  The importance of semi-arid ecosystems in the global carbon cycle as sinks
for CO2 emissions has recently been highlighted. Africa is a carbon sink and
nearly half its area comprises arid and semi-arid ecosystems. However, there
are uncertainties regarding CO2 fluxes for semi-arid ecosystems in Africa,
particularly savannas and dry tropical woodlands. In order to improve on
existing remote-sensing based methods for estimating carbon uptake across
semi-arid Africa we applied and tested the recently developed plant phenology
index (PPI). We developed a PPI-based model estimating gross primary
productivity (GPP) that accounts for canopy water stress, and compared it
against three other Earth observation-based GPP models: the temperature and
greenness model, the greenness and radiation model and a light use efficiency
model. The models were evaluated against in situ data from four semi-arid sites
in Africa with varying tree canopy cover (3 to 65 percent). Evaluation results
from the four GPP models showed reasonable agreement with in situ GPP measured
from eddy covariance flux towers (EC GPP) based on coefficient of variation,
root-mean-square error, and Bayesian information criterion. The PPI-based GPP
model was able to capture the magnitude of EC GPP better than the other tested
models. The results of this study show that a PPI-based GPP model is a
promising tool for the estimation of GPP in the semi-arid ecosystems of Africa.
