Sustainable biomass sourcing through agroforestry systems based on poplar and eucalyptus as fast-growing trees

Global biomass demand is expected to increase during the next decades, which bears risks of accelerating food insecurity and biodiversity loss. Therefore, guidelines and standards have been developed to ensure sustainable biomass sourcing as feedstock for material or energetic purposes. This review addresses the question of how fast-growing trees in agroforestry systems increase biomass production and serve as sustainably sourced biomass feedstock. The Land Equivalency Ratio (LER), a measure of productivity gains by agroforestry, was positively correlated with the number of trees per hectare (R = 0.561) and with the spacing between tree rows (R = 0.26). The former corresponds to a high wood yield of the given agroforestry systems, while the latter corresponds to high crop yields within the agroforestry systems. The LER of tree windbreak systems (spacing between tree rows >100 m) was 1.1-2.1. Tree windbreak systems adhered to principles of sustainable biomass sourcing, while other agroforestry systems often provided lower food crop yields compared to the corresponding crop monoculture. Still, such agroforestry systems help to diversify incomes, have the potential to protect croplands against erosion, and improve the microclimate. Depending on local conditions, biomass from those agroforestry systems can be considered sustainable, too.