Can leaf quality explain the influence of shade tree species on the fertility of cocoa farms? Marie Sauvadet and colleagues summarise their recent research.

Cocoa, a major commodity worldwide, is largely produced by smallholder farms in developing countries. With limited access to synthetic fertilisers and pesticides, smallhold farmers traditionally lean on cocoa ecology to ensure their harvest thorough the years.

Cocoa naturally grows under the shade of higher trees, which protect the plant against climatic fluctuation and diseases, and maintain top soil fertility through litterfall. Therefore, by growing cocoa under these bigger trees (called shade trees), the farmers reduce cocoa stress and nutrient demand.

Either remnants from former forest or planted by the farmers, the shade trees in cocoa agroforests are very diverse and are selected not only for the shade they provide, but also the provision of additional goods for the local populations. For instance, some shade trees, such as Iroko, produce high-quality timber, while others, such as Safou, provide food for the farmers.

Shade trees have contrasting impacts on soil fertility in cocoa farms. Some species directly compete with cocoa for light, water and nutrients, which may decrease their growth and yield. Others can increase soil fertility and compensate at least partially for the lack of fertilisers. For instance, nitrogen-fixing tree species like Albizia adianthifolia have the ability to use nitrogen from the atmosphere; the fixed nitrogen moves to the tree, then to the soil through plant residues recycling (for instance litterfall). When the plant residues decompose, the fixed nitrogen then becomes available to cocoa.

Using shade tree species with the most benefits for farmers and soil fertility is thus a promising way to ensure cocoa farms sustainability, but how do we know which species are the best? Tens of shade tree species can currently be found in cocoa farms. While farmers have often a detailed perception of shade tree influence on cocoa, there is still little information on their specific effects on soil fertility.

An emerging trend in agroecology is to consider plant functional attributes to understand their effects on the environment: is this species able to use the nitrogen from the atmosphere? Does it produce a lot of roots that will allow it to take up more nutrients? How many leaves will it restitute to the soil? Is this litter easy to decompose for soil organisms?

Based on this principle, we investigated whether the functional attributes of shade tree species could be used to understand their effects on soil fertility. We worked in cocoa farms from the Bokito district (Central Cameroon). Bokito farmers grow cocoa on poor soils with little access to fertilisers, and rely heavily on the use of shade trees. We studied five common shade tree species in this area: two fruit tree species (Canarium and safou), two timber tree species (Iroko and Kapok), and one N-fixing tree species (Albizia adianthifolia).

These species presented contrasting morphology; the Kapok was by far the biggest tree species, both in height (> 30 m) and width (> 300 cm trunk diameter), while safou was the smallest (12 m height and 36 cm diameter). All the species had similar root biomass in the most superficial soil layer, but restituted different amount of leaf litter to the soil. This amount was the highest under Iroko canopy, high under the Kapok tree, safou and Albizia, and low under Canarium and unshaded cocoa (i.e. under no shade tree).

Further analyses showed that the leaf litters collected under Iroko, Kapok and Albizia had high contents of nutrients essential for cocoa growth (such as nitrogen and phosphorus) and were at the same time easily decomposable by soil organisms. In opposition, litters collected under Canarium, safou and cocoa alone contained less nutrients and/or were less palatable to soil organisms due to their acidity and high tannin contents.

Differences in leaf litter quality were mirrored by contrasting soil fertility under cocoa and shade tree species. Soils under cocoa alone presented the lowest level of fertility, which was barely improved under Canarium and safou. On the other hand, soil under Iroko, Kapok and Albizia presented higher nitrogen and phosphorus availability for plant growth. However, in the case of Albizia, this increase was followed by local soil acidification that could have been caused by Albizia nitrogen-fixing activity. On the opposite, Kapok tree, whose leaf litters were enriched in calcium, decreased soil acidity and allowed higher soil carbon content.

Overall, our results show that shade tree species selection based on their functional traits appears as a promising practice to adequately manage soil fertility. Combined with socio-economical consideration on the importance on the additional good produced by shade trees (for instance timber for Iroko), shade tree traits could be used to design a decision framework to identify the best shade tree for cocoa farms sustainability.

Read the full article, Cocoa agroforest multifunctionality and soil fertility explained by shade tree litter traits, in Journal of Applied Ecology.

The work was carried out by a group of researchers from CIRAD, IRD, CNRS and ICRAF within the framework of the CGIAR Research Program on Forests, Trees and Agroforestry (FTA).