Analysis of the Ricinodendron heudelotii × Theobroma cacao L. Interaction in Traditional Agroforestry Systems in Côte d’Ivoire

Abstract

The objective of this study was to improve cacao (Theobroma cacao) production through its association with a woody species, such as Ricinodendron heudelotii, in western Côte d’Ivoire. To do this, a design of two transects (10 m and 20 m) was installed around the species in 5 localities (Buyo, Duekoué, Guéyo, San-Pedro, and Soubré). The diameter at the breast height of the tree, the height, the number of fruits, the above biomass, and its carbon stock were measured. Results showed morphological variability of R. heudelotii according to the localities. The presence of the species within cacao trees reduces the rate of pod rot, stabilizes the rate of pods eaten away, and increases the biomass production and the carbon storage of cacao trees. The distance between the two species had no impact on the vigor and the yield of cacao trees. However, cacao density in the 10 m line was reduced compared to that of the 20 m. Therefore, the integration of R. heudelotii into cacao agroforestry systems could improve cacao production. That is why the reduction of cacao density, as well as the use of more suitable varieties of cacao, should be considered for the sustainability of this system.

1. Introduction

Ivorian forests are subject to various pressures; the main ones are abusive logging [1], illegal occupation of protected areas [2], and cash crop plantations in forests, such as cacao (Theobroma cacao), which actively contributes to Ivorian forest degradation. Indeed, Côte d’Ivoire has lost 90% of its humid forest since independence, principally driven by extensive cocoa farming [3,4]. For example, in 20 years, 80% of the total forest area of the South-East zone has been cleared for cocoa cultivation, as well as for the department of Duékoué in 17 years [4,5]. According to [6], in 2020, tree cover loss in the state was 252% greater than in the first decade of the 21st century and 109% more than the average tree cover loss from 2011–2019. Agroforestry is a dynamic natural resource management system that is built on an ecological foundation integrating trees into farms and the rural landscape, thereby diversifying and sustaining production to improve the social, economic, and environmental conditions of all land users [7,8]. Indeed, the studies of [9], which concluded that “cacao agroforestry systems (CAS) seem to have almost completely disappeared from the Ivorian landscape”, have been contradicted by several authors who have highlighted the presence of these systems in the center-western Côte d’Ivoire [10,11]. More recently, the work of [12] showed the existence of cacao-based agroforestry systems in Kokumbo in the Lacs region of Côte d’Ivoire. Numerous authors have defined basic agroforestry criteria which have made it possible to identify five main production systems [12,13], and three of which exist in Côte d’Ivoire [5,14,15]. These systems have been classified based on floristic diversity and vegetation structure. These are simple CAS (open canopy and a high proportion of exotic species, such as banana (Musa paradisiaca) and avocado (Persea americana), mixed CAS (open canopy and a low density of local species), and complex CAS (closed canopy and a high density of local species).

Agroforestry systems based on cacao trees, which constitute a traditional form of production whose functioning is similar to that of a forest, would contribute to the rate of carbon sequestration. Pure culture systems produce less cacao while being more sustainable and environmentally friendly. They also provide a range of important environmental services, such as biodiversity conservation, maintenance of soil fertility, and carbon storage [16]. By generating many ecological services, CAS are perceived as sustainable [17] and are therefore eligible for the mechanism for reducing emissions from deforestation and forest degradation [18,19]. A study conducted by [20] on agroforestry systems in eastern Cameroon showed that cacao trees store less carbon than other associated trees. The average carbon content of these systems is 107 t C ha⁻¹ and depends, above all, on the associated trees which contribute more than 95%.

In Côte d’Ivoire, agroforestry practices seem very old; however, data concerning the complete evaluation of the ecosystem services of agroforestry systems, in particular those on the rate of sequestered carbon, are almost non-existent [21]. A draft assessment of carbon levels in cacao agroforestry systems was recently initiated in the Department of Lakota Centre-western Côte d’Ivoire [14]. In this zone, mature trees in agroforestry systems have an aboveground biomass ranging from 195.4 (young farms) to 751.2 kg ha⁻¹ (old farms).

Furthermore, several valuable local fruit species, such as Beilschmiedia mannii, Garcinia kola, Irvingia wombolu, and, notably, Ricinodendron heudelotii, exist in fallows and cacao farms and are appreciated by populations. Indeed, the relative abundance of Ricinodendron heudelotii, commonly called “Akpi”, and its seed production are higher in secondary forests than in primary forests in Central and West Africa, suggesting the possibility and the easy integration of the species into agroforestry programs aimed at creating tree plantations in association with local crops, such as cacao and coffee, in areas where the forest has been destroyed [22]. This approach would allow farmers to benefit not only from the sales of the harvest of their crops, but also from those of the fruits of R. heudelotii from their farms [21]. Very little information exists on the species associated with the cacao tree in Côte d’Ivoire and, in particular, R. heudelotii. This situation aroused the interest of this study on the integration of the species in cacao-based agroforestry system models to generate knowledge on its production, its interaction with cacao trees, and its impact on the cacao yield. The hypothesis of this study is that the agroforestry systems model based on T. cacao and R. heudelotii would provide information on improving the yield of producers’ cacao plots and would reduce the ecological impacts linked to deforestation. The purpose of this study was to improve cacao farming by association with R. heudelotii for a sustainability production in Côte d’Ivoire. More specifically, it involved analyzing the dendrometric parameters of the species in the localities of Buyo, Duékoué, Guéyo, Soubré, and San-Pedro, evaluating its influence on the density and vigor of cacao trees, quantifying the biomass and carbon stock produced by this agroforestry system, and assessing the potential yield of R. heudelotii fruits and cacao depending on the proximity of cacao trees to the species. The sites were chosen primarily for the importance of the species for farmers, which was measured by the abundance of R. heudelotii in cacao plantations and by the use of the species or its products by local populations [23].

2. Materials and Methods

2.1. Study Area

The study was conducted in Buyo, Guéyo, Soubré, Duékoué, and San-Pedro in western and southwestern Côte d’Ivoire from 2019 to 2020 (Figure 1). Buyo, Guéyo, and Soubré are located in the Nawa Region in southwestern Côte d’Ivoire. The weather is characterized by a dry season (December–March) and two rainy seasons (April–June and September–November). The mean temperatures were between 26 and 28 °C. The average rainfall was between 1300 and 1600 mm/year for 115 days of rain [24]. The soil was ferralitic with dense, humid, intermediate evergreen forest vegetation that has been reduced over time in favor of huge plantations of perennial crops, such as cacao [24].

San-Pedro has a relief formed by uplands and hills with a maximum altitude of 600 m and plains, and soil made up of numerous lowlands, suitable for both cash crops and food crops [25]. The climate is of the humid tropical type, with an average rainfall of 1740 mm per year and an annual temperature oscillating between 26.4 and 27 °C [25].

Duékoué is located in a forest and mountainous area in the west of the country [26] and benefits from a humid tropical climate characterized by a rainy season from April to October, with 1182.8 mm average precipitation, and a drought, which covers the period from November to March [27]. The mean temperature is 25 °C [28] with 98% relative humidity, with dense evergreen forest type vegetation [27]. The soil is clay-sandy [29].

2.2. Material and Methods

2.2.1. Plant Material

A survey was carried out in the study areas to identify cacao plantations containing one or more R. heudelotii trees (Figure 2). However, only farms in full production, in which there were one or more trees of R. heudelotii, were retained. Thus, 66 trees of R. heudelotii were identified in cacao plantations, including 25 in Guéyo, 11 in San-Pedro, 10 in Soubré, 10 in Buyo, and 10 in Duékoué.

2.2.2. Methods

• Characteristics of Ricinodendron heudelotii trees in traditional agroforestry systems

For each R. heudelotii tree identified, height and circumference measurements at breast height (or 1.30 m from the ground) were taken. The values of the circumferences collected on each tree were then converted into diameters by estimating that the shape of the section of the trunk was circular, according to the following formula:

$$\mathrm{D}= \mathrm{C}/\mathsf{\pi }, \mathrm{with} \mathrm{C}=\mathrm{Circumference} \left(\mathrm{cm}\right), \mathrm{D}=\mathrm{Diameter} \left(\mathrm{cm}\right), \mathrm{and} \mathsf{\pi }=3.14.$$

The estimation of R. heudelottii above biomass and its carbon sequestration rate was then assessed.

- Calculation of R. heudelotii biomass

The R. heudelotii biomass was calculated using the allometric model of [30]. This was chosen for our study because it incorporates the dendrometric parameters existing at tree level (height, diameter, wood density):

AGB = exp [−2.977 + 0.94 ln(Wi × (DBHi²) × Hi)]

with AGB = aerial biomass of individual trees (kg), Hi = tree height (m), Wi = wood density (g.cm⁻³), and DBHi = diameter at breast height (cm). The DBHi was measured at 1.3 m from the ground [31] and the wood density of Ricinodendron is 0.40 g.cm⁻³, according to [32].

- Above carbon sequestration rate

Estimating the carbon stock of a plant in an ecosystem depends on knowing the dry biomass. The carbon stock is linked to the biomass by the relationship:

$\mathrm{C} \left(\mathrm{t} \mathrm{C}/\mathrm{ha}\right)= \mathrm{CF} \times \mathrm{AGB}$ (Kg), where CF is the biomass to carbon conversion factor. It has been reported that the carbon contained in the dry biomass of a tree is 50% [33,34].

Productivity was evaluated from the fruit load of the identified R. heudelotii trees. It involved picking up and counting the fruit that fell from each tree. This activity was carried out during the tree production period, from March to October. The frequency of collection was three rounds per week for each tree. These fruits were weighed to determine the fruit mass per tree.

An analysis of the effect of R. heudelotii on neighbor T. cacao was conducted. In each identified farm, lines of 10 and 20 m in radius were delineated around each R. heudelotii, taking it as the center of the disc, delimiting the line. On each cacao, and within the 10 and 20 m lines, agro-morphological and yield measurements were carried out. This design was similar to the one used by [35] during their study on the impact of the cacao swollen shoot disease.

• Agro-morphological parameters of the cacao density of cacao trees near the R. heudelotii trees

Cacao density was assessed by counting the cacao trees present in each belt. The measurement of parameters, such as the diameter at the collar and the height, was performed. These parameters allowed us to calculate cacao vigor [36]:

$$\mathrm{Vigor} \mathrm{of} \mathrm{cacao} =\left(\mathrm{Height} \mathrm{of} \mathrm{cocoa} \mathrm{trees} \left(\mathrm{m}\right)\right)/\left(\mathrm{Diameter} \mathrm{at} \mathrm{the} \mathrm{collar} \mathrm{of} \mathrm{cocoa} \mathrm{trees} \left(\mathrm{m}\right)\right)$$

The calculation method used for R. heudelotii trees was the same for cacao. However, for cacao trees, the diameter considered is that at the collar at 20 cm from the ground, and the density of the wood considered is 0.42 g.cm⁻³ [31].

- For Biomass: $\mathrm{AGB} = \exp \left[-2.977+0.94 \ln \left(\mathrm{Wi} \times \left(\mathrm{DBHi}²\right)\times \mathrm{Hi},\right)\right]$

with AGB = aerial biomass of individual trees (kg), Hi = height of cacao trees (m), Wi = tree density wood (g.cm⁻³), and DBHi = diameter at the collar (cm).

- For carbon stock: $\mathrm{C} \left(\mathrm{t} \mathrm{C}/\mathrm{ha}\right)= \mathrm{CF} \times \mathrm{AGB} \left(\mathrm{Kg},\right)$

with C = carbon stock of trees and CF = biomass to carbon conversion factor.

• Determination of cacao tree productivity

Cacao yield was evaluated by determining their pods’ load. In fact, in addition to quantifying the number of fruits present on each tree, we counted the quantity of gnawed and rotten fruits per cacao tree present in the strips. Their rates (%) were then calculated using the following formulas:

$$\mathrm{Bitten} \mathrm{pod} \mathrm{rate} =\left(\mathrm{Number} \mathrm{of} \mathrm{bitten} \mathrm{pods}\right)/\left(\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{pods}\right)\times 100$$

and

$$\mathrm{Rotten} \mathrm{pod} \mathrm{rate}=\left(\mathrm{Number} \mathrm{of} \mathrm{rotten} \mathrm{pods}\right)/\left(\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{pods}\right)\times 100$$

2.2.3. Statistical Analysis

The general linear model (GLM) analysis of variance of the SAS software (9.4) was used to determine the cacao tree–R. heudelotii interaction according to the Student-Newman-Keuls test at the 5% threshold. Multivariate analyses were performed with five morphological parameters and two performance parameters using Xlstat 2013 software. Thus, Principal Component Analysis (PCA) and Hierarchical Ascending Classification (HAC) were carried out to study all seven morphological and yield variables simultaneously. The analyses characterized the links between the variables and grouped the most productive R. heudelotii trees and those which were less so on the basis of their similarity according to the different variables.

3. Results

3.1. Dendrometric Parameters and Productivity of Ricinodendron heudelotii

3.1.1. Morphological Characters, above Biomass, and Its Sequestered Carbon

The analysis of the variance of the morphological characters, the biomass, and the carbon sequestration of the R. heudelotii revealed significant differences between the localities for all the parameters (p < 0.0001). The means of these various parameters were classified in a homogeneous way according to each locality (

Table 1. Dendrometric parameters of R. heudelotii.

Localities	Height (m)	Circumference (cm)	DBH (cm)	Biomass (kg)	Stock of CO2 (t C/ha)
Guéyo	24.56 a*	320.68 b	102.12 b	574.7 a	287.3 a
San-Pedro	19.41 b	237.00 c	75.47 c	250.22 b	125.1 b
Soubré	17.82 b	385.30 a	122.70 a	548.7 a	274.3 a
Buyo	16.12 b	310.60 b	98.91 b	327.08 b	163.5 b
Duekoué	13.31 c	185.50 c	59.074 c	113.3 b	56.6 b
Means	19.69	294.51	93.79	362.8	181.3
Pr > F	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

* In the same column, the means followed by the same letter are statistically identical at the 5% (Student-Newman-Keuls test)/DBH: Diameter at breast height.

).

For all the morphological parameters (height, circumference, dbh), the highest values were observed in the localities of Buyo, Guéyo, and Soubré. With an overall average of 19.69 m, the height of the R. heudelotii varied from 13.31 m (Duékoué) to 24.56 m (Guéyo). Trees with large circumferences were recorded in Soubré (385.30 cm), while low values were observed in Duékoué (185.5 cm) and San-Pedro (237.0 cm).

With regard to the quantity of biomass and the carbon stock of R. heudelotii trees, the highest values were observed in Guéyo with 574.7 kg for the biomass and 287.3 t C/ha for the carbon stock, followed by trees from Soubré (548.7 kg for biomass and 274.3 t C/ha for carbon stock) and Buyo (327.08 kg for biomass and 163.5 t C/ha for carbon stock). The lowest values were recorded at Duékoué, with a biomass of 113.3 kg and a carbon stock of 56.6 t C/ha.

3.1.2. Productivity of Ricinodendron heudelotii in Traditional CAS

The statistical description of the productivity of R. heudelotii showed significant differences at the locality level for the parameters (p < 0.05). Results indicated that the locality of Soubré recorded the highest values for the number and mass of Akpi fruits, with 4869 fruits and 4869 kg, respectively (

Table 2. R. heudelotii fruits number and mass by locality.

Localities	Number of Fruits	Mass of Fruits (g)
Soubré	4869 a*	140,478 a
Guéyo	2139 ab	62,899 ab
Buyo	1312 ab	37,080 b
Duékoué	759 b	20,727 b
San-Pedro	300.8 b	2374.8 b
Means	588	58,074.07
Pr > F	0.0235	0.0163

* In the same column, the means followed by the same letter are statistically identical at the 5% (Student-Newman-Keuls test).

). Low values were recorded at San-Pedro with 300.8 fruits and 2374.8 kg.

The potential elite trees of Ricinodendron heudelotii were analyzed. Indeed, the principal component analysis (PCA) and ascending hierarchical classification (HAC) were performed on 51 fruit-bearing trees according to morphological and yield parameters. The results revealed the strongest correlations between the variables.

The matrix of Pearson correlation coefficients (

Table 3. Correlation matrix between morphological and yield parameters.

Parameters	Nbfr	Masf	Haut	Circ	Biom	CO2	Diam
Nbfr	1 *	0.988	0.115	0.607	0.676	0.676	0.439
Masf		1	0.115	0.615	0.683	0.683	0.418
Haut			1	0.390	0.348	0.348	0.016
Circ				1	0.984	0.984	0.664
Biom					1	1.000	0.650
CO2						1	0.650
Diam							1

* Values in bold are different from 0 at significance level α = 0.05. Nbfr = number of fruits; Masf = mass of fruit; Haut = height; Circ = circumference; Biom = biomass; CO₂ = carbon stock; Diam = diameter.

) revealed the strongest links between morphological parameters and those of the R. heudelotii tree yield. Thus, the largest positive correlations related the number of fruits to the mass of fruits (0.988), the biomass to the circumference (0.984), and the carbon stock to the circumference (0.984). Several other positive correlations were also highlighted between the number of fruits and parameters such as circumference (0.607), biomass (0.676), CO₂ stock (0.676), and diameter (0.439). Correlations between fruit mass and height (0.115), circumference (0.615), biomass (0.683), CO₂ stock (0.683), and diameter (0.418) were also observed. Height was positively correlated with circumference (0.390) and CO₂ (0.348). Diameter was also correlated with circumference (0.664), biomass (0.650), and CO₂ (0.650).

Information provided by the three components, retained on the basis of their eigenvalues greater than or equal to 1, was 84.45%. Axis 1 with an eigenvalue of 5.618, expressing 70.23% of the total variability, was the most important for the characterization of elite trees. The parameters that contributed to this axis were the number of fruits, the mass of fruits, the circumference, the diameter, the biomass, and the stock of CO₂. Axis 2, with an eigenvalue of 1.138, corresponding to 14.22% of the total variability, is represented by the height of the trees. The representation of R. heudelotii morphological and yield parameters along axes 1 and 2 (Figure 3) showed two groups of trees.

The hierarchical ascending classification (HAC) (Figure 4) indicated three large groups of elite trees according to their morphological and yield parameters. The first group was composed of the most productive R. heudelotii trees from the localities of Guéyo (Koakpi3, Koakpi7, and Koakpi9), Buyo (Kbakpi1, Gbakpi2, Gbakpi3, Gbakpi4, Gbakpi5, Gbakpi6, Gbakpi7, Gbakpi8, Gbakpi9, and Gbakpi10), Soubré (Pbakpi1, Pbakpi2, Pbakpi3, Pbakpi4, Pbakpi5, Pbakpi6, Pbakpi7, Pbakpi8, Pbakpi9, and Pbakpi10), and Duékoué (Diakpi2, Diakpi4, Diakpi6, and Diakpi7). It was characterized by the number of fruits (Nbfr), the mass of fruits (Masf), and the diameter of the trees (Diam). The second group was made up of R. heudelotii from the localities of San-Pedro (Yeakpi1, Yeakpi2, and Yeakpi4), Duékoué (Diakpi1, Diakpi3, Diakpi5, Diakpi9, and Diakpi10), and Guéyo (Koakpi4, Koakpi5, and Koakpi10). This group was characterized by the height (Haut), the circumference (Circ), the carbon stock (CO₂), and the amount of tree biomass (Biom). The third group was composed of the least productive R. heudelotii trees from San-Pedro (Yeakpi3 and Yeakpi7), Guéyo (Koakpi1, Koakpi2, Koakpi6, and Koakpi8), and Duékoué (Diakpi8). This group was not characterized by any of the studied parameters.

3.2. Ricinodendron heudelotii–Theobroma cacao interaction

3.2.1. Density and Vigor of Cacao Trees around R. heudelotii Trees

The analysis of the variance of the density of cacao trees revealed a significant difference according to the proximity of the cacao trees to R. heudelotii trees (p < 0.001). However, no statistical difference was observed for the vigor of cacao trees according to the distance from R. heudelotii tree (p > 0.2218) (

Table 4. Vigor and density of cacao trees around R. heudelotii.

Distance T. cacao–R. heudelotii	Cacao Density	Cacao Vigor
0–10 m	34.57 b*	0.1150 a
10–20 m	76.66 a	0.1144 a
Means	55.61	0.1146
Pr > F	<0.0001	0.2218

* In the same column, the means followed by the same letter are statistically identical at the 5% (Student-Newman-Keuls test).

).

The cacao density of 36 trees within a radius of 10 m from R. heudelotii trees was lower than that of the radius of 20 m with 77 cacao trees. The average values observed for the vigor of cacao trees of 0.115 and 0.114 in the radii of 10 and 20 m, respectively, were statistically identical.

3.2.2. Biomass and Carbon Sequestration of T. cacao Trees near R. heudelotii Trees

The analysis of the variance of biomass and carbon stock of cacao trees showed significant differences according to the proximity of the companion tree (p < 0.05) (

Table 5. Biomass and carbon stock of cacao trees.

Distance T. cacao–R. heudelotii	Biomass (kg)	Stock of Carbon (t C/ha)
0–10 m	102.4 a*	51.21 a
10–20 m	85.6 b	42.81 b
Means	94.0	47.01
Pr > F	0.0026	0.0026

* In the same column, the means followed by the same letter are statistically identical at the 5% (Student-Newman-Keuls test).

). The biomass and carbon content values recorded in the 10 m radius (102.4 kg for the biomass and 51.21 t C/ha for the carbon stock) were higher than those of the 20 m radius (171.28 kg for biomass and 85.64 t C/ha for carbon stock).

3.2.3. Productivity of T. cacao around R. heudelotii Trees

The productivity of cacao trees was not affected by their proximity to R. heudelotii trees for parameters such as the number of pods (p > 0.80) and the rate of gnawed pods (p > 0.63). However, the number of rotten pods showed a significant difference (p < 0.05) between cacao trees. The number of pods of 20.09 in the 10 m radius and 19.88 in the 20 m radius were statistically identical, just like the rate of gnawed pods. However, the average rate of decay is high in the radius of 20 m with 28.2% compared to that of 10 m with 24.9% (

Table 6. Number of pods, rate of gnawed and rotten pods per line.

Distance T. cacao–R. heudelotii	Number of Pods	Rate of Gnawed Pods (%)	Rate Rotten Pods (%)
0–10 m	20.09 a*	6.07 a	24.93 a
10–20 m	19.88 a	5.78 a	28.23 b
Means	19.95	5.87	27.12
Pr > F	0.80	0.63	0.04

* In the same column, the means followed by the same letter are statistically identical at the 5% (Student-Newman-Keuls test).

).

4. Discussion

4.1. Morphological Characteristics and Productivity of R. heudelotii

The study revealed a variability both in terms of dendrometric parameters and in terms of biomass production and its carbon sequestration depending on the area where R. heudelotii is found. However, the highest values were observed in the localities of Guéyo, Soubré, and Buyo. This could be due to the ecology of these areas which is different from others. This area has an ecology characterized by high rainfall (1.300 to 1.600 mm/year) and dense humid forest vegetation gradually reduced to cacao plantations [24], a habitat in which R. heudelotii fully flourishes. These results are in line with those of [37], which showed that R. heudelotii is typical of humid, secondary forests. It then adapted to areas of fallow, savannah, and cacao farms due to the disappearance of forests. According to [38], the reason why the species is found almost everywhere is due to its ease of adaptation to the environment in which it develops. According to these authors, R. heudelotii is one of the forest species of tropical Africa used as a model for studying the seasonal variations of trees growing in unstable areas that have undergone strong anthropogenic activities or natural disturbances, such as bush fires. This variation in morphology goes hand in hand with the amount of biomass and carbon stored by trees, the highest values of which are found in the same localities. There could, therefore, be a link between the dendrometric characteristics of trees and their capacity to store biomass and carbon. The more the tree grows, the more its capacity to store carbon increases. These assertions are consistent with those of [39], who showed, during a study on the carbon stock of R. heudelotii in Cameroon, that height and diameter at breast height (DBH) have a positive influence on stored carbon. For these authors, tall trees with a large diameter sequester more carbon than those with low parameters. However, [40] added that morphological parameters alone are not enough to determine which tree stores carbon best. Consideration should also be given to the density and age of the stand.

This study allowed us to determine that a tree of R. heudelotii can have an average height of 19 m with 2.94 m in circumference and 0.93 m in diameter in the localities studied. The average values of biomass and its carbon stock (181.3 t C/ha) observed in our study proved to be lower than those found by [41] in cacao-based agroforestry systems on Congo cacao trees, which was 313.56 t/ha. This difference is probably due to the diversity of environments and tree densities which seem different from that of our study. It is also caused by the allometric equations used, which are different.

In terms of productivity, R. heudelotii trees in the Nawa region (Buyo, Guéyo, and Soubré) have the best fruit yields compared to other localities. Our study highlighted the relationship between the yields obtained and the morphological parameters (height, circumference, diameter, biomass, and carbon stock) in order to select the most productive trees (elite trees). This study showed that there is indeed a relationship between the morphology of trees and their yield. Trees such as Koakpi7, Pbakpi6, Koakpi8, or even Koakpi10 are characterized by a large diameter, a great capacity to store biomass and its carbon, as well as an excellent fruit yield. They could, therefore, be used in the implementation of the cacao-based agroforestry systems. These results are in line with those of [42] on the study of the financial profitability of cacao agroforests in central Cameroon. He claimed that as the tree grows, its yield becomes quite large.

4.2. Relationship Ricinodendron heudelotii × Theobroma cacao

The results showed that the density of cacao trees varied from one radial to another. There are few cacao trees near the R. heudelotii tree (34 cacao trees in the 10 m strip) compared to 76 in the 20 m strip. This could be explained by the fact that R. heudelotii has a fairly large trunk which would force the producer to reduce the density of cacao trees near it. According to the information received from the farmers, the low density around the associated tree facilitates access for harvesting almonds and avoids competition between R. heudelotii and cacao trees for light. According to them, when the cacao trees are present near the companion species, they tend to rise more quickly in search of the sunlight, which they are deprived of because of the shade of the R. heudelotii crown. This makes it difficult to harvest the pods formed at height. Despite this difference in density, the vigor of the cacao trees is in no way affected by the presence of the tree. Cacao trees showed the same vigor both when they are close to the tree and when they are 20 m away from it. These results are consistent with those of [43], who maintained that the trees associated with cacao trees had no impact on the morphological characteristics of these. According to him, the morphological characteristics of the cacao tree make it possible to set up an agroforestry system and to manage it over the long term. In addition, the cacao tree is a typical shade plant due to its origins and the physiological characteristics of its photosynthetic apparatus. Furthermore, ref. [44] stated that in a good part of cacao-producing countries, such as Côte d’Ivoire, cacao farming is based on an unsustainable technical model where the cacao tree is grown without shade, to the detriment of forest species. For these authors, it would be important to associate this crop with woody species. Reducing cacao density near associated trees would not prevent cacao trees from developing normally. The establishment of a sustainable cacao-based agroforestry system that benefits everyone will, therefore, have to take into account the density of cacao trees near these associated trees.

Results also showed that R. heudelotii does not affect the yield of pods or the rate of gnawed pods produced by the cacao trees. The number of gnawed pods was 24 and did not vary significantly at the level of the two lines. These results are different from those of [45], who argued that the presence of R. heudelotii in cacao-based agroforestry systems should help increase the yield of the cacao farm because it improves soil fertility.

At the pod level, however, a positive influence of the companion tree was observed for the rate of rot. The average rate recorded at the level of cacao trees located within the radius of 10 m is 24%, and that obtained at the level of 20 m is 28%. Therefore, R. heudelotii appears to be a species that favors the reduction of the rate of pod rot. These results are contrary to those of [46], who mentioned that cacao trees located close to trees are subject to shade pressure from companion trees, thus leading to pod rot which results in a poor yield of cacao trees. Additionally, ref. [47] added that shading modifies the amount of light, temperatures, and air movements in the cacao farm and directly affects the photosynthesis, growth, and yield of the cacao tree. The stability of the yield and the decrease in the rate of pod rot can be explained by the fact that there are several varieties of cacao, some of which are able to flourish better under shade from associated trees than without shade. In addition, in most of the cacao-based agroforestry systems studied, farmers use unimproved varieties of cacao from various origins at 71% in Côte d’Ivoire [48]. Among this diversity of varieties, the one that is most encountered in the farms, which were the subject of our study, is the Amelonado variety, commonly called “French cacao”. This variety and its hybrids are, according to the study carried out by [49], the most resistant to the shade of associated trees. Indeed, they manage to develop fully in complex agroforestry systems. This variety would therefore be a brake on the increase in pod rot and therefore on the increase in the yield of marketable cacao. It would therefore be a wise choice for farmers who would benefit from using it in their cacao-based agroforestry systems.

The average biomass and the average stock of carbon stored by the cacao trees present in the 10 m strip were clearly higher than the values of those encountered in the 20 m strip. R. heudelotii trees would, therefore, have a positive influence on the production of biomass and on the capacity of cacao trees to sequester carbon. These results agree with those of [50], who showed that agroforestry made it possible to increase CO₂ capture while reducing greenhouse gas emissions associated with deforestation and intensive agriculture. According to him, one agroforestry hectare provides goods and services equivalent to 5 or 20 ha of deforested land.

The average carbon stock of cacao trees, which was 47.01 t C/ha in this study, is lower than that of R. heudelotii (181.3 t C/ha). This result is similar to that of [20], but differs somewhat in the values found. Indeed, the study of these authors showed that cacao trees store less carbon than associated trees. However, the value observed in our study remains lower than that obtained by these authors (107 t C/ha). This difference is probably due to the diversity of environments and densities of trees, which seems different from that of our study, and also due to the allometric equations used, which are different. Moreover, with the same yield in the two sections of the study design, one might think that the species would not impact the cacao yield, or that the interaction between it and the cacao tree would depend on the cacao variety used or the planting material. This association of species is a favorable asset for reducing the pressures exerted on natural forests by actively participating in the reduction of greenhouse gas emissions associated with deforestation and migratory agriculture. In addition, the producers confided that they integrate R. heudelotii into agricultural systems for its kernels, which are consumed and marketed. This species provides them with an additional income to cacao, which allows them to meet their needs. In view of these results, we can recommend the use of R. heudelotii as a companion species in cacao-based agroforestry systems. However, it would be desirable to use varieties of cacao trees that are resistant to the effect of shade from associated trees. The fruits of the best trees that have been listed in the Nawa region must be used for the establishment of seedlings that will be used in agroforestry systems. The density of cacao trees around the associated trees should be reduced as much as possible in order to facilitate access and the harvesting of kernels from the trees.

5. Conclusions

The objective of this study was to improve cacao farming by combining woody species, such as Ricinodendron heudelotii, for production sustainability in Côte d’Ivoire. It appears from this study that the association of R. heudelotii, a ligneous species, with cacao trees actually could contribute to the improvement of production. It does not interfere with the good growth and yield of cacao trees. However, the density is reduced around the companion tree within a radius of 10 m. The presence of R. heudelotii in cacao farms helps to reduce the rate of pod rot. This woody species has also improved above biomass production and the ability of cacao trees to sequester carbon. R. heudelotii deserves to be used in cacao agroforestry systems. However, these results do not allow us to say this with certainty because several factors, such as the age of the species, that of the plantation, and the different varieties of cocoa tree and their density, were not taken into account in this study. That is why the study should be continued and completed with the aim of consolidating the results obtained and drawing appropriate and definitive conclusions as to the effective implementation of such a system. Thus, it will be necessary to:

• conduct studies over several years on plantations of different ages to determine the influence of R. heudelotii on the morphology of cacao trees according to their age;
• evaluate the growth site of T. cacao and R. heudelotii on energy storage in the biomass of woody plants;
• assess the root system of the species and the cacao trees;
• study the influence of R. heudelotii on different cacao varieties in order to select those best suited for the design of sustainable agroforestry systems;
• study the influence of climate variability on the behavior of R. heudelotii trees by dendrochronological analyses;
• study the impact of light transfer on R. heudelotii and cacao trees;
• perform isotopic analysis of carbon stock to see fluctuating climate variability in study areas;
• compare the carbon stock of traditional cacao-based agroforestry systems with those of secondary forests;
• characterize the types of harmful R. heudelotii capable of harming the cacao trees;
• assess the income that the species could bring to producers;
• domesticate the species;
• extend the study to all cacao-growing areas of Côte d’Ivoire in order to control all its contours.