Species interactions dynamics in poplar forestry and agroforestry systems: from water and nitrogen use efficiencies to tree productivity
Résumé
In recent years, plantations of fast-growing tree species have emerged as a possible way to meet the increasing demand for biomass for renewable energy in Europe. Agroforestry and mixed forest plantations including fast-growing tree species could be an attractive option for maintaining high productivity while optimizing the sustainable use of nutrient and water resources. Our objective was to assess the performance of poplar trees (Populus deltoides ´ P. nigra, Dorskamp clone) when associated with a N2-fixing-species either herbaceous (succession alfalfa and clover; agroforestry) or woody (alder, forest mixture) compared to poplars in monoculture in an experimental plantation set up in 2014 in northeastern France. Annual poplar aboveground biomass production was monitored from tree to plantation levels over nine growing seasons. During the early stages, interspecific competition between crops and poplars predominated. A facilitation process in the agroforestry treatment, due to soil nitrogen enrichment by the N2-fixing crops, led to higher aboveground biomass production at tree level in agroforestry poplars than in the forest mixture and the monoculture from the 6th growing season. Poplar grown in mixture with alder exhibited similar growth dynamic to poplar in monoculture. The aboveground yield at plantation scale was similar for the three investigated treatments although tree density was halved in the agroforestry system. Tree productivity is closely related to water and nitrogen-use efficiencies, i.e. the quantity of biomass produced per unit of water or nitrogen used (WUE and NUE respectively). Poplar trees in both mixture types showed higher WUE than those in monoculture during the seventh growing season. The more WUE was integrated in time (leaf < wood < tree), the more the differences among treatments were marked. In addition, at tree level, poplar NUE was significantly higher in the forest mixture than in then agroforestry and monoculture systems, due to a decrease in litter production and a decrease in litter nitrogen concentrations in the forest mixture. The higher biomass production and lower water consumption occurring when poplar is associated with a N2-fixing crop make such mixtures promising in a context where water could become a limiting resource. Our results also underline the importance of litter management practices and crop rotation strategies to promote soil nutrient restoration and maximize poplar productivity. Understanding the determinants of poplar productivity, NUE and WUE in different systems is indeed of practical importance for land managers to devise strategies to optimize resource use and improve agricultural sustainability.