Impacts of tree species identity and species mixing on ecosystem carbon and nitrogen stocks in a boreal forest
FOREST ECOLOGY AND MANAGEMENT 2020, 458 (117783)
Blasko R, Forsmark B, Gundale MJ, Lundmark T, Nordin A

Abstract
Forest management practices, such as selection or mixing of particular tree species, may enhance forests’ carbon (C) sinks and resilience against climate change. While a majority of research on this subject has focused on aboveground production, far less is known about how these management decisions impact belowground C storage, as well as the C and nitrogen (N) stocks of the whole ecosystem. We used a well-replicated 60-year-old experiment in boreal Sweden comparing monocultures and a mixture of the two dominant coniferous species: Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris (L.), set up at a site that was assessed as equally suitable for the growth of either species. Our aim was to evaluate the species identity and species mixing effects on ecosystem C and N stocks. We measured total standing volume, aboveground tree biomass, fine-root biomass, C and N pools in tree biomass and soil, litterfall inputs, and soil CO2 emissions. Our results show major differences in C allocation and growth patterns between spruce and pine. We found almost twice as high total standing volume and litterfall inputs in the pine stands than in the spruce stands. Higher proportion and amount of needle biomass resulted in larger amounts of N retained in the canopy and smaller accumulation of C and N in the humus in the spruce compared to pine stands. The C sinks in aboveground tree biomass and soil were larger in the pine compared to spruce stands at this site. In addition, a significantly higher soil CO2 efflux rate and fine-root biomass in the spruce compared to pine stands suggested greater tree internal allocation of C belowground to roots and ectomycorrhizal fungi in response to stronger N limitation. We found no significant mixing effect in the mixed stands, given the levels of the measured variables did not exceed levels of the most productive monoculture, with an exception of higher SOC stocks in the deeper (10–20 cm) mineral soil layer in the mixed stands. Our results do not support the idea of higher productivity and C sinks of forest mixtures compared to the best performing monoculture on the given site suggesting that these tree species are not complementary from a forest management perspective. However, in many cases the mixed stands performed equally well as the best monoculture, indicating that management for multi-species stands may not result in any loss in C uptake and storage.

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