Wood Yields From Agroforestry Practices in Africa - A spatial explicit assessment of short rotation woody crop yields
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The world’s population continues to grow, requiring increased amounts of food, feed, fibre and fuels. In order to meet these demands, the current available lands should be used more efficiently and more arable lands should become available. In agroforestry systems short rotation woody crops (SRWCs) are planted alongside food crops and are proven to be more efficient compared with traditional agriculture. Combining these perennial woody crops with food crops on the same land has several benefits for the local environment. As a result that, the food production can increase substantially while fuelwood in grown in the same area. The benefits of agroforestry can also play a role in the conversion of degraded lands into arable lands. Because agroforestry can be a good solution for the expected land scarcity problems, it is essential to quantify the actual yield potential of biomass in such systems. The Bioenergy Simulator of the International Renewable Energy Agency (IRENA) calculates the yield potential of short rotation woody crops, however this model doesn’t take the local climate and soil conditions into account. Therefore, the aim of this research is to estimate the yield potential of short rotation woody crops for bioenergy production in Africa, while taking local soil climate and soil conditions and agroforestry practices into account. A total of 15 nitrogen-fixing SRWCs have been analysed in this research. In order to calculate the yield potential of these species, a method has been developed that peels back from the theoretical to technical yield potential. For each species, first the constraint free biomass production potential has been calculated. This potential is based upon simple biophysical processes such as photosynthesis and respiration and can be calculated with temperature, precipitation and solar irradiation data. For all 15 species a climate and soil suitability analysis has been done. This constraint free yield is then reduced by the limitations imposed due the climate and soil conditions. The remain yield are considered as the theoretical yield potential. The technical yield potential is calculated by excluding all non-suitable land use system for the production of SRWCs. With the technical yield potential of all species known, an analysis has been done in order to select the right species in an area. The results show that all 15 species are suitable to grow in Africa. The species achieve yields ranging from 2 t/ha up to 16 t/ha and the average total suitable land available for a species is 355 million hectare (Mh). Leuceana Leucocephala is the species that has the largest technical production potential on itself with 410 million tons (Mt) per year, while the average production is 171 Mt per year. However, this research shows that it is of importance to choose the right species in an area and thereby the total production potential of SRWCs in agroforestry systems in Africa can increase significantly. Five of the 15 species are considered to be less suitable to grow in Africa, compared the yield potentials of the other species. The results of the best performing species analysis are analysed per suitable land use system. The analysis shows that agricultural lands achieve the highest yields and that 95% of the total arable lands in Africa are suitable for SRWC production. The largest total production potential can be achieved on Grasses and Shrub lands of which 30% of the total pasture land in Africa is suitable for SRWC production. There is a small area of sparsely vegetated lands where high yields yield of SRWCs can be achieved and there is a very low potential on Bare lands. The total suitable land when picking the right species is 555 Mha on which a total of 684 Mt SRWCs can be produced per year in Africa. Since 95% of the total arable land in Africa is suitable for the implementation of agroforestry systems, the potential effects on food production can be enormous. Especially agricultural lands in arid regions can benefit from these systems. On top of that, this research shows that more than 40 Mha of marginal lands have the potential to be restored with agroforestry systems.