Tree temperatures, volatile organic emissions, and primary attraction of bark beetles

Bark beetles (Scolytidae) may be attracted by organic compounds emitted from tree barks and appear able to identify weakened trees, but the exact relationship between tree health and attraction is not known. This paper deals with possible connections between reduced sapflow, volatile emissions, and beetle colonisation of injured spruce trees. The hypothesis tested assumes that the cooling effect of sapflow on the phloem of healthy trees is sufficient to keep temperature-dependent emissions of terpenes below those of stressed trees with reduced transpiration. Temperature profiles from the bark surface to the sapwood were measured on three trees over several weeks. One tree in a closed stand gave the natural temperature profile and the flow of xylem sap. The external temperature of a 1 m long stem zone of a second tree in the closed stand was kept constant by artificial heating to study the effects of sapflow on phloem temperature uncoupled from interaction with external heat fluctuations. In a third tree the effect of interrupting sapflow on temperature profiles and emissions of ethanol and monoterpenes was studied by transection of the sapwood. Although sapflow had a significant cooling effect on the cambium temperature, it resulted in a maximum temperature difference of not more than 2.5 °C between a tree with high sapflow and one without transpiration, which does not appear sufficient for a substantial increase in emissions that might attract beetles to stressed trees. A strong correlation between bark temperature and emissions was corroborated and the daily maximum of emissions coincided with the maximum of sapwood temperature. Interruption of the sapflow gradually changed the emission spectrum, with pinene increasing relative to limonene, and ethanol detected in appreciable amounts only after several months. The emission spectrum was not identical with the terpene content of the bark. These findings raise the possibility that at least part of the emissions do not originate in the bark but in the sapwood. A survey of literature data on sapwood ethanol contents indicates a connection between sapflow, oxygen contents of xylem water and the emission of ethanol.