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.