Shade trees have limited benefits for soil fertility in cocoa agroforests

Abstract

A B S T R A C TAgroforestry is often promoted as a sustainable agricultural practice that can ameliorate causes of declining yields, such as soil degradation. However, despite the often-stated potential of agroforestry, quantitative data on the benefits of shade trees are limited to relatively few cropping systems, particularly maize and coffee. Furthermore, agroforests are not cost-free and the benefits of agroforests might not be sufficient to outweigh these costs in all cropping systems or environments. Here we quantify costs and benefits of agroforests for cocoa production in Ghana, West Africa. Specifically, we quantified the ability of shade trees to increase soil carbon stocks and soil fertility (i.e. total soil carbon, nitrogen and phosphorus, available phosphorus and potassium, cation exchange capacity, soil aggregation, pH, and foliar nitrogen and phosphorus concentrations), and investigate if these benefits are sufficient to outweigh the negative effects of shade trees on cocoa growth and yields. We measured cocoa yields, soil fertility and carbon-sequestration under individual shade trees, and in 30 × 30 m plots that were distributed along a gradient of shade-tree cover (plot-scale). We found localized positive effects of individual shade trees on soil carbon and nitrogen content, as well as soil aggregation. However, we found no evidence for positive effects of agroforests via improved soil fertility or carbonsequestration with increasing shade-tree cover at the plot scale, a scale that more closely matches the scale at which agroforests are managed. Cocoa growth was lower under individual shade trees and decreased with increasing shade-tree cover in plots, and cocoa yields also decreased with increasing shade-tree cover. Our results indicate that the benefits of agroforestry for soil fertility and carbon sequestration in cocoa cultivation systems might not be as extensive as believed, and may not be sufficient to compensate for short-term costs to production.

Structural characteristics determine productivity in complex cocoa agroforestry systems

“…Both the cocoa yield and the incident light were a logistic function of the planting distance from the shade trees. Our results were also in line with those of Blaser et al (2017) who, in Ghana, showed that cocoa growth was lower under shade trees and decreased with increasing shade-tree cover, as it was also observed for cocoa yield. The correlation between the unproductive adult cocoa tree rate on one hand, and the density of associated trees or the number of associated species on the other hand (Table 2D) confirmed that competition for water, light, and nutrients was a factor limiting yield build-up in cocoa stands (Besse 1972;Lachenaud and Mossu 1985).…”

Cocoa agroforest multifunctionality and soil fertility explained by shade tree litter traits

“…Shade tree management in cocoa plantation has been discussed for decades regarding their benefits and disadvantages for cAFS (Andres et al, 2016;Blaser et al, 2017;Niether et al, 2019;Sanchez, 1995). The microclimatic regulation introduced by shading decreases diseases' outbreaks, which are known to hamper both cocoa yield and sustainability (Andres et al, 2016).…”

“…Shade tree effects on cocoa yield, nutritional status and soil fertility has been studied both at the community (Blaser, Oppong, Yeboah, & Six, ; Niether, Schneidewind, Fuchs, Schneider, & Armengot, ) and at the species level (Isaac, Timmer, & Quashie‐Sam, ; Wartenberg et al, ). Depending on the study, observed effects are explained by a variety of individual or community properties such as above‐ground biomass (Isaac, Timmer, et al, ; Niether et al, ; Wartenberg et al, ), leaf biomass nutrient concentration (Isaac, Timmer, et al, ; Wartenberg et al, ) or canopy architecture and/or cover (Blaser et al, ; Isaac, Timmer, et al, ; Wartenberg et al, ). Nonetheless, to our knowledge, there is no study considering the specific relationships between shade tree functional traits and agroecosystem functioning in cAFS.…”

Valorisation strategies for cocoa pod husk and its fractions

“…Conversion of native forest systems to cocoa cultivation frequently depletes SOM [24] as a result of physical disturbance and lack of replenishment of organic matter exported from the system with each crop [25]. Such decline in SOM has been linked to declines in cocoa productivity and bean quality with time since land conversion to cocoa farms [26].…”