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

Increased tropospheric ozone (O) 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 O, consequent effects on the carbon budget have been largely unexplored. Here, we studied the effects of elevated O (80 nmol mol) on diurnal variation of BVOC emissions and gas exchange of CO 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 O 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, O-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 O in both species. Decreases in Pn indicated O stress. O-induced monoterpene emissions from aboveground were the main volatile defense response. Elevated O had little effect on BVOC emissions from belowground parts of either species in spring or summer. In spring, elevated O 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 O resulted in a net CO-C loss at the soil-plant system level of Scots pine. During this process, O-induced BVOC-C loss can represent a significant fraction of carbon exchange between the atmosphere and Scots pine. In Norway spruce, the effects of O 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 O stress.