Volatile organic compound emission rates from mixed deciduous and coniferous forests in Northern Wisconsin, USA

Biogenic emissions of volatile organic compounds (VOC) from forests play an important role in regulating the atmospheric trace gas composition includi ng global tropospheric ozone concentrations. However, more information is n eeded on VOC emission rates from different forest regions of the world to u nderstand regional and global impacts and to implement possible mitigation strategies. The mixed deciduous and coniferous forests of northern Wisconsi n, USA, were predicted to have significant VOC emission rates because they are comprised of many genera (i.e. Picea, Populus, Quercus, Salix) known to be high VOC emitters. In July 1993, a study was conducted on the Chequameg on National Forest near Rhinelander, WI, to identify and quantify VOC emitt ed from major trees, shrubs, and understory herbs in the mixed northern for ests of this region, Emission rates were measured at various scales - at th e leaf level with cuvettes, the branch level with branch enclosures, the ca nopy level with a tower based system, and the landscape level with a tether ed balloon air sampling system. Area-average emission rates were estimated by scaling, using biomass densities and species composition along transects representative of the study site. Isoprene (C5H8) was the primary VOC emit ted, although significant quantities of monoterpenes (C10H16) were also emi tted. The highest emission rates of isoprene (at 30 degrees C and photosynt hetically active radiation of 1000 mu mol m(-2) s(-1)) were from northern r ed oak (Quercus rubra, > 110 mu g(C)g(-1) h(-1)); aspen (Populus tremuloide s, > 77); willow (Salix spp., > 54); and black spruce (Picea mariana, > 10) . Emission rates of hybrid poplar clones ranged from 40 to 90 mu g(C)g(-1) h(-1) at 25 degrees C; those of Picea provenances were generally <10, and e mission rates of a hybrid between North American and European spruces were intermediate to parental rates. More than 30 species of plants were surveye d from the sites, including several from previously unstudied genera such a s Alnus, Chamaedaphne, Ledum, Tilia, Rubus, and Sphagnum. Based on the meas ured isoprene concentrations in the daytime atmospheric surface layer and m ixed layer, area-averaged fluxes of isoprene were estimated to be about 1 m g(C)m(-2) h(-1). This estimate agrees reasonably well with model prediction s. Our results indicate that mixed forests in the Lake States region of the USA are a significant source of reactive VOC to the atmosphere. Accurate e stimates of these emissions are required for determining appropriate regula tory air pollution control strategies. Future studies are needed to extrapo late these estimates to other landscapes and to better understand the facto rs controlling observed variations in VOC emissions. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.