and shrubly layers.The high existence of young individuals in the lower shrubly stratus indicates appreciable recruitment of the population between 5.1-20 m in the height.

Diameter of the Canopy
The global canopy diameter structure of the Hyphaene thebaica parklands follows a «J» distribution with a maximum of trees concentrated in 5.1-10 m as well as in the two other parklands.They show a belt shape (Figure 8).The belt shape is pronounced for B. aegyptiaca and mixed Acacia senegal -A.nilotica parklands.The number of tree species with a canopy diameter more than 30m is scarce.The number of individuals by diameter reduces with the increasement of canopy diameter.In general, these two sahelian parklands present belt shape suggesting that regeneration of young individuals and maintenance of old trees are difficult.

Ecological Services and Economic Value
The carbon stock ranges from 5.60 tC/ha in Balanites aegyptiaca parkland to 21.10 tC/ha in Hyphaene thebaica parkland (Table 3).There is a significant difference among the parklands (0.03<0.05), suggesting that they operate differently.The ecological services of the Chad sahelian parklands fluctuates significantly from 20.25 tCO 2 /ha in Balanites aegyptiaca parkland to 77.36tCO 2 /ha in Hyphaene thebaica parkland (0.0005 <0.001), confirming the heterogenous carbon sequestration potential of the parklands.Concerning their economic aspect, farmers of the sahelian parklands of Chad will receive in total 1205.2$, if carbon credit is paid.The highest economic value is observed in Hyphaene thebaica parkland.

Typology of Parklands
The three types of agroforestry parklands distinguished are well known in the sahelian zone.Compared to sudanian parklands, those of Chad sahelian zone are less diversified.The specific richness of these parklands is low compared to what have been reported in others tropical ecosystems [20][21][22].Nevetheless, this low specific richness value is higher than that registered in parklands reported in Doufelgo -Togo [4] and in Cameroon [2].However, their density is appreciable because higher than that highlighted in Prosopis africana parrklands (7,89 trees/ha) of Niger [23].The demography analysis has shown that these parklands exhibited a homogenous structure.Companion tree species found are natural except Azadirachta indica, Mangifera indica, Moringa oleifera and Citrus lemon introduced by sahelian farmers.Diverse functions are linked to plant species retained deliberately in parklands among which the common are food, regeneration of soil fertility, fodder production for animals, etc.Each of these species presents its proper structure depending to the use made by farmers.The average trees density varies significantly and the recovery flutuates according to the ecoligical requirement of crops (heliophilous, scyaphilous).

Floristic Diversity and Structural Characteristics
In general sahelian parklands show a low diversity with Shannon index ranges from 1.62 to 2.91 bits with indexes of Simpson and Pielou of the same trend, is a characteristic of the arid zone plant population.This trend express a homogenization or sahelization of the vegetation and mainly of the parklands [9].Lack of diversity in these parklands is linked to the cultural practices of the area.The spatial structure of parklands is affected by specific crop management [3].Different types of species are met in sahelian parklands: exotics and naturals.The low floristic richness observed at the level of parklands would be due to the use of species by the human populations for the satisfaction of their daily need as well as the ecology of crops produced.
The demography of sahelian parkland exhibits a normal distribution of diameter independently of the parkland type.As above mentionned, this distribution indicates that young and old trees in the system have difficulties to grow.These findings agree with those reported on Borassus akeassi parklands in Benin [24].

Carbon Sequestration Potential and Economic Value
Several works have shown the function of agroforestry systems as an opportunity to reduce CO 2 concentrations in the atmosphere by increasing carbon stocks in agricultural lands [7,[25][26][27][28][29]. Globally, Chad sahelian parklands store an average of 10.98 tC/ha.This value is low compared to that reported in other tropical'parklands [1,2,22].Nevertheless, Hyphaene thebaica parkland showed a leading function in mitigating climate change.Its sequestration potential represents 54.19% of the biomass stored in these parklands.Despite certains phenomenons which destroy agroforestry parklands in general, those of the Chad sahelian zone are economically profitable and appear as an attractive alternative in terms of timber production, household income source and the fight against global warming.The total carbon captured by sahelian parklands is 120.52 tCO 2 /ha.This finding disagrees the one reported in Tandjile-Chad [11], in Benin [30] and in northen Cameroon [2,21,31].Chad sahelian parklands have a slip carbon sequestration capacity.In general, parklands play different major functions among which food, soil fertility restoration, fodder production for animals and several others ecological services.

Conclusion
Three different physiognomic types of agroforestry parklands existing in the Chad sahelian zone were characterized: Hyphaene thebaica parkland, Balanites aegyptiaca and mixed Acacia senegal -A.nilotica parkland.They are well known, well represented and well structured.Their physiognomy varies according to the specific flora and dominant species as well as objectives of the farmers.Two of them are monospecific (Hyphaene thebaica, and Balanites aegyptiaca parklands) and the third is mixed (A.senegal -A.nilotica).Hyphaene thebaica parkland plays a leading role in mitigating climate change by storing 54.19% of the biomass of the Chad sahelian parklands.The economic value is 1205.2$USA.Valorisation of agroforestry systems will contribute to the biodiversity preservation as well as improving carbon sequestration potential and of cause supply data which will facilitate the Chad access to carbon market.To better know the functionning of the Chad sahelian parklands, the soil, litter and herbaceous species carbon stock needs to be estimated as well as crop production.

Figure 2 .
Figure 2. Dendrogram of parkland repartition in the Sahelian zone of Chad.

Figure 6 .
Figure 6.Repartition of individuals following the diameter.

Figure 7 .
Figure 7. Repartition of individuals following the height.

8 .
Repartition of trees according to canopy diameter.

Table 1 .
Typology of agroforestry parklands in the sahelian zone of Chad.

Table 3 .
Ecological services and economic value of Chad sahelian agroforestry parklands.