Tree carbon fluxes and stocks in West African cocoa plantations

The study was conducted at 15 sites across Côte d’Ivoire, covering a South-North climate and vegetation gradient from evergreen to semi-deciduous forests. These sites were selected to represent varying cocoa field characteristics, including structural complexity (ranging from monocultures to agroforests), cocoa field age (young to mature), and cocoa tree productivity (low to high). Each site had 10 cocoa plots, ranging from 0.3 to 5 hectares. The carbon stocks of all trees, including remnant, spontaneous, and planted trees, have been estimated using the BIOMASS R package. This tool provides a robust and widely accepted methodology for calculating tree biomass and carbon stocks, ensuring accurate and reliable estimates for this dataset.

Methods
Study Sites

Study plots have been selected from 15 locations throughout the cocoa production area (the forest zone) of Côte d’Ivoire (Fig. 1). These sites are situated along a South-North climate gradient, with mean annual temperatures ranging from 22.6°C to 26.2°C and annual rainfall varying between 1,900 mm and 1,100 mm. This climate gradient corresponds to a South-North vegetation gradient, transitioning from evergreen forests to semi-deciduous forests. To capture the diversity of cocoa fields, we identified 10 cocoa fields at each site, focusing on three key criteria: (i) structural complexity, ranging from nearly full-sun cocoa monocultures to complex cocoa agroforests; (ii) age of the cocoa fields, encompassing both young (< 20 years) and mature (> 20 years) plots; and (iii) cocoa tree yield, varying from low productivity (< 300 kg) to high productivity (> 300 kg). The study plots vary in size from 0.3 to 5 hectares and serve as the sampling units, corresponding to the management units of the farmers.

Data Collection

All cocoa fields were mapped using GPS technology for precise delineation. Each companion tree with a minimum diameter at breast height (dbh) of 10 cm was identified to the species level. The dbh and the height of each tree was recorded. The origin of each tree—classified as planted, remnant, or spontaneous— and the age of each planted and spontaneous tree was recorded based on the farmer's declaration. Four socio-environmental factors of interest were identified for this study:

Two social factors related to the cocoa farmer: botanical knowledge and land ownership status of the cocoa field, obtained through interviews with the farmers.
Two field-related factors: cocoa tree density and previous land use. Cocoa tree density was estimated by counting the number of cocoa trees in a 1000 m² area within each cocoa field, and previous land use was obtained through interviews with the farmers.

The aboveground biomass of each individual tree was estimated and then converted to Carbon, using the computeAGB() function from the BIOMASS package in the R software. Carbon Stocks per plot and per cohort was calculated by adding the stock of each tree.