Why models underestimate West African tropical forest productivity

Tropical forests dominate terrestrial photosynthesis, yet there are major contradictions in our
understanding due to a lack of field studies, especially outside the tropical Americas. A recent
field study indicated that West African forests have among the highest forests gross primary
productivity (GPP) yet observed, contradicting models that rank them lower than Amazonian
forests. Here, we explore possible reasons for this data-model mismatch. We found the in situ
GPP measurements higher than multiple global GPP products at the studied sites in Ghana.
The underestimation of GPP by models largely disappears when a standard photosynthesis
model is informed by local field-measured values of (a) fractional absorbed photosynthetic
radiation (fAPAR), and (b) photosynthetic traits. Satellites systematically underestimate
fAPAR in the tropics due to cloud contamination issues. The study highlights the potential
widespread underestimation of tropical forests GPP and carbon cycling and hints at the ways
forward for model and input data improvement