Forests with mixed tree species affect the functioning of soil microorganisms indirectly via root and litter traits in European forests

Gillespie, Lauren; Hättenschwiler, Stephan; Milcu, Alexandru; Wambsganss, Janna; Shihan, Ammar; Fromin, Nathalie

Two permanent forest plots at the Finnish site used for soil sampling in the SoilForEUROPE project in June 2017. A single tree species stand with Pinus sylvestris (left) and a mixed stand with the three species Picea abies, Pinus sylvestris and Betula pendula (right). Photo credit: L. Gillespie.
Two permanent forest plots at the Finnish site used for soil sampling in the SoilForEUROPE project in June 2017. A single tree species stand with Pinus sylvestris (left) and a mixed stand with the three species Picea abies, Pinus sylvestris and Betula pendula (right). Photo credit: L. Gillespie.

Forest soils harbour a highly diverse community of microorganisms, such as bacteria and fungi, that play a crucial role in forest functioning and stability. Indeed, these microorganisms control the rate nutrients and carbon are released from dead organic matter, such as leaf litter, and made available to organisms. They concurrently release greenhouse gases, notably CO2 and N2O, into the atmosphere. However, the activity of these soil microorganisms and their subsequent influence on ecosystem functioning are dependent on many factors, including the plant community. Forests composed of one versus multiple tree species (mixed forests) can affect soil microbial activity differently in various ways including: different leaf litters, which contain different chemical compounds and nutrients, different compounds secreted from tree roots, different tree-microorganism symbioses, and modification of soil properties.
In four mature, natural forest ecosystems across Europe, we sought to understand how forests composed of a single versus mixed tree species forests differently affect the functioning of soil microorganisms through their different leaf litter and root traits. We hypothesized that mixed tree species forests promote higher soil microbial activity.
We found that, across different forests ranging from Mediterranean to boreal forests, mixed forests modified soil microorganism activity differently, compared to single tree species forests, through their different leaf litter and root traits and potentially through modifying soil properties. However, mixed forests did not always promote higher soil microbial activity. These results help us to better understand how tree species mixing influences forest processes driven by the soil microbial community. But the studied system is complex and disentangling the effects of individual parameters is difficult at the large spatial scale of our study. For soil microbial functioning, the consequences of changes in tree species composition in response to species loss, climate change, or management decisions may be largely determined by the modification of leaf litter and root traits represented by the newly established tree communities and their modifications to soil properties.

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