Canopy herbivory altering C to N ratios and soil input patterns of different organic matter fractions in a Scots pine forest

Herbivorous insects can affect ecological processes in forested ecosystems such as nutrient and matter cycling especially during outbreak situations. However, the knowledge about their contribution to the quality and flows of energy and matter in forests is still imperfect. In this paper we report on the herbivore-affected C to N ratios in different fractions of organic matter cascading from the canopy to the forest floor during a pine lappet (Dendrolimus pini L.) mass infestation. Throughout a four months period we monitored the C and N fluxes with throughfall, and the C/N ratios of insect excrements (faeces) and pine needles in an 80-year-old Scots pine forest. Compared to the control, herbivore defoliation significantly magnifies C and N input fluxes by two to three times amounting to 95 kg TOC and 5.9 kg TN ha−1 in addition. Concurrently NO3-N fluxes diminished and the C/N ratios in throughfall solutions increased during peaking frass activity. Compared to fresh needle biomass, the C/N ratios in insect faeces triple during peaking frass activity resulting in values between 70 and 100. This study demonstrates the importance of herbivorous insect’s pests on element cycling as they act as a short-time phenomenon altering the nutrient quality and quantity reaching the forest floor and potentially affecting below-ground processes.