A review and synthesis of monoterpene speciation from forests in the United States

The monoterpene composition (emission and tissue internal concentration) of major forest tree species in the United States is discussed. Of the 14 mos t commonly occurring compounds (alpha-pinene, beta-pinene, Delta(3)-carene, d-limonene, camphene, myrcene, alpha-terpinene, beta-phellandrene, sabinen e, rho-cymene, ocimene, alpha-thujene, terpinolene, and gamma-terpinene), t he first six are usually found to be most abundant. Expected regional varia bility based on the monoterpene composition fingerprints and corresponding toe species distribution and abundance is examined. In the southeast, alpha -pinene and beta-pinene seem to dominate monoterpene emissions, while in th e northern forests emissions are distributed more evenly among the six majo r compounds. In some parts of western forests, beta-pinene and Delta(3)-car ene can be more abundant than alpha-pinene. Among the other eight compounds , beta-phellandrene and sabinene occasionally are significant percentages o f expected local monoterpene emissions. Ocimene and rho-cymene are estimate d to be more common in regions dominated by deciduous broadleaf forests, al though total emission rates are generally lower for these forests relative to those dominated by conifers. These percentages are compared with monoter pene composition measured in ambient air at various sites. Estimated monote rpene emission composition based on local forest species composition agrees fairly well with ambient measurements for the six major compounds. The pas t assumption that alpha-pinene composes approximately 50% of total monoterp ene emissions appears reasonable for many areas, except for possibly the no rthern coniferous forests and some areas in the west dominated by true firs , spruce, and western pines (lodgepole and ponderosa pines). The oxygenated monoterpenes such as camphor, bornyl acetate, and cineole often compose hi gh percentages of the monoterpenes within plant tissues, but are much less abundant in emission samples. Even after adjusting for lower vapor pressure s of these compounds, emission rates relative to the hydrocarbon monoterpen es are often lower than would be expected from their internal concentration s. More study is warranted on monoterpene emission rates and composition, e specially from the spruces, true firs, hemlocks, cedars, and some deciduous species such as the maples. Non-invasive canopy level and whole ecosystem flux studies are also needed to establish uncertainty estimates for monoter pene emission models. (C) 2000 Elsevier Science Ltd. All rights reserved.