The influence of tree species mixture on ecosystem-level carbon accumulation and water use in a mixed boreal plantation

Throughout the world, huge areas are occupied by monospecific tree plantations in order to fulfil the increasing demand for industrial wood production. The use of mixed-species plantations has been recently advocated as a potential compromise to maintain high wood production while conserving other ecosystem services. Yet, little is known about the impact of tree species mixture on the carbon and water balance of mixed plantations. We combined data of above-ground biomass, xylem sap flux density, and carbon and oxygen isotope composition of leaves and needles in a boreal plantation to test whether different levels of species richness would impact tree- and plot-level biomass production, transpiration and water use efficiency. Data were recorded in 22 plots (7 for transpiration) (20 × 20 m) randomly allocated to monocultures and to two-, three-, and five-species mixtures of Scots pine (Pinus sylvestris), Norway spruce (Picea abies), Silver birch (Betula pendula), Common alder (Alnus glutinosa) and Siberian larch (Larix sibirica). At tree level, for a given species, we found significant differences among mixtures in transpiration and water use efficiency, whereas above-ground biomass was not impacted by species mixture. At plot level, a large variability in functioning among mixtures was observed, but increasing species richness did not enhance productivity, transpiration and water use efficiency. The presence of high performing species in mixtures such as birch resulted in changes in canopy structure that led to changes in micro-environmental conditions. These changes contribute to explain the variability in transpiration and water use efficiency among mixtures. We concluded that mixing local-adapted species in a region under non-limiting soil water conditions did not provide beneficial interest for ecosystem-level carbon and water balance at this ontogenic stage.