title: Ecosystem-scale volatile organic compound fluxes during an extreme drought in a broadleaf temperate forest of the Missouri Ozarks (central USA).
abstract: Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegetation and the surrounding air. These BVOCs play key ecological and atmospheric roles that must be adequately represented for accurately modeling the coupled biosphere-atmosphere-climate earth system. One key uncertainty in existing models is the response of BVOC fluxes to an important global change process: drought. We describe the diurnal and seasonal variation in isoprene, monoterpene, and methanol fluxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC fluxes were dominated by isoprene, which attained high emission rates of up to 35.4 mg m(-2)  h(-1) at midday. Methanol fluxes were characterized by net deposition in the morning, changing to a net emission flux through the rest of the daylight hours. Net flux of CO2 reached its seasonal maximum approximately a month earlier than isoprenoid fluxes, which highlights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Nevertheless, both processes were strongly suppressed under extreme drought, although isoprene fluxes remained relatively high compared to reported fluxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, confirming the complex processes driving biogenic methanol fluxes. The fraction of daytime (7-17 h) assimilated carbon released back to the atmosphere combining the three BVOCs measured was 2% of gross primary productivity (GPP) and 4.9% of net ecosystem exchange (NEE) on average for our whole measurement campaign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The meganv2.1 model correctly predicted diurnal variations in fluxes driven mainly by light and temperature, although further research is needed to address model BVOC fluxes during drought events.
authors.affiliation: Department of Earth System Science, University of California, Irvine, CA, 92697, USA.
authors.forename: Roger
authors.identifier: http://orcid.org/0000-0002-2078-9956
authors.initials: R
authors.lastname: Seco
authors.affiliation: Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Karl
authors.affiliation: Pacific Northwest National Laboratory, Richland, WA, USA.|Department of Civil and Environmental Engineering, Washington State University, Pullman, WA, USA.
authors.forename: Alex
authors.initials: A
authors.lastname: Guenther
authors.affiliation: Department of Forestry, University of Missouri, Columbia, MO, 65211, USA.
authors.forename: Kevin P
authors.initials: KP
authors.lastname: Hosman
authors.affiliation: Department of Forestry, University of Missouri, Columbia, MO, 65211, USA.
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Pallardy
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA.
authors.forename: Lianhong
authors.initials: L
authors.lastname: Gu
authors.affiliation: U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Research Triangle Park, NC, 27711, USA.
authors.forename: Chris
authors.initials: C
authors.lastname: Geron
authors.affiliation: Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, 80301, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Harley
authors.affiliation: Department of Earth System Science, University of California, Irvine, CA, 92697, USA.
authors.forename: Saewung
authors.initials: S
authors.lastname: Kim
mesh_terms: D002070:Butadienes / Q000378:metabolism*; D055864:Droughts; D065928:Forests; D045782:Hemiterpenes / Q000378:metabolism*; D000432:Methanol / Q000378:metabolism*; D008923:Missouri; D008962:Models, Theoretical; D039821:Monoterpenes / Q000378:metabolism*; D010420:Pentanes / Q000378:metabolism*; D014197:Trees / Q000378:metabolism*
chemical_list: D002070:Butadienes; D045782:Hemiterpenes; D039821:Monoterpenes; D010420:Pentanes; C005059:isoprene; D000432:Methanol
keywords: VOC; biogenic emissions; drought; isoprene; isoprene volcano; megan; methanol; monoterpenes