Monoterpene and Isoprene Emission in Norway Spruce Forests

The isoprenoids isoprene and monoterpenes are hydrocarbons with important ecological functions. In conifers monoterpenes are synthesised mainly for protection against insects. In a ca. 90 year old Norway spruce stand in the Bavarian Forest National Park, Germany, and on the Wank mountain near Garmisch-Partenkirchen, Germany, the formation of isoprene and monoterpenes in needles of Picea abies [L.] Karst., the emission into the atmosphere, the diurnal and seasonal cycles of emission and immission, the horizontal and vertical distribution of the source strength in and above the canopy and the fate of isoprenoids in the atmosphere were studied. The work was performed together with meteorologists and air chemists. Controlling factors in both, isoprene emissions and monoterpene emissions from intact needles are leaf temperature and photosynthetic active radiation (PAR). On the basis of these two parameters emission factors and emission algorithms were calculated. The sun crown of the trees is the dominating source for isoprenoids in the forest during the physiologically active period of the year. In 1991, 88% of the monoterpens were emitted in July and August. The contribution of the trunk compartment to total forest emission ranges from 1% to 64% and is not clear yet. The contribution of the soil is negligible (3%). The emission pattern and the emission rates of monoterpenes dramatically change after injuring of needles and trunks by opening the resin ducts. The α-pinene fluxes of the forest calculated using the gradient approach and the enclosure technique are similar and indicate that the sun crown of the forest is the main source of the isoprenoids transported in upper air layers. Around noon, however, the gradient approach resulted in deposition of α-pinene, although this compound is emitted from the twigs in large amounts due to high temperature and high solar insulation. At the site the ozone forming potential through photochemical isoprenoid degradation should be of minor importance, since the terpene/NOx ratio in ambient air is mostly < 1 during the day. The interdisciplinary investigations of reactive trace gases released into the atmosphere by Norway spruce ecosystems demonstrate the complexity of the interactions soil, tree, canopy, atmosphere, man and animal.