| Summary: | The VOC emission rates of 16 Betula pubescens and 15 Betula pendula trees were measured during the 1997 growing season in southern Finland. Since the monoterpene emission rates of Betula pubescens showed large variations between the tree specimen, these measurements were continued in 2000 with two different clones. Both Betula pubescens and Betula pendula had low monoterpene emission rates early summer, but after the leaves were fully grown, darker and harder, Betula pendula initiated high monoterpene emission rates, whereas Betula pubescens showed large tree-to-tree variations. One of the studied clones initiated higher emission rates after the leaves had matured, whereas the emission rates of the other clone decreased a little. Betula pubescens also emitted linalool and sesquiterpenes. Linalool was the dominant emitted compound in June but later in summer the linalool emissions declined. Also the monoterpene emission pattern changed in the course of the growing season; at the time of leaf expansion the emission was composed of different monoterpenes, but later sabinene and trans-ocimene dominated. The seasonal changes in the emission rates were analysed using the Effective Temperature Sum (ETS, accumulated temperature above 5 deg.C). Monoterpene emissions, as well as those of linalool and sesquiterpenes, were dependent on temperature. When light was prevented from reaching the cuvette, the monoterpene emissions decreased, but sesquiterpene emissions did not. Physical disturbance of the leaves increased the emission rates of cis-3-hexen-1-ol, cis-3-hexenylacetate, 2-hexenal and 1-hexanol considerably.
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