Impact of timber felling on the ambient monoterpene concentration of a Scots pine (Pinus sylvestris L.) forest

The biogenic VOC emissions from the forests in Finland are dominated by monoterpenes. Annually, 4000–5500 km2 of forests are felled, which corresponds to 2–3% of the total forest land. Damaging plant organs has been found to increase the terpene emissions of conifers, and therefore tree cuttings can be expected to increase monoterpene emissions from a managed site. The aim of this study was to evaluate the effects of felling of Scots pine timber on the ambient monoterpene concentration of forest air below the canopy layer. To achieve this, three experimental plots (diameter 25 m) established in an even aged Scots pine stand (ca. 40 years) were felled manually in June 8–9 2004 with different intensities: one was clear-cut and two thinned with removal of 30% and 60% of the tree basal area. The felled biomass was left at the plots for the measurement period of June–September. The aerial monoterpene concentration was increased 2–3 fold by clear-cutting during the first 7 weeks after the felling. The increment of concentration due to the clear-cut was greater than in the thinnings, which also increased the concentration significantly compared to the control plot in untouched forest. The last concentration observations in late-September (15 weeks after felling) showed negligible differences between the plots. The amount of logging residue left at the site was respectively largest at the clear-cut plot, and the residue is considered to be the most important factor explaining the increment of the monoterpene concentration. The aerial monoterpene concentration cannot directly be used to describe the monoterpene flux of the site, and the actual increment of emitted monoterpenes due to the fellings remain unclear. However, the significant increase in the ambient concentration induced by the felling implies that there is a great potential impact on local or even regional atmospheric chemistry.