Basal emission rates of isoprene and monoterpenes from major tree species in Japan: interspecies and intraspecies variabilities

Uncontrolled terpenoid emissions from forest trees in Japan may have contributed to high O₃ concentrations observed in urban and suburban areas. To estimate ozone formation via a series of reactions between NO_x and terpenoids using atmospheric chemistry models, it is important to produce terpenoid emission inventories by collecting all reported emission data for the major tree species in Japan and examining their reliability. In this review, we first describe three different plant terpenoid emission types, i.e., isoprene-emitting type, monoterpene-emitting type with storage tissues and organs, and monoterpene-emitting type without storage tissues and organs. Second, we describe various methods for measuring plant terpenoid emissions, including a recently developed simplified method, and explain their reliability. We emphasized that applicable measurement methods depend on the terpenoid emission types. Data obtained using static chamber methods should not be considered because they have the highest uncertainty resulting from normal chamber materials that are not specific to terpenoid measurements and lack humidity control. Finally, we show the absolute values of the collected emission rates and describe their variability. The deciduous oak species, Quercus serrata and Quercus mongolica var. crispula, and bamboo species, Phyllostachys pubescens and Phyllostachys bambusoides, are strong isoprene emitters. Among the monoterpene emitters, four evergreen broadleaf trees, including three Quercus species, had the highest basal emission rate (BER). The monoterpene storage type conifers Larix kaempferi and Pinus densiflora have relatively lower BERs. Emission data are not available for Castanopsis cuspidata, and seasonal changes in emission rates have not been reported for several major tree species in the top 20 rankings. Within species, the reported emission rates of some tree species differed by threefold. These differences may be attributed to the reliability of the measurement and analytical systems, tree age, leaf morphology, environmental conditions, and genetic diversity. We emphasize the need for reliable measurements to achieve a more precise terpenoid emission inventory for major tree species in Japan.