Plants are natural resources essential for human food and care
[1]
. However, many plant species have disappeared and others have become scarce during the last century due to human activities such as logging, agricultural expansion, urbanization, etc. in Africa
[2]
.
Ivory Coast is not exempt from this specific erosion phenomenon caused by the expansion of cocoa cultivation. This speculation is said to be responsible for the disappearance of the forest. According to
[3]
-
[5]
, this country has lost almost all of its humid tropical forest area from 1960 to 2015, mainly due to the scale of cocoa cultivation
[6]
.
The pure cultivation of cocoa trees has historically been dominant, but it is currently at an impasse because it is not very sustainable. This practice has contributed to 14% of deforestation, significantly reducing forest areas and, consequently, the specific richness of the soils
[6]
-
[9]
. Moreover,
[10]
and
[5]
state that nearly 80% of the forest area in the southwest of Côte d’Ivoire has been cleared for cocoa cultivation. It is therefore urgent to identify solutions to stabilize cocoa-growing areas, reduce pressure on forests and adapt to climate change. A balance can be found between cocoa trees and a mixture of trees chosen by farmers for their various uses, while maintaining a suitable cocoa yield in the long term
[11]
. Agroforestry is a sustainable resource management solution for cocoa farming. It consists of integrating trees into farms, which helps maintain and diversify agricultural production in order to promote the pillars of sustainable development
[12]
.
Recent research shows that many forest species are conserved by cocoa farmers for their multiple uses such as food, timber, firewood, their economic and medicinal importance
[13]
-
[15]
, as well as the agro-ecological benefits of these species such as soil moisture conservation, shading, and soil fertilization
[16]
[17]
. Ricinodendron heudelotii (Baill.) Pierre ex Pax (Euphorbiaceae) and Irvingia gabonensis (Aubry-Lecomte ex O’Rorke) Baill. (Irvingiaceae) are among the species that have a positive impact on cocoa farming and are prized by local populations
[18]
. These two species are used for multiple purposes, including the exploitation of their wood and seeds, which constitutes a real threat to their survival in their natural habitat
[19]
and
[20]
. This threat is all the more concerning as the International Union for Conservation of Nature has classified them on the red list of threatened species in the “Least Concern” category for R. heudelotii
[21]
and “Near Threatened” for I. gabonensis
[22]
. The sustainable management of R. heudelotii and I. gabonensis is therefore of paramount importance both economically, socially and environmentally. Research carried out for the domestication and conservation of these two species has shown many challenges related to the irregular germination and low germination rate of seeds due to their dormancy
[23]
. In addition, strategies for improving seed germination and vegetative propagation techniques by cuttings have been developed using synthetic phytohormones. However, these products are not affordable for farmers due to their high cost, thus limiting initiatives to produce these plants. However, the use of wildlings by farmers appears to be an alternative for the production of plants of these two species. However, the conditions necessary to obtain vigorous plants remain poorly documented. It is with this in mind that this study is being carried out, with the aim of contributing to the production of vigorous seedlings of these two species in view of their sustainable conservation through agroforestry practices.
2. Materiel and Methods2.1. Study Site
This study was carried out in the Soubré department, located in the Nawa region in southwestern Côte d’Ivoire (
Figure 1
). The Soubré department is located between longitudes 6˚19' and 6˚57' West and latitudes 5˚26' and 6˚13' North. The climate is sub-equatorial, characterized by two rainy seasons occurring between April-June and September-November, and two dry seasons occurring between July-August and December-March, with abundant rainfall between 1600 mm and 1800 mm and a temperature ranging between 26˚C and 32˚C throughout the year
[24]
. The vegetation consists of gallery forests, secondary forests, marshy formations, fallow land, and industrial plantations dominated by cocoa
[25]
.
2.2. Vegetative Material
The plant material used in this study consisted of wildlings or resprouts of Irvingia gabonensis and Ricinodendron heudelotii of varying heights. The collar diameter of the plants was 0.50 cm and 0.86 cm for I. gabonensis and R. heudelotii, respectively.
Figure 1. Study area.
2.3. Methods
The collection of resprouts was carried out in the months of March and April 2023. The presence of rain during this period facilitated the uprooting of the plants from the soil. Thus, resprouts of I. gabonensis and R. heudelotii of varying heights were collected under mother trees in fallow land and cocoa plots in Soubré.
On the wet plots, the regrowth was directly uprooted using a palm knife while avoiding damage to the root system. In less humid terrain, a small basin about 10 centimeters deep was dug with a hoe around the plant and then filled with water to facilitate uprooting.
After collection, these resprouts were carefully packaged in coolers to facilitate their transport to the experimental plot.
The preparation of the resprouts consisted of classifying them into three height classes: H1: [0 - 15 cm], H2: [15 - 30 cm], and H3: [30 - 50 cm], in order to evaluate the effect of height on the recovery and growth of the wildlings. Subsequently, the wildlings of each height class were subjected to defoliation treatment retaining 0, 2, 3, and 4 leaves on each wildling, starting from the collar towards the apical bud. This was done to determine the effect of the presence or absence of leaves on the recovery and growth of the plants.
To study the influence of light on the recovery and growth of young plants, nurseries were set up in two different environments. The first environment was illuminated without shading, and the second environment was shaded. The shade in the latter environment came from the canopy of large trees under which the nursery was installed. According to the data from SODEXAM (Airport, Aeronautics and Meteorological Development and Operating Company of Côte d’Ivoire) the average temperatures of the two growing environments are 23˚C and 29˚C for the shaded environment and the illuminated environment respectively during the study period (April-June 2023) in the experimental area. In both environments, the nurseries were established over an area of 200 m2 (20 m long × 10 m wide).
The black polyethylene bags, measuring 15 cm long × 6 cm wide and containing the substrate made of previously sterilized forest topsoil, were arranged according to the experimental design (
Figure 2
).
Figure 2. Diagram of the experimental setup.
The trial was established using a factorial block design with three replicates. The first factor was plant height, with three levels: H1 [0 - 15 cm], H2 [15 - 30 cm], and H3 [30 - 50 cm]. The second factor, defoliation treatment, consisted of four levels, namely the retention of 0, 2, 3, and 4 leaves on the plants. The third factor was light exposure of the environment, with two levels: illuminated environment and shaded environment. In each environment, 12 treatments were carried out (3 height classes × 4 methods of defoliation treatment) for each species. For each treatment, five wild seedlings were used with three replicates, resulting in a total of 180 wild seedlings per species per growing environment (12 treatments × 5 seedlings × 3 replicates). Across both species, a total of 360 wild seedlings were used during the trial. The experimental setup consisted of three blocks. Within each block, three subplots corresponding to the height classes were established. Each subplot included five wild seedlings subjected to the four defoliation treatment (0, 2, 3, and 4 leaves) (
Figure 2
).
The trial monitoring involved regularly watering the plants every two days. Weeds were manually removed from the planting bags to prevent any competition with the wild seedlings. Data collection consisted of recording the number of living and dead plants through simple counting. The height of growing plants was measured every 14 days using a measuring tape, from the collar to the sheath of the last leaf. As for the appearance of new leaves, it was observed every 14 days, and the leaves were counted through simple counting. The collected data were used to evaluate the following parameters: (i) the survival rate of the plants, (ii) the height growth of the plants, and (iii) the number of newly emerged leaves. The survival rate was determined as the ratio of the number of living seedlings to the total number of seedlings at the end of the experiment, using the formula below:
With TS: Survival Rate; N: Total number of plants placed in bags for a treatment; nm: number of dead plants.
The height growth of the plants was calculated at the end of the experiment using the following formula: ACH = (Final height − Initial height).
The number of leaves was determined by counting every 14 days.
Analysis of Variance (ANOVA) was used to compare growth parameters based on the different factors studied. Additionally, MANOVA with the Wilks’ test was applied to simultaneously compare growth data from the two environments (shaded and illuminated). The statistical analysis of the data was performed using RStudio Version 3.5.1.
3. Results3.1. Effect of Height on Survival, Growth, and Leaf Emergence of Irvingia Gabonensis and Ricinodendron Heudelotii Seedlings
Effect of height on the survival rate of seedlings
The analysis of
Table 1
shows that for both species, the survival rate of the seedlings did not vary significantly according to the height classes of the plants. The survival rate is relatively high for the different height classes of R. heudelotii seedlings. It is 80%, 83%, and 88% for the height classes of [15 - 30] cm, [30 - 50] cm, and [0 - 15] cm, respectively. Statistical analysis reveals no significant difference (P = 0.34) between these results. Height has no impact on the survival rate of R. heudelotii seedlings. On the other hand, I. gabonensis seedlings had a low survival rate for all height classes, with rates ranging from 37% for the first two classes ([0 - 15] and [15 - 30]) to 53% for the [30 - 50] cm height class. These results are not statistically different (P = 0.17) and indicate that height has no influence on the survival rate of the plants.
Effects of height classes on stem growth
The height growth of the seedlings in the nursery varied between species (
Table 1
). At R. heudelotii, seedling height significantly influenced their growth (P < 0.001), unlike I. gabonensis. R. heudelotii seedlings in the [0 - 15 cm] height class exhibited the highest growth, with an increase of 8.6 cm, compared to 5 cm and 4.85 cm for the [15 - 30 cm] and [30 - 50 cm] height classes, respectively. At I. gabonensis, the growth of the seedlings was the same (1 cm) regardless of the height class.
Effects of height class on the number of leaves produced
Analysis of
Table 1
showed that for each species, the number of leaves produced does not vary significantly based on the height of the seedlings (P < 0.05). At R. heudelotii, all height classes have approximately the same number of leaves (4 leaves). In contrast, I. gabonensis seedlings produced 1 leaf.
<xref ref-type="bibr" rid="scirp.144590-"></xref>Table 1. Survival rate, growth rate, and the number of leaves of the species.
Species
Height class (cm)
Survival rate (%)
Increase of the stem (cm)
Number of leaves issued
[0 - 15]
37
1
1
Irvingia gabonensis
[15 - 30]
37
1
1
[30 - 50]
53
1
1
P
0.17
<0.001
<0.05
[0 - 15]
80
8.6
4
Ricinodendron heudelotii
[15 - 30]
83
5
4
[30 - 50]
88
4.85
4
P
0.34
<0.001
<0.05
3.2. Effect of Defoliation Treatment on the Survival, Growth, and Leaf Production of Irvingia gabonensis and Ricinodendron heudelotii Seedlings
Data analysis of the effect on survival
The survival rate of seedlings from both species in the nursery varied according to the defoliation treatment method (
Figure 3
). At R. heudelotii, the survival rate of seedlings was above 80% and statistically identical regardless of the defoliation treatment method. On the other hand, the survival rate of I. gabonensis seedlings varied significantly based on the defoliation method (P < 0.001). The highest survival rate (59%) was observed for seedlings without leaves (0 leaves). The lowest survival rate was recorded for seedlings with 4 leaves. The survival of I. gabonensis seedlings decreased with the increase in the number of leaves.
Effect of defoliation on the growth of seedlings
The growth of R. heudelotii is rapid for each defoliation method. The defoliation treatment method with 4 leaves shows the fastest growth, with an average increase of 8 cm. For the defoliation treatment methods of 0 leaves, 2 leaves, and 3 leaves, growth height ranges from 4 cm (0 leaves) to 6 cm (2 leaves). These statistically significant results indicate the influence of defoliation on height growth (P = 0.000). The average height of I. gabonensis seedlings is almost negligible, with an average growth of 1 cm. The defoliation treatment method of 0 leaves results in a higher average growth of 1.3 cm. These differences are not highly significant (P = 0.02), as shown in
Figure 4
.
Effect of defoliation treatment on leaf production of seedlings
The number of leaves for Ricinodendron heudelotii was higher for the different defoliation treatment methods, with an average of 3.5 leaves. defoliation treatment has no influence on leaf production. The average number of leaves was low (0.6 leaves) for Irvingia seedlings. This low number is consistent across the different defoliation methods, with the 0-leaf showing better results compared to the 2-leaf, 3-leaf, and 4-leaf defoliation treatment methods, as shown in
Figure 5
.
Figure 3. Survival rate of wildlings of Irvingia gabonensis and Ricinodendron heudelotii based on the defoliation treatment.
Figure 4. Stem height based on the defoliation treatment.
Figure 5. Number of leaves based on the defoliation treatment.
3.3. Effect of Environment on the Survival, Growth, and Leaf Production of Irvingia gabonensis and Ricinodendron heudelotii Seedlings
Effect of environment on the survival of seedlings
The survival rate as a function of the environment is specific to the species. Both environments recorded survival rates above 80% for R. heudelotii seedlings. In contrast, I. gabonensis seedlings recorded a lower survival rate, ranging from 41% in the illuminated environment to 43% in the shaded environment. Statistical analysis shows that the environments have no significant influence on the survival rate of the seedlings (
Table 2
).
Effect of environment on the growth of seedlings
The growth of R. heudelotii is rapid in both environments, with an average of 6 cm. The shaded environment records the highest growth, with an average height of 7.4 cm compared to 4.95 cm in the illuminated environment. Statistical analysis shows a significant impact of the environment on height growth (P = 0.00). The growth of I. gabonensis is slow, and there is no growth difference between the two environments (P = 0.99), as shown in
Table 2
.
Effect of environment on the number of leaves
Analysis of
Table 2
shows that leaf production in seedlings varied between species depending on the lighting conditions in the nursery. At I. gabonensis, the number of leaves produced (0.6) was the same in both shaded and illuminated environments. However, in R. heudelotii, the highest number of leaves (4.2) was recorded in the illuminated environment.
3.4. Combined Effect of Height and Defoliation Treatment on Growth and Leaf Production of Irvingia gabonensis and Ricinodendron heudelotii Seedlings
- Ricinodendron heudelotii
Analysis of
Figure 6
shows that height growth of seedlings was significantly influenced (P < 0.001) by the interaction of the three factors (height x defoliation x environment). The highest height growth (16 cm) was recorded in the shaded environment with seedlings having a height between 0 and 15 cm and possessing four leaves. In contrast, seedlings with a height between 30 and 50 cm and having 2 leaves, grown in the illuminated environment, showed the slowest growth, with an average of 3.08 cm.
- Irvingia gabonensis
Analysis of
Figure 7
shows that height growth of seedlings was significantly influenced (P < 0.001) by the interaction of the three factors (height × defoliation × environment). The highest height growth (2 cm) was recorded in the illuminated environment for seedlings in the height classes [15 - 30 cm] and [30 - 50 cm], with 0 and 2 leaves, respectively. In contrast, no growth was observed in the different environments.
<xref ref-type="bibr" rid="scirp.144590-"></xref>Table 2. Survival rate, growth rate, and number of leaves based on the environment.
Species
Environment
Survival rate (%)
Increase of the stem (cm)
Number of leaves issued
Irvingia gabonensis
[shaded]
43
1
0.6
[lit]
41
1
0.6
P
>0.05
>0.05
>0.05
Ricinodendron
[shaded]
81
7.4
3.5
heudelotii
[lit]
82
4.95
4.2
P
>0.05
<0.05
<0.05
Figure 6. Height growth of Ricinodendron heudelotii seedlings based on the interaction of environment, size, and defoliation method.
Figure 7. Height growth of Irvingia gabonensis seedlings based on the interaction of environment, size, and defoliation method.
4. Discussion
The growth and survival of plants in the nursery, stem height plays a crucial role. Several scientific studies have examined this complex relationship between stem height and plant survival rates, considering various environmental and biological factors
[26]
. Indeed, stem height is closely related to the plant’s vigor. Plants with taller stems often have more developed root systems, allowing them to anchor better and absorb water and nutrients more efficiently for growth. The high survival rate (80%) recorded in our results for Ricinodendron heudelotii seedlings, compared to those of Irvingia gabonensis, could be explained by the species’ performance. R. heudelotii is known for its rapid growth rate
[26]
. Its seedlings, therefore, seem to be better adapted to transplantation and exhibit rapid vegetative growth recovery, which likely contributed to the higher survival rate in the nursery.
The poor growth performance of I. gabonensis seedlings observed in this study is consistent with the fact that I. gabonensis is initially slow-growing compared with R. heudelotii
[27]
. In addition, some authors have examined the effect of stem height on the survival rate of tree seedlings in a nursery
[28]
. Their results revealed a significant correlation between stem height at the time of transplantation and seedling survival. Taller seedlings exhibited higher survival rates post-transplantation, suggesting that stem height can be an important indicator of plant health and resilience in the nursery. However, it should also be noted that excessively tall stems may present disadvantages. Similarly, others have reported that vegetable plants with excessively long stems had significantly lower survival rates than plants with medium height stems. This study underlines the importance of controlling stem growth to ensure successful plant production in the nursery
[29]
.
A taller stem is often associated with faster vertical growth in many plant species. This is generally due to an increase in the leaf surface area exposed to light, which promotes more intensive photosynthetic activity, leading to higher biomass production. As a result, plants with taller stems tend to have more significant height growth in the nursery. A positive correlation between the initial stem height of maize seedlings and their subsequent height growth over time was observed by authors as
[30]
. Seedlings with taller stems showed faster height growth compared to those with shorter stems, suggesting that stem height can be an early indicator of future growth. These results align with our findings for Ricinodendron heudelotii seedlings, showing a similar pattern between initial plant height and stem growth.
Additionally, defoliation treatment which involves removing some of the leaves from a plant, is a common practice in nurseries for various reasons, such as managing plant vigor, stimulating branching, and reducing water loss and disease risk. However, the effects of defoliation treatment on plant survival rates in nurseries can vary depending on several factors
[31]
. Size can be a determining factor in the survival rates of certain species
[31]
. Proper defoliation practices can influence the ability of plants to mobilize their resources efficiently, thereby enhancing their adaptation to the nursery environment. Demonstrated that selective pruning of apple seedlings in nurseries resulted in a significant increase in survival rates after field transplantation
[32]
. These results align with those observed for Irvingia. gabonensis seedlings, which show an interaction between pruning methods and survival rates, but contrast with those for Ricinodendron heudelotii.
Reducing the number of leaves could help the plant reduce water loss through evapotranspiration, thus preserving necessary reserves for growth despite the environmental change. These results contradict those of a number of authors who have shown that tomato leaf reduction leads to a reduction in the growth of other plant organs and a decrease in photosynthetic activity
[33]
. The high survival rate of R. heudelotii seedlings in both study environments could be attributed to the good health and physiological condition of the plants, as well as favorable environmental conditions for recovery. Seedling growth requires good conditions and care, including soil moisture, leaf reduction and shading
[34]
. Light is a fundamental element in nurseries, and light intensity can have a significant impact on plant survival. Optimal light conditions promote healthy development and increase the chances of survival
[35]
.
Furthermore, the positive effect of the light environment on leaf emergence highlighted in our results can be explained by the attraction of the small bud (the gemmule), from which the leaves emerge, towards the light. Moreover, thanks to photosynthetic activity, the leaves ensure their development by accumulating biomass. This can be attributed to the high level of photosynthetic activity facilitated by the intensity of incident light. Indeed, light has an impact on leaf initiation in Thunbergia Atacorensis (
[36]
Asseh et al., 2017). Furthermore, growth rate and leaf initiation in young cucumber plants increased with the amount of daily light energy
[37]
.
Regarding the results for I. gabonensis seedlings, they showed no difference in growth and leaf production between the two environments. These results, in addition to contrasting with those of R. heudelotii seedlings, also differ from those of
[36]
(Asseh et al. 2017). The work of these two authors on the growth of Thunbergia Atacorensis gave similar results under the same conditions. When it comes to optimizing plant survival rates in the nursery, it is essential to consider the combined effects of several factors, including stem height, growing medium and defoliation. These factors interact in complex ways to influence the growth, vigor and resistance of nursery plants. The interaction of these factors had no influence on the survival rate of R. heudelotii and I. gabonensis plants.
These results contradict those obtained by
[38]
Chen et al., (2020), who studied the combined effects of stem height, light, and defoliation on the survival rate of maize seedlings in nurseries. Their results showed that plants with taller stems, grown under adequate light intensity and subjected to moderate defoliation, had the highest survival rates. These plants exhibited better root development, greater stress resistance, and improved ability to absorb nutrients and water from the soil, which promoted their survival in the nursery. On the other hand, studies have also shown that the interactions between stem height, growing medium, and defoliation can vary depending on the plant species studied. Indeed,
[39]
Garcia et al., (2018) evaluated the effects of these factors on the survival rate of fruit tree seedlings in nurseries. Their results showed species-specific responses, emphasizing the importance of considering the needs of each crop when managing nursery cultivation.
However, it is worth noting that the results of this work, based on observations over a short period of two months, could be improved by similar studies over a longer period, possibly 3 to 4 months for fast-growing species like and 6 to 8 months for slow-growing species like Irvingia gabonensis
[40]
.
5. Conclusion
This study demonstrated that the propagation of Ricinodendron heudelotii and Irvingia gabonensis is possible through wildings. However, the ability of R. heudelotii to adapt quickly in nurseries, particularly with regrowth of 0 to 15 cm in height and having 4 leaves in shaded environments, makes it an ideal choice for agroforestry projects. In contrast, for I. gabonensis, the shoots measuring 15 to 30 cm in height with 0 leaf and those 30 to 50 cm in height with 2 leaves placed in a well-lit environment show good growth potential. The proper development of the plants after transplantation requires specific morphological, physiological, and ecological conditions for each species. A better understanding of the interactions between the morphological, physiological, and ecological factors of these species could significantly improve the production of their plant material through nursery regrowth, contributing to their sustainable conservation through agroforestry. However, this requires in-depth complementary studies on their environmental impact in association with cocoa cultivation and the growth of young plants in plantations.
Novelty Statement
Irvingia gabonensis and Ricinodendron heudelotii are forest species known to rural populations for their multiple uses, which unfortunately pose a threat to their sustainable conservation. In the face of difficulties in the natural regeneration of these two species, this study showed that the production of their plant material in nurseries is possible from shoots taken at an appropriate height, in a favorable lighting environment after suitable treatment; with a view to their domestication.
Acknowledgements
The first author acknowledges Felix Houphouët Boigny University for the academic training and the NGO Water and Honey Switzerland for the partial funding of this study.
Author Contributions
Affessi Alain Jiani GITTE and Souleymane SANOGO planned the experiments which are practiced by Brou Christian KOUAME, Jonas Patrick DAO interpreted the results, Affessi Alain Jiani GITTE made the write up statistically analyzed the data and made illustrations.
Data Availability
Data presented in this study will be available on a fair request to the corresponding author.
Ethics Approval
This work was carried out in accordance with the ethics and deontology of the Université Félix Houphouët Boigny of Ivory Coast.
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