Phenology and gross primary production of two dominant savanna woodland ecosystems in Southern Africa

Accurate estimation of gross primary production (GPP) of savanna woodlands is needed for evaluating the terrestrial carbon cycle at various spatial and temporal scales. The eddy covariance (EC) technique provides continuous measurements of net CO2 exchange (NEE) between terrestrial ecosystems and the atmosphere. Only a few flux tower sites were run in Africa and very limited observational data of savanna woodlands in Africa are available. Although several publications have reported on the seasonal dynamics and interannual variation of GPP of savanna vegetation through partitioning the measured NEE data, current knowledge about GPP and phenology of savanna ecosystems is still limited. This study focused on two savanna woodland flux tower sites in Botswana and Zambia, representing two dominant savanna woodlands (mopane and miombo) and climate patterns (semi-arid and semi-humid) in Southern Africa. Phenology of these savanna woodlands was delineated from three vegetation indices derived from Moderate Resolution Imaging Spectroradiometer (MODIS) and GPP estimated from eddy covariance measurements at flux tower sites (GPPEC). The Vegetation Photosynthesis Model (VPM), which is driven by satellite images and meteorological data, was also evaluated, and the results showed that the VPM-based GPP estimates (GPPVPM) were able to track the seasonal dynamics of GPPEC. The total GPPVPM and GPPEC within the plant growing season defined by a water-related vegetation index differed within the range of ±6%. This study suggests that the VPM is a valuable tool for estimating GPP of semi-arid and semi-humid savanna woodland ecosystems in Southern Africa.