Carbon dynamics, net primary productivity and human‐appropriated net primary productivity across a forest–cocoa farm landscape in West Africa

Abstract

Terrestrial net primary productivity (NPP) is an important metric of ecosystem functioning; however, there are little empirical data on the NPP of human‐modified ecosystems, particularly smallholder, perennial crops like cocoa (Theobroma cacao), which are extensive across the tropics. Human‐appropriated NPP (HANPP) is a measure of the proportion of a natural system's NPP that has either been reduced through land‐use change or harvested directly and, previously, has been calculated to estimate the scale of the human impact on the biosphere. Additionally, human modification can create shifts in NPP allocation and decomposition, with concomitant impacts on the carbon cycle. This study presents the results of 3 years of intensive monitoring of forest and smallholder cocoa farms across disturbance, management intensity, distance from forest and farm age gradients. We measured among the highest reported NPP values in tropical forest, 17.57 ± 2.1 and 17.7 ± 1.6 Mg C ha−1 year−1 for intact and logged forest, respectively; however, the average NPP of cocoa farms was still higher, 18.8 ± 2.5 Mg C ha−1 year−1, which we found was driven by cocoa pod production. We found a dramatic shift in litterfall residence times, where cocoa leaves decomposed more slowly than forest leaves and shade tree litterfall decomposed considerably faster, indicating significant changes in rates of nutrient cycling. The average HANPP value for all cocoa farms was 2.1 ± 1.1 Mg C ha−1 year−1; however, depending on the density of shade trees, it ranged from −4.6 to 5.2 Mg C ha−1 year−1. Therefore, rather than being related to cocoa yield, HANPP was reduced by maintaining higher shade levels. Across our monitored farms, 18.9% of farm NPP was harvested (i.e., whole cocoa pods) and only 1.1% (i.e., cocoa beans) was removed from the system, suggesting that the scale of HANPP in smallholder cocoa agroforestry systems is relatively small.

Human Appropriated Net Primary Productivity of Complex Mosaic Landscapes

“…Calculations described by Morel et al (2019) indicate that HANPP for cocoa could be estimated with relatively high accuracy from cocoa tree density and number of shade trees (Figure 5). Values for HANPP varied from −4.6 to 5.2 Mg C ha-1 yr-1 or across the plots, with the negative values being recorded at a particularly shade-tree dense, young cocoa farm, and an older farm managed largely for timber.…”

“…In order to determine the HANPP of such a mosaic landscape, detailed measures of HANPP of cocoa must be determined. Recent work by Morel et al (2019) generated detailed site-level data of NPP and HANPP in cocoa farms in the Central Region of Ghana. The study reported a strong relationship between cocoa farm characteristics, namely shade tree density and cocoa tree density, and HANPP.…”

“…This study was carried out in the Central Region of southern Ghana around six communities bordering Kakum Conservation Area (5 • 33 ′ 42.712 ′′ N, 1 • 20 ′ 55.234 ′′ W; see Figure 2). The area of study was further subdivided into three distinct regions, each inclusive of two focal villages and the surrounding land use areas, following the sampling design of Morel et al (2019). These regions are: AB (including the villages of Aboabo and Asorefie), HM (including the villages of Homaho and Nsuoekyi), and KA (including the villages of Kwame Amoabeng and Tufokrom).…”

“…The work was carried out as a part of the larger ECOLIMITS project which collected data between 2014 and 2017 to better understand the relationship between poverty and ecosystem services (Hirons et al, 2018b). The project intensively monitored two forest plots within Kakum National Park as well as eight cocoa farm plots under varying levels of management intensity for NPP and HANPP over 3 years as well as collected above ground biomass and cocoa yield data for 28 additional plots (Morel et al, 2019). Each of the intensive plots was 60 × 60 m (0.36 ha), and field measurements followed the Global Ecosystems Monitoring (GEM) protocol (Marthews et al, 2014).…”

“…These components were also measured for cocoa trees in addition to the estimated NPP of cocoa pod production. Detailed descriptions of the full methods used to generate the NPP of cocoa farms as well the background forest NPP can be found in Morel et al (2019). NPP values for the cocoa farms were then compared to NPP values collected for the conservation area forest plots (∼17 Mg C ha −1 yr −1 ) to generate an estimate of HANPP of the cocoa farms.…”