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

Research output: Contribution to journalArticle

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.

Details

Authors
  • Alexandra C. Morel
  • Michael Adu Sasu
  • Stephen Adu-Bredu
  • Marvin Quaye
  • Christine Moore
  • Rebecca Ashley Asare
  • John Mason
  • Mark Hirons
  • Constance L. McDermott
  • Elizabeth J.Z. Robinson
  • Emily Boyd
  • Ken Norris
  • Yadvinder Malhi
Organisations
External organisations
  • University of Oxford
  • Zoological Society of London
  • Nature Conservation Research Centre (NCRC Ghana)
  • Forestry Research Institute of Ghana (FORIG)
  • University of Reading
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Forest Science
  • Earth and Related Environmental Sciences

Keywords

  • carbon cycling, Ghana, HANPP, net primary productivity, smallholder, Theobroma cacao
Original languageEnglish
JournalGlobal Change Biology
Publication statusE-pub ahead of print - 2019
Publication categoryResearch
Peer-reviewedYes