R eports Ecology, 94(2), 2013, pp. 267–272 Ó 2013 by the Ecological Society of America Moth herbivory enhances resource turnover in subarctic mountain birch forests? MAARIT KAUKONEN, 1,4 ANNA LIISA RUOTSALAINEN, 1 PIIPPA R. WA ¨ LI, 1 MINNA K. MA ¨ NNISTO ¨ , 2 HEIKKI SETA ¨ LA ¨ , 3 KARITA SARAVESI, 1 KAROLIINA HUUSKO, 1 AND ANNAMARI MARKKOLA 1 1 Department of Biology, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland 2 Finnish Forest Research Institute, P.O. Box 16, 96301 Rovaniemi, Finland 3 Department of Environmental Sciences, University of Helsinki, Niemenkatu 73, 15140 Lahti, Finland Abstract. Massive moth outbreaks cause large-scale damage in subarctic mountain birch forests with a concomitant decrease in carbon flux to mycorrhizal fungi and an increased deposition of dissolved carbon and nutrients as moth frass into soil. We investigated impacts of moth herbivory along three replicated gradients with three levels of moth herbivory (undamaged, once damaged, repeatedly damaged) on soil nutrient levels and biological parameters. We found an increase in soil nutrients and in the biomass of enchytraeid worms, which are key faunal decomposers. Fungi : bacteria ratio and C:N ratio decreased in humus with increasing severity of herbivory. Our findings suggest enhanced resource turnover in mountain birch forests due to massive moth herbivory. This may provide a shortcut for carbon and nutrient input to subarctic soils, which largely bypasses the main routes of carbon from plants to soil via mycorrhizal and litter-decomposing fungi. Moreover, a temporal shift occurs in carbon allocation to soil, providing decomposers an opportunity to use an early- season peak in resource availability. Our results suggest a hitherto unappreciated role of massive insect herbivore attacks on resource dynamics in subarctic ecosystems. Key words: Betula pubescens; carbon allocation; Epirrita autumnata; Finnish Lapland; humus and soil chemistry; moth herbivory; mountain birch; Operophtera brumata; resource turnover; soil decomposers; subarctic ecosystems. INTRODUCTION Above- and belowground communities of forest ecosystems are tightly connected. For example, micro- bial activity in the rhizosphere is controlled by carbon flux from photosynthesizing plants (Kuzyakov and Cheng 2001). Because the growth of soil organisms is generally controlled by carbon availability (Bardgett and Wardle 2010), a major reduction of carbon flux belowground, such as the removal of canopy foliage due to herbivory, is likely to have a marked impact on soil- inhabiting organisms. Intensive moth herbivory on tree canopies results in massive deposition of moth feces (frass) with high quantities of dissolved N and C compounds into the soil (Frost and Hunter 2004). Canopy herbivory has been found to change understory plant community composition as well as alter litter input and quality (Wardle et al. 2004, Mitchell et al. 2011). Changes both in litter quality and frass deposition can affect soil decomposition processes and the nutrient mineralization–immobilization balance in soil (Hunter 2001). In arctic communities, intensive mammalian herbiv- ory on aboveground plant parts has been reported to result in either acceleration (Stark and Grellmann 2002) or deceleration (Olofsson and Oksanen 2002) of nutrient cycling rates. Whether herbivory accelerates or deceler- ates nutrient cycling may be crucial to the recovery of ecosystems after herbivore infestation. If soil nutrient availability increases due to frass deposition, compen- satory growth of the damaged trees could be facilitated (Anttonen et al. 2002). Outbreaks of the foliage-feeding autumnal moth, Epirrita autumnata (Borkh.), on mountain birch, Betula pubescens ssp. czerepanovii (N. I. Orlova) Ha¨met-Ahti, forests appear at approximately 10-year intervals in the Fennoscandian subarctic (Nuorteva 1963). Recently, winter moth, Operophtera brumata (L.), also has spread Manuscript received 5 June 2012; revised 13 September 2012; accepted 14 September 2012. Corresponding Editor: D. A. Wardle. 4 E-mail: maarit.kaukonen@oulu.fi 267