Ecosystem carbon storage two decades after afforestation in Norway spruce and silver birch monocultures and mixtures on abandoned agricultural land in hemiboreal Estonia

In silviculture, tree species mixing is seen as a potential measure to increase ecosystem carbon (C) sequestration due to higher resulting productivity compared to monocultures. Currently, there is a lack of studies on how monocultures and mixtures of the two main boreal tree species, Norway spruce and silver birch, compare regarding C sequestration and storage in planted stands. We evaluated ecosystem C stocks, stand productivity and soil nutritional status in spruce and birch monocultures as well as mixed stands with 50/50 mixing proportion two decades after afforestation of abandoned agricultural lands with homogeneous soil fertility. We found that Norway spruce monocultures had the highest mean annual increment (MAI), highest ecosystem C stock, and the highest periodic annual increment (PAI). In silver birch stands, C was mostly stored in the stems of the trees, whereas Norway spruce monocultures had a significantly higher share of C stored in the canopy. While the growth of individual silver birches was not influenced by the stand type, Norway spruce was suppressed by the inter-specific competition in the mixtures, where the average tree stem MAI was 56 % lower than in the monocultures. In general, the SOC and soil nutrient concentrations were similar in the mixed-species stands and monocultures. Spruce monocultures had a higher SOC stock in the forest floor. In conclusion, we did not confirm a higher productivity of mixed stands compared to monocultures, but different patterns of C allocations indicate the need for longer observations to identify the silvicultural system with the best climate benefit. • Mixed forests are favored over monocultures in the changing climate. • Productivity and C storage advantages of mixtures are unclear in Northern Europe. • We studied silver birch and Norway spruce monocultures and mixtures. • The highest productivity and C stocks were found in spruce monoculture. • Ecosystem C stock allocation differed between studied stand types.