Investigations of the emissions of monoterpenes from Scots Pine

Plants produce and emit a large number of volatile organic compounds (VOC) such as isoprene and monoterpenes (C10H16). Monoterpene emission rates from Scots pine (Pinus sylvestris), a typical central European conifer, were measured under ambient conditions within the scope of this work. The studies focused on diurnal and seasonal cycles of monoterpene emissions, branch-to-branch and plant-to-plant variability of emission rates, and on the transferability ofresults from laboratory (studies of Dr. J. Wildt and coworkers) and outdoor measurements. Generally, no significant differences between the results obtained under laboratory and ambient environmental conditions were found. Under both laboratory and ambient conditions, monoterpene emissions were found to increase with needle temperature at a rate of 5 % to 16 % per Kelvin and fonowed under otherwise unchanged conditions an Arrhenius type dependence on temperature. The temperature dependence of emissions was without a clear seasonal trend and without significant differences from plant-to-plant. Only in the laboratory a dependence of emission rates on photosynthetic active radiation (PAR) was found (increase of 20-30% at a constant temperature, saturation in the light dependence at about 15 % of fun sunlight). Under outdoor conditions, a PAR dependence was not detected. Seasonal variations of a single branch and branch-to-branch variations in the spectrum of emitted monoterpenes were small, but different individual Scots pines emitted a completely different spectrum of monoterpenes. The temperature normalized standard emission rates were found to be highly variable. Values for the sum of monoterpenes ranged between 0.06 and 0.65 µg g(dW)-lh-1 (microgram monoterpenes per gram dry weight of needles and hour) for young pines and between 0.24 and 3.7 µg g(dW)-lh-1 for the adult pine. The variations of the standard emission rates from the same plant at different times of the year were on the same order of magnitude as the plant-to-plant variability. Stress to the plant was a possible explanation for these variations, but this effect could not be described quantitatively. ßased on the results a monoterpene flux was calculated for a forest in Southern Germany (Hartheimer Wald, near Freiburg), ranging between 54-941 ng m-2s-l at T = 30°C. Future laboratory studies should focus on stress effects and their impact on VOC emissions. The effect of stress on VOC emission rates must be quantified and included in the existing models for better predictions of emission rates and fluxes

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