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Abstract
The major phytoncides found in coniferous and broad-leaved forests are α-pinene and isoprene, respectively. The amounts of these phytoncides vary between seasons, increasing from June to September for the summer, and decreasing for the winter. The patterns of diurnal variation differ between the different substances; the amounts of monoterpenes, such as α-pinene, are higher during the night than during the day whereas the amounts of isoprene are the opposite. In a forest, the horizontal distribution of phytoncides during the day tends to be more concentrated near the center of the forest than its edges and their vertical distribution tends to be more concentrated closer to the ground.
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Monoterpene mixing ratios and fluxes were measured above a ponderosa pine plantation in the Sierra Nevada mountains from July to October 1998. Data were obtained during a variety of weather regimes including periods of extreme heat and dryness as well as during rain. Monoterpene emissions were highly elevated during and after the rain events, and the expected exponential increase of emissions with temperature did not occur during extremely hot and dry conditions, suggesting an influence of ambient humidity levels on monoterpene emissions. Therefore, we propose a modified emission algorithm based on responses to both temperature and humidity.
Volatile (VOC) flux from leaves may be expressed as GSΔP, where GS is stomatal conductance to specific compound and ΔP partial pressure gradient between the atmosphere and substomatal cavities. It has been suggested that decreases in GS are balanced by increases in ΔP such that stomata cannot control VOC emission. Yet, responses of emission rates of various volatiles to experimental manipulations of stomatal aperture are contrasting. To explain these controversies, a dynamic emission model was developed considering VOC distribution between gas and liquid phases using Henry's law constant (H, Pa m3 mol-1). Our analysis demonstrates that highly volatile compounds such as isoprene and monoterpenes with H values on the order of 103 have gas and liquid pool half-times of a few seconds, and thus cannot be controlled by stomata. More soluble compounds such as alcohols and carboxylic acids with H values of 10-2-101 are controlled by stomata with the degree of stomatal sensitivity varying with H. Inability of compounds with high solubility to support a high partial pressure, and thus to balance ΔP in response to a decrease in GS is the primary explanation for different stomatal sensitivities. For compounds with low H, the analysis predicts bursts of emission after stomatal opening that accord with experimental observations, but that cannot be currently explained. Large within-leaf VOC pool sizes in compounds with low H also increase the system inertia to environmental fluctuations. In conclusion, dynamic models are necessary to simulate diurnal variability of the emissions of compounds that preferably partition to aqueous phase.
Seasonal variations of the leaf oil contents and the amounts of terpene emission of Cryptomeria japonica, Chamaecyparis obtusa, Abies sachalinensis, Thuja occidentalis, Thuja standishii, Cupressus sempervirens and Cinnamomum japonicum were studied. The leaf oil contents and the amounts of terpene emission were high in summer and low in winter. It was suggested that the terpene emission varied with leaf oil production. The amounts of terpene emission from C. obtusa were higher at 30°C than 15°C either in the light or in the dark and they were also higher in the light than in the dark. The amounts of terpene emission increased by the forced emission method by which three seedlings on pots were rotated by touching each other's leaves and the amounts of terpenes reached 3 to 35 μg/(g(grams dry weight) · h).The individuals of varieties of C. obtusa seedlings whose quantities of terpene emission were high were selected to propagate these individuals.
The mechanism of monoterpene emission from Pinus densiflora was studied using an environmentally controlled gas cabinet. It was found that monoterpene emission rate increases exponentially with temperature and is also influenced by light. These observations were explained reasonably by a mechanism whereby monoterpene emission rate depends on the monoterpene amount in the leaf oil and its saturated vapor pressure.