Primary Production in African Drylands : Quantifying Supply and Demand Using Earth Observation and Socio-ecological Data
Abstract: The human-environment connection in the mostly rural drylands of sub-Saharan Africa forms a complex, interlinked system that provides ecosystem services. This system is susceptible to climatic variability that impacts the supply of its products, and high population growth, which impacts the demand for these products. When plants remove carbon dioxide from the atmosphere through the process of photosynthesis, they use some of this carbon to maintain plant cellular structure. The rest is stored as plant tissue and forms plant biomass. The annual accumulation of this plant biomass is called net primary production (NPP). On an annual basis, NPP supplies the provision of crops, animal feed and pasture. The societal implications of reduced NPP can be severe, possibly leading to crop failure and eventual food insecurity. The trends in NPP supply over sub-Saharan Africa between 2000 and 2013 are significant over 32% of the area (4.7 million km2). However, they are concentrated in three distinct regions: the western Sahel (2 g C m-2 yr-1), central Africa (30 g C m-2 yr-1) and parts of Zambia, Malawi and Mozambique (-25 g C m-2 yr-1). In contrast, the mean overall trend in NPP demand is 3.5 g C m-2 yr-1, though in urban areas it averages approximately 50 g C m-2 yr-1. The tradeoffs between NPP supply and demand (i.e. change in one quantity relative to another) are locally constrained and linked to the prevailing climate, population growth and net migration. The demand-supply balance of NPP is influenced by climate, such as variability caused by the El Niño – Southern Oscillation. The greatest sensitivity to El Niño occurs in Southern Africa. Here, a +1oC shift in the Niño 3.4 index (as a measure of El Niño) causes a mean change in the NPP supply of -6.6 g C m-2 yr-1. Despite the fact that there were more La Niña events than El Niño events during the period of this study, the negative impact of El Niño on Southern Africa is strong enough to tip the balance toward the negative. Climatic variability influences the rate of carbon uptake and in sub-Saharan drylands all plants undergo photosynthesis at the expense of losing moisture to the atmosphere. The two main moisture related biophysical limitations, plant available water and vapor pressure deficit, together limit plant carbon uptake by influencing the greening and browning phases of vegetation phenology. The combination of Earth observation data (Land Surface Temperature, Enhanced Vegetation Index, and shortwave infrared surface reflectance) in a multiple regression model was able to explain 89% of the variability of in situ measured carbon uptake across three Sahelian sites. Testing the new Plant Phenology Index (PPI) to get better estimates of sub-Saharan carbon uptake showed that a PPI-based model was able to capture the magnitude of in situ carbon uptake relatively well (R2 = 0.75, 1.39 g C m-2 d-1) compared to the other tested models. However, the performance of PPI in these semi-arid systems can be further improved through the inclusion of total chlorophyll content as it is a principal factor influencing carbon assimilation.
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