Short-term impacts of energy wood harvesting on ectomycorrhizal fungal communities of Norway spruce saplings

The ISME Journal Short-term impacts of energy wood harvesting on ectomycorrhizal fungal communities of Norway spruce saplings Karoliina Huusko 0 3 Oili Tarvainen 1 3 Karita Saravesi 0 3 Taina Pennanen 2 3 Hannu Fritze 2 3 Eero Kubin 1 3 Annamari Markkola 0 3 0 Department of Biology, University of Oulu , Oulu , Finland 1 Oulu Unit, Finnish Forest Research Institute , Oulu , Finland 2 Vantaa Unit, Finnish Forest Research Institute , Vantaa , Finland 3 Oulu , P.O. Box 3000, Oulu FI-90014 , Finland The increased demand for harvesting energy wood raises questions about its effects on the functioning of the forest ecosystems, soil processes and biodiversity. Impacts of tree stump removal on ectomycorrhizal fungal (EMF) communities of Norway spruce saplings were studied with 454-pyrosequencing in a 3-year field experiment replicated in 3 geographical areas. This is possibly the most thorough investigation of EMF communities associated with saplings grown on sites subjected to energy wood harvesting. To separate impacts of tree stump and logging residue removal on EMF and plant variables, we used three harvesting treatments with increasing complexity from patch mounding alone (P) to patch mounding combined with logging residue removal (RP), and patch mounding combined with both logging residue and stump removal (SRP). Saplings grown in uncut forests (F) served as references for harvesting treatments. A majority of sequences (492%) and operational taxonomic units (OTUs, 55%) were assigned as EMF. EMF OTU richness, fungal community composition or sapling growth did not differ between harvesting treatments (P, RP and SRP), while EMF OTU richness, diversity and evenness were highest and sapling growth lowest in the undisturbed reference forests (F). The short study period may partially explain the similarities in fungal and sapling variables in different harvesting treatments. In conclusion, our results indicate that neither stump removal nor logging residue removal have significant additional negative impacts on EMF communities or growth of Norway spruce saplings in the short-term compared with the impacts of more conventional harvesting methods, including clear cutting and patch mounding. The ISME Journal (2015) 9, 581-591; doi:10.1038/ismej.2014.154; published online 29 August 2014 Introduction In recent years, the practice of energy wood harvesting has rapidly increased due to economic and environmental considerations. The European Union has agreed to reduce greenhouse gas emissions and increase the overall share of renewable energy production by 20% by the year 2020 (EU, 2009) . National targets for biofuel production, for example, in Finland and Sweden, are set as high as 38% and 49%, respectively (EU, 2009) . The growing market for wood fuel has led to a rapid increase in the use of logging residue (that is, branches and foliage), and more recently also that of tree stumps and roots (Bjo? rheden, 2006; D??az-Ya?n? ez et al., 2013) . However, combined impacts of residue and stump harvesting, especially on soil processes and biota, are still poorly understood. Compared with stem harvest only, collecting also stumps and logging residues intensifies the loss of both soil nutrient and carbon (C) stocks (Palviainen et al., 2010; Walmsley and Godbold, 2010) , which may have cascading effects on soil microbiota and further on the new regenerating tree cohort. Most boreal forest trees form an association with ectomycorrhizal fungi (EMF) in their roots. Nutrient and water availability of trees is greatly improved by EMF symbionts, whereas EMF gain photosynthetic C from trees (Smith and Read, 2008) . EMF have an essential role in forest functioning, as they fuel the soil subsystem with recently fixed C (Godbold et al., 2006; Clemmensen et al., 2013) . In addition, many EMF are capable of enzymatic uptake of organic nitrogen (N). This is crucial, as N is considered to be the most limiting nutrient in boreal forests. Despite the large stock of organically bound N in forest soil, organic N is otherwise mainly inaccessible for trees (Lindahl et al., 2007; Read and Perez-Moreno, 2003) . Besides enzymatic uptake, EMF may specialize in nutrient mobilization over long distances (Agerer, 2001) . Thus, maintaining high EMF diversity in roots has been shown to have an impact on the host tree productivity in the field (Korkama et al., 2006; Saravesi et al., 2011) . In more controlled laboratory conditions, tree seedling growth increased with higher number of EMF associated with their roots, but the effect was dependent on tree species and nutrient status in the soil (Jonsson et al., 2001), as well as on functional complementarity of the EMF community (Velmala et al., 2014) . Silvicultural practices such as clear cutting and site preparation have a significant impact on EMF communities of establishing seedlings. However, remaining tree stumps and roots may retain EMF fungi for some years after their host trees have been harvested (Hagerman et al., 1999; Heinonsalo and Sen, 2007; Heinonsalo et al., 2007) . The time scale for turnover of mycorrhizal roots, sclerotia and hyphae, as well as preservation of fungal inoculum potential in roots and soil may vary depending on fungal species (Jones et al., 2003) . Compared with the substantial impacts of clear cutting on soil microbiota, collecting aboveground logging residues seems to have a lesser impact, as no significant longterm effects have been reported either on EMF or saprotrophic fungal communities (Mahmood et al., 1999; Allme?r et al., 2009) . On the other hand, reports on the effects of stump harvesting on EMF are scarce and partially contradictory. Stump removal decreased the number of EMF morphotypes in Douglas fir roots with a simultaneous reduction in growth and N content of seedlings (PageDumroese et al., 1998). In contrast, stump removal increased Norway spruce seedling growth, but had either no effect on EMF morphotypes (Kataja-aho et al., 2012) or reduced EMF richness in roots (Menkis et al., 2010) . Hence, impacts of stump removal and collecting logging residues on belowground microbial communities, especially on EMF, require further studies. Our aim was to estimate how energy woodharvesting practices contribute to EMF communities and growth of Norway spruce (Picea abies (L). Karst) saplings, on a south-to-north geographical gradient spanning from the southern boreal to middle boreal vegetation zones. The focus was on possible changes in root fungal diversity and community composition due to altered inoculum potential of disturbed forest soil, and further, on potential impacts on sapling growth. We expected differences in EMF along an increasing complexity of energy wood-harvesting practices, including patch mounding, logging residue removal and stump removal. Further, we expected stump removal to decrease EMF inoculum potential, possibly resulting in lower EMF diversity and, consequently, reduced (...truncated)