N2-fixing species benefit biomass production in agroforestry mixtures depending on spatial scale and plantation age but not in the mixed forestry system

Abstract

Forest plantations play a crucial role in addressing the growing demand for wood-based products and biomass for renewable energy. However, their expansion is often limited by competition with food crops. Agroforestry, which integrates trees within agricultural plots, and mixed forest plantations have emerged as promising ways to reconcile these needs, especially when they include nitrogen-fixing species. This study evaluates the performance of mixed forest and agroforestry plantations including nitrogen-fixing species, such as alder and alfalfa, compared to monocultures. Based on an experimental trial in northeastern France, we compared the biomass production, and its time course, for poplar (Populus nigra × P. deltoides) and alder (Alnus glutinosa) trees in monocultures to their biomass production when associated together in the same stand, and when associated to crops (poplar with alfalfa and alder with grass) at three different scales (tree, plantation, and cropping system). After nine years of growth, at the tree level, both poplar and alder biomass was higher in the agroforestry systems than in the forest mixture or the monocultures. However, at the plantation level, the wood yield of the poplars and alders did not significantly differ between the mixtures and the monocultures at the end of the rotation. At the cropping system level, the two agroforestry systems exhibited a Land Equivalent Ratio significantly superior to 1, indicating higher productivity in these systems compared to crop and tree monocultures. The two agroforestry systems including nitrogen fixators showed very promising results, while the poplar and alder association was disappointing.