Stand type affects fluxes of volatile organic compounds from the forest floor in hemiboreal and boreal climates

The forest floor is a significant contributor to the stand-scale fluxes of biogenic volatile organic compounds. In this study, the effect of tree species (Scots pine vs. Norway spruce) on forest floor fluxes of volatile organic compounds (VOC) was compared in boreal and hemiboreal climates. Monoterpenoid and sesquiterpene flux rates were determined during 2017–2018 using dynamic (steady-state flow-through) chambers placed on permanent soil collars on boreal and hemiboreal forest floors, where the canopy was formed by Scots pine ( Pinus sylvestris ), Norway spruce ( Picea abies ) or dominated by Scots pine with small coverage of Norway spruce and birches ( Betula pendula and Betula pubescens ). The total monoterpenoid fluxes were higher from the Scots pine forest floor (boreal 23 μg m −2 h −1 ) and from the mixed forest floor (hemiboreal 32 μg m −2 h −1 ) compared to the Norway spruce forest floor in both boreal (12 μg m −2 h −1 ) and hemiboreal (9 μg m −2 h −1 ) climates. Due to higher litterfall production, the forest floor seems to be a greater source of monoterpenoids and sesquiterpenes in the hemiboreal mixed stand dominated by Scots pine compared to the boreal Scots pine stand, although the difference was not statistically significant. Forest floor VOC fluxes followed a similar seasonal dynamic in different forest stands, with the highest flux rates in spring and summer. Significant VOC sources in the boreal forest floor were synthesis and release from vegetation and living roots together with litter decomposition of fungi and other microbes, where VOCs are released from needle storage pools. Ground vegetation in the hemiboreal forest is scanty under the dense tree canopy, meaning soil processes, such as litter decomposition, microbial metabolism and root release, were likely the dominating VOC sources. VOC fluxes from the hemiboreal forest floor were reduced by increases in soil moisture. This study indicates that if the warming climate changes tree species’ abundance and stand biomass, by increasing tree growth and coverage of broadleaf species, it may affect VOC fluxes from the forest floor and impact the total VOC emissions from northern soils.