Interception of Photosynthetically Active Radiation on Cocoa Plantations in Mexico

Cocoa (Theobroma cacao L.) is the main source of income for 40-50 million people in producing areas located primarily in Africa, Asia, and Central and South America (within the tropical belt extending 20 degrees north and south from the equator) (Nadurille, 2010). 

World production is distributed among the countries of South America, Central America, the Caribbean, Africa, Asia and Oceania, plus Mexico, and all of these countries have tropical wet forests. Cocoa cultivation originated in the Americas but the exact place of origin cannot be pinpointed (Batista, 2009). The cocoa is a native Amazonian species from neotropical lowlands and tropical rain forests and is now grown in more than 50 countries in the humid tropics (Motamayor et al., 2008).

 The cocoa requires shading to efficiently perform its physiological processes. Traditionally cocoa has been grown under the protection of fast-growing species and other leguminous trees, including palm trees and tropical fruit trees. The association of timber trees with cocoa production systems maximizes the economic benefits to the farmer, making the system more sustainable in terms of conserving natural resources such as soil, water and biodiversity (Vanhovea et al., 2016). 

Besides protecting the crop, shade trees offer additional benefits and services to the agroforestry system, such as nitrogen fixation (in the case of leguminous trees) and the production of leaf litter that becomes soil organic matter, which buffers against extreme weather conditions, such as temperature, wind and humidity. Cocoa associated with larger species creates stable conditions especially against the sun’s heat effect, promotes nutrient recycling and therefore contributes to the sustainability of this system (Santana and Cábala 1987). In addition, cocoa producers generally benefit from shade tree species by using them as a source of energy (firewood) and tropical fine woods. This allows the system to be more sustainable from an economic standpoint, as it provides additional income (Somarriba et al., 2014), and ecologically contributes to the protection and conservation of soil and water resources, as well as to carbon sequestration and oxygen release (Abou Rajab, 2016). 

Batista (2009) mentions that it is possible to cultivate cocoa without shade and even increase yield, but the disadvantages that this would entail in the long term have not been assessed. By removing the shade trees from cocoa plantations, it ceases to be a diversified agroecosystem and becomes a monoculture and is therefore no longer sustainable under current conditions; moreover, such a system requires the use of new technologies and application of crop inputs such as irrigation and chemical fertilizers.

 Another factor that determines the shading level is pruning, is done to maintain the cocoa tree’s proper shape, facilitate crop management tasks and decrease the presence of pests and diseases. The types of pruning performed on cocoa plantations include shape pruning, maintenance pruning and restoration and sanitation pruning (López et al., 2016). 

Photosynthetically active radiation (PAR) is the radiation that crops use to perform their vital functions in allphenological stages. Unlike most crops, cocoa grows well under shaded conditions. The crop uses only a portion of the photosynthetically active radiation that reaches the plantation, as the rest is intercepted by the shade trees; therefore, correct management of the shading level on cocoa plantations is of utmost importance (Batista, 2009). The most accurate and practical method to indirectly estimate shading is the use of a ceptometer, which estimates the PAR at various heights of the plant to determine the interception of radiation when penetrating the foliage in an agroforestry system, and it can be used in studies which require quantitatively relating the effect of shade on production. 

The level of shading or its opposite, the level of sunlight that penetrates the plantation, determines the photosynthetic rate of agricultural crops in the lower strata, their growth, their demand for nutrients and water, the dynamics of pests and diseases and ultimately the commercial production. Shade trees on cocoa plantations provide environmental, economic and social benefits to the home, the environment and society (Almeida et al. 2002). 

Despite their importance, no studies have been conducted in Mexico related to shading levels on cocoa plantations. Consequently, the objectives of this study were: i) to characterize the shading level on cocoa plantations in the Chontalpa region of Tabasco, Mexico to generate possible recommendations related to agronomic management of the crop; ii ) to measure the PAR in three strata of the cocoa agroecosystem to determine the shade percentage in producer plots and iii) to determine the interception percentage of the shade and cocoa trees (shade percentage).