Diurnal variation in BVOC emission and CO2 gas exchange from above- and belowground parts of two coniferous species and their responses to elevated O3

Increased tropospheric ozone (O₃) concentrations in boreal forests affect the emission of biogenic volatile organic compounds (BVOCs), which play crucial roles in biosphere-atmosphere feedbacks. Although it has been well documented that BVOC emissions are altered in response to elevated O₃, consequent effects on the carbon budget have been largely unexplored. Here, we studied the effects of elevated O₃ (80 nmol mol⁻¹) on diurnal variation of BVOC emissions and gas exchange of CO₂ from above- and belowground parts of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) and further investigated effects on the carbon budget. In spring, elevated O₃ decreased BVOC emissions and net photosynthesis rate (Pn) from above-ground parts of both species. As BVOC emissions have a causal relationship with dormancy recovery, O₃-induced decreases in BVOC emissions indicated the inhibition of dormancy recovery. Contrary to the spring results, in summer BVOC emissions from aboveground parts were increased in response to elevated O₃ in both species. Decreases in Pn indicated O₃ stress. O₃-induced monoterpene emissions from aboveground were the main volatile defense response. Elevated O₃ had little effect on BVOC emissions from belowground parts of either species in spring or summer. In spring, elevated O₃ decreased the proportion of carbon emitted as BVOCs relative to that assimilated by photosynthesis (the proportion of BVOC-C loss) at the soil-plant system levels in both species. In summer, elevated O₃ resulted in a net CO₂–C loss at the soil-plant system level of Scots pine. During this process, O₃-induced BVOC-C loss can represent a significant fraction of carbon exchange between the atmosphere and Scots pine. In Norway spruce, the effects of O₃ were less pronounced. The current results highlight the need for prediction of BVOC emissions and their contributions to the carbon budget in boreal forests under O₃ stress.