PNNL

Abstract

Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegeta-tion and the surrounding air. These BVOCs play key ecological and atmospheric roles that must be adequately repre-sented for accurately modeling the coupled biosphere–atmosphere–climate earth system. One key uncertainty in existing models is the response of BVOC ?uxes to an important global change process: drought. We describe the diur-nal and seasonal variation in isoprene, monoterpene, and methanol ?uxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC ?uxes were domi-nated by isoprene, which attained high emission rates of up to 35.4 mg m??2 h??1 at midday. Methanol ?uxes were characterized by net deposition in the morning, changing to a net emission ?ux through the rest of the daylight hours. Net ?ux of CO2 reached its seasonal maximum approximately a month earlier than isoprenoid ?uxes, which high-lights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Never-theless, both processes were strongly suppressed under extreme drought, although isoprene ?uxes remained relatively high compared to reported ?uxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, con?rming the complex processes driving biogenic methanol ?uxes. 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 cam-paign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The MEGANv2.1 model correctly predicted diurnal variations in ?uxes driven mainly by light and temperature, although further research is needed to address model BVOC ?uxes during drought events.