High Monoterpenoid Emissions from Scots Pine Litter Controlled by Moisture.

Biogenic volatile organic compounds (VOCs) play important roles in atmospheric chemistry, yet most studies have focused on canopy emissions. Decomposition of forest litter, a major below-canopy VOC source, can substantially influence atmospheric oxidation and aerosol formation. Scots pine (Pinus sylvestris L.), one of the most widely distributed tree species across the boreal zone, produces terpene-rich litter that may represent a significant but understudied VOC source. Here, we incubated fresh needle litter under controlled temperature and moisture levels to quantify VOC and CO2 fluxes. Monoterpenoids overwhelmingly dominated emissions (91%), with oxygenated species such as camphor ((+)-2-bornanone) and 2,5-bornanedione being the most abundant. Moisture was the main control: water addition increased monoterpenoid fluxes by 5- to 7-fold relative to drier treatments, suggesting a potential role of microbial activity. Temperature had a weaker but compound-specific influence, the strongest for sesquiterpenoids. Isoprene emission rates increased, while oxygenated VOC emission rates declined over time, indicating a transition from stored-pool release to microbial processes. Specifically, the strong correlation between monoterpenoid and CO2 fluxes suggests shared microbial processes and highlights the key role of moisture in VOC release from decomposing pine litter. This relationship also offers a practical basis for how CO2 flux data could potentially be used to help constrain monoterpenoid emissions from pine-dominated forest floors.