Local knowledge and effects of associated tree species litter fall on soil health under cocoa systems in the humid forests of Cameroon

Table of Contents

ABSTRACT

1. Introduction

2. Material and methods
2.1. Study sites
2.2. Inventory of associated tree species and local knowledge acquisition
2.3. Determination of chemical content of litter fall of associated tree species
2.4. Statistical Analyses

3. Results
3.1. Diversity and frequency of associated tree species
3.2. Farmer’s knowledge on litter fall production
3.3. Chemical quality of the litter fall of the associated tree species
3.4. Comparison between farmers’ ranking and measured nutrient contents

4. Discussion

5. Conclusion

ACKNOWLEDGEMENTS

REFERENCES

AKONNUI Ferdinand ² ; Bidzanga Nomo ¹ ; ABANG A MBANG Joseph ¹ , AMBANG Zachee ² .

ABSTRACT

Investigations were conducted to assess farmers’ perceptions of the effects of associated tree species on soil fertility status under cocoa-based systems in the Evodoula district one of the oldest cocoa producing basin of southern Cameroon characterized by a high population density and land use intensity. Farmers were asked to rank, based on their knowledge of the fertilizing patterns of associated species, ten trees species with the most positive effects on soil fertility. The ranking was tested with respect to the amount of litter fall and the quantity of minerals released (N, P, K, Ca and Mg) as factors affecting the nutrient recycling process. In decreasing order of importance, the top ten species identified were Milicia excelsa, Alstonia boonei, Terminalia superba, Irvingia gabonensis, Ceiba pentandra, Ficus mucoso, Musanga cecropoides, Coula edulis, Eribroma oblungum and Zanthoxylum heitzi.

The mean litter fall ranged from 19.03 to 0.52 kg/ha and N was the main nutrient in the leaf litter of the different tree species with its concentration varying from 29.81 to 53.56 g/kg and a mean of 42.31 g/kg. P was present in very low concentrations (from 0.4 to 1.96 g/kg) while K varied widely from 0.76 to 6.82 g/kg. Mg was quantitatively the second element in the leaf litter, with values ranging from 8.84 to 37.29 g/kg. A strong correlation (P < …) was found to exist between the farmers’ ranking and the chemical composition of the litter , promoting thereby the assertion that the integration of farmers’ knowledge in global science could contribute in improving the understanding of the phenomenon as well as the adoption rate amongst end users.

Key words: cocoa, litter fall, fertility, nutrient content, local knowledge.

1. Introduction

In many tropical landscapes, agroforestry systems (AFS) are the major agro ecosystems that resemble natural forest [5; 14] as they combine a diversity of animal and plant species to provide socio-economic and ecological services [7; 19].

The cocoa tree (Theobroma cacao) is commonly grown under shade trees in order to reduce physiological and hydric stress [24]. Cocoa growers in major production basins have, based on their knowledge of the forest and cocoa agronomic management skills, developed cultural practices that associate the crop with other indigenous and exotic tree species [12; 6; 14]. In southern Cameroon, such systems are largely based on old cocoa orchards, indicating thereby the feasibility of a sustainable cocoa-based agroforestry model [14]. Such systems have been found to generally result from the clearing of primary or secondary forests, followed by eventual thinning of the understored and introduction during the establishment phase of cocoa seedlings and other annual crops like banana and plantain. The associated trees are generally used to regulate shade and parasitic pressure [21; 4], as well as to provide ecological services, especially in the field of biodiversity conservation and natural resources management [6; 26; 14]. The result is generally a complex multi-strata and multi-species agro-forest associations whose composition, structure and species richness reflect the ecological and socio-economical needs and interests of the local populations [6; 7; 14; 20; 27].

Although not always recognized by agronomists [9; 6], trees in cocoa-based agroforests have more uses for local farmers than just providing a suitable microclimate for cocoa trees [25]. Agricultural development authorities in Cameroon have encouraged farmers to diversify their income by maintaining and introducing economically-useful plant species (for timber, medicine, fruits, etc.) in the cocoa fields [26]. Some of such species are known for their fertilizing capacities through nutrient recycling. Yet, little information is available on the contribution of such species to the productivity of soils under cocoa fields. It is therefore hypothesized that trees species in cocoa-based farms play a major role in the improvement of soil conditions, hence the productivity of the entire system.

Soil fertility in tropical humid forests is mostly biological. Tree species, vegetation and soil fauna activities are biological indicators of fertile soils [7; 11], through the process of nutrient cycling. However, most resource-poor famers lack the technical means or the mastery of classic indicators to determine the nutrient content of the soils they cultivate. Hence, their appreciation of the soil’s fertility status is mostly based on rather subjective attributes like crop productivity, soil structure and texture; and particularly the vegetation and soil fauna activities [8; 6]. They are knowledgeable about the tree-soil interactions as well as the actions of micro-organisms and climate on litter fall decomposition. Indeed, the complementarities between local knowledge on agroforestry and scientific knowledge on the soil’s fertility under non-cocoa based systems [7].

However, the productivity of cocoa-based agro-forests is constrained in most producing countries, especially with respect to the ageing cocoa systems and the depletion of the soils’ fertility , and in extreme cases, the abandon of plantations, the volatile and highly fluctuating prices of the cocoa produce that leads cocoa producers to associate with alternative crops (such as rubber and palm oil) and pressure from diseases and pests (pod rot ,Capsid and Swollen Shoot) [7].

Nevertheless, these systems have resisted to these constraints and remained productive indicating therefore that the farmers probably master a mechanism that restores soil’s fertility.

This study investigates the effects of associated tree litter fall on soil health under cocoa-based systems based on farmers’ knowledge. If the pertinence of the knowledge’s is established, such measures could be introduced in management plans for cocoa- based systems in order to enhance their ecological and economic performance.

2. Material and methods

2.1. Study sites

The study was conducted in Evodoula District, located in one of the oldest cocoa-producing basins in south Cameroon (Fig. 1). Evodoula lies approximately on latitude 4°5'0"E and Longitudes 11° 12' 0"N, covers a surface area of about 19 km², has a high population density (~ 76 inhabitants/km2) and a high land use intensity. Diversification within cocoa fields is less common.

illustration not visible in this excerpt

Fig. 1. Map of Cameroon map showing study location in its agro-ecological zone

The location of the study falls in the tropical climate regime, characterized by a bimodal rainfall pattern, with average rainfall ranging between 1500 and 2000 mm per annum [12]. The longer rainy season occurs from March/April to May/June, although more rain falls in the shorter period of September - November. The longer dry period in the year extends from December to February, whilst the July to August period, though dry, has sporadic rainfall. In this period, the temperatures are lowest and air saturation relatively high [6]. Above ground temperatures range between 22 and 25 ° C. The vegetation is semi-deciduous evergreen and degrades from the gradient Equator to north [6].

The study area has mostly red ferallateritic, acidic, clayey soils with a low nutrient retention capacity. Physical and hydraulic characteristics of these soils reveal, by their variation in the ferralitic field, the existence of a zonal arrangement due to climate and human activity.

2.2. Inventory of associated tree species and local knowledge acquisition

The mean surface area of the cocoa orchards of the studied zones was 2 ha. A systematic inventory of non-cocoa tree species was done in 15 cocoa-based systems to determine tree species frequency and distributions. The sampling method follows that used by [3; 19]. In each cocoa-based system studied, each tree species (forest, exotic as well as palm tree) was counted, numbered, identified separately and their density per plot estimated. Species identification was based on vernacular names with the assistance of the farmers and correspondences with the scientific names were established from literature review.

Afterwards, a semi-structured questionnaire was administered to each owner of the cocoa-based systems sampled. Information collected elucidated the farmers’ perceptions of the functional attributes of litter fall from the respective tree species and their impact on soil fertility as well as the ranking (in decreasing order of importance) of the ten plant species of the system with the highest fertility potential. A generalized ranking was obtained by calculating the mean value of the position occupied in the individual farmers’ ranking. The empirical classification by farmers of the fertility potential of these ten species was then compared to the classification of the same species based on their respective nutrient contents.

2.3. Determination of chemical content of litter fall of associated tree species

Litter fall of the associated tree species ranked by farmers was collected daily, put into labeled sacks, air dried and taken to the laboratory for compositional analysis of macro-nutrients (nitrogen, phosphorus, potassium, calcium, and magnesium. Analytic procedures followed those of [1]. All analyses were carried out at the Soil, Plant and Water Laboratory of IRAD in Nkolbisson Yaoundé in conformity with standard analytical procedures [1].

2.4. Statistical Analyses

Total litter macro-nutrient and contents of the trees species were obtained by summing the proportion of the respective elements analyzed.

Descriptive analyses were used to determine differences in concentrations of nutrient elements in litter fall between trees species.

3. Results

3.1. Diversity and frequency of associated tree species

Results on the diversity of non-cocoa tree species showed that there were 122 different non-cocoa trees species, belonging to 37 families, in the cocoa-based systems of the study locations. The families identified were mostly of the Sterculiaceae (28 species), Moraceae (22), Mimosaceae (15), Apocynaceae (13), Anacardiaceae (11), Euphorbiaceae (10), Meliaceae (10), Rutaceae (10), Bombaceae (8), Burseraceae (8) and Musaceae (8).

On average, each cocoa-based agro-forest had 48 tree species per hectare associated with cocoa. The total number of trees depended on the farm sizes although the frequency distribution of the associated species was heterogeneous within the sample plots. Although the vegetation of the study sites had been exposed to an advanced level of degradation, several indigenous (1,165) and exotic tree species (491) were still present in the different cocoa agro-forests. For the indigenous species, the most represented were the Milicia excelsa, Mansonia altissima, Albizia adianthifolia, Ficus mucuso, Ficus exasperata, Pycnanthus angolensis, Musanga cecropoides and Terminalia superba while the exotic species were particularly fruit species such as Mangifera indica, Persea americana and Dacryodes edulis.

3.2. Farmer’s knowledge on litter fall production

Survey results showed that all cocoa-based systems studied were dominated by the presence of indigenous trees species. The latter produced more litter than the exotic trees species and the organic matter they produced played a favorable role in maintaining the soils’ fertility. Here, the farmers believe in their protection of the soil against erosion, maintenance and improvement of the soils’ physico-chemical properties (structure; porosity; water retention, and soil nutrient), modification of the soil temperature, and the rapid decomposition of organic matter.

Based on questionnaires analysis, farmers attributed the fertility potential of certain tree species based on the observation of the cocoa production around the tree, leaf size, arrangement, leaf consistency and the positive influence of some trees species (good trees) on the soil fertility. It is understood that the trees species introduced or maintained by farmers are those proximity to the cocoa trees are observed to improve on the later’s productivity through the quality of litter fall and shade, and eventual improvement of the soils’ fertility. The ranking of the tree species with respect to their fertility potential, by local farmers, in increasing order of importance is shown in Table 1.

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¹ Institute of Agricultural Research for Development (IRAD), P. O. Box 2067 Yaoundé, Cameroon

² Department of Plant Biology, Faculty of Science, University of Yaoundé 1.