Reviewed: Asigbaase M, Sjogersten S, Lomax BH, Dawoe E. Tree diversity and its ecological importance value in organic and conventional cocoa agroforests in Ghana. PLOS One. January 2019;14(1):e0210557. doi: 10.1371/journal.pone.0210557.
Deforestation for agricultural expansion threatens valuable tree species worldwide. Demand for cacao (Theobroma cacao, Malvaceae) is rising and large price increases are expected while land suitable for its production is predicted to shrink in Ghana and Côte d’Ivoire (Ivory Coast) due to climate change.
Cacao agroforestry is a production system in which farmers integrate shade trees in the same plot with food crops. In addition to providing vital shade, interplanting and/or conserving trees in situ suppresses weeds and insects, returns soil nutrients taken up by cacao trees, and improves microclimate resiliency and water retention. Producers are able to increase their income with such food, fruit, and timber products. However, the trend to replace native trees with food crops may lead to the use of agrochemicals. Smallholders often cannot bear the costs of chemical fertilizers and pesticides, so the trend to monoculture often leads to larger farms owned by fewer farmers. Higher costs of chemical-dependent farming drive the need for more land area and thus, deforestation.
Organic standards prohibit the use of synthetic chemicals. Most of Ghana’s cacao production occurs under de facto organic conditions dating back to the crop’s introduction in 1870, but organic agricultural certification is relatively new. Organic cacao monoculture relies on organic agrochemicals, while organic cacao agroforests use shade tree diversity to obtain the same results.
Diversified, organic agroforestry practices that conserve native trees and enhance biodiversity have been recommended for Africa. The large size of cacao-producing areas and their significant overlap with global biodiversity hotspots make quantifying the benefits of these practices essential. The authors compared the potential of organic cacao agroforestry to conserve native floristic diversity with conventional cacao agroforests (i.e., those that use synthetic agrochemicals). Indices of shade tree species richness (the number of different species in a defined area) and species diversity (the number and relative abundance of species in a defined area) were estimated from 84 organic and conventional plots. Species importance value index (“a measure of how dominant a species is in a given ecosystem”) and conservation status were used to evaluate the conservation potential of shade trees in plots studied.
Cacao Farm Details
This study was conducted in seven randomly selected cacao-producing communities in Suhum Municipality in the Eastern Region of Ghana. Cacao trees were introduced to Ghana in the Eastern Region, where Suhum is home to some of the country’s oldest organic cacao farms. Vegetation is mostly secondary forests, fallows, and cultivated areas. Shade-grown cacao systems are mixed with varying proportions of upper canopy shade and, increasingly, food and fruit trees. Farms are generally two hectares (4.9 acres) or smaller. Most have the recommended 12-18 shade trees per hectare that provide 30%-40% canopy cover. Major management practices include shade control, fertilizer application, pod harvesting, bean processing, and pest, weed, and disease control.
For this study, organic farms were defined as those managed for at least five years with only certified organic inputs. All organic farms in this study were registered and certified by Control Union, an international certification organization. In chosen communities, organic and conventional farms were randomly selected from lists provided by local cacao regulators. Farms were characterized as young (≤ 15 years old), mature (16-30 years old), and old (≥ 31 years old) cacao systems. Fourteen farms from each age group per farm type were selected (42 organic + 42 conventional = 84 total). Farms had similar management practices, land preparation methods, and cropping histories, and most were in neighboring communities.
Cacao yields on both types of farms ranged from 400-800 kg per hectare annually. Data were collected between April and August 2016. Within each farm, a 25 x 25 meter plot was randomly established. Within each determined plot, trees and shrubs were identified to species level, use of shade trees was determined, and the circumference and diameter of every cacao or shade tree greater than 5 cm was measured. In addition, every stem of multi-stemmed plants was measured. To compare the spatial structure of a plot, shade-providing species were grouped by whether or not they were maintained for food/fruit. To assess shade tree management, they were grouped by use for domestic (e.g., food and medicine), ecological/agroforestry (e.g., shade and nitrogen fixation), or economic (e.g., timber) needs. Residual plots and two-way analysis of variance (ANOVA) were used to assess differences between farm types overall and at each stage of maturity.
In organic plots, 1,140 individual shade trees were counted in total; in conventional plots, 494 were counted. In organic young cacao systems, 41 species from 18 families were found; in conventional young cacao systems, there were 36 species from 18 families. Organic old cacao systems hosted 38 species from 19 families, and conventional old cacao systems had 27 species from 17 families.
The most abundant trees on both types of farms in all age groups were pioneer species used for timber, domestic construction, and/or medicine. (Pioneer species are the first species to colonize a certain environment.) Most were native species. Four of six food/fruit species found on all organic farms also grew on all studied conventional farms, but relative abundance of food/fruit species ranged from 77.5%-79.8% per hectare on organic farms and 45.4%-63.8% per hectare on conventional farms. Relative abundance of nitrogen-fixing plants was 6% per hectare on organic and 2.6% per hectare on conventional farms. Shade tree abundance averaged 5.10 per hectare on organic and 3.48 per hectare on conventional farms.
Organic farms had significantly higher mean species richness (P = 0.004) and significantly higher scores on the Shannon diversity index, Simpson’s reciprocal index, and Margalef’s diversity index (P < 0.001 for all). However, some medicinal and forest species were “notably important” on conventional farms. Despite higher total basal stem area in organic plots, conventional plots had similar shade tree stem density. Conventional young cacao systems may be trending toward greater species diversity to meet socio-economic goals. While some of the documented trees were found to have conservation concerns (14 species in the organic farms and 10 species in the conventional farms), 75% of the recorded species have not yet been assessed.
According to the authors, these results suggest that organic cacao farms “may contribute more significantly to native species conservation than conventional farms due to their high shade tree diversity and the maintenance of relatively higher levels of tree species with conservation concern.” Therefore, manipulation of relative species density that does not compromise native species should be encouraged and perhaps required for organic certification, the authors concluded.