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Trampling and Spatial Heterogeneity Explain Decomposer Abundances in a Sub-Arctic Grassland Subjected to Simulated Reindeer Grazing

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Abstract

Mammal grazing is composed of three mechanisms—removal of foliar tissue (defoliation), return of nutrients via dung and urine (fertilization), and trampling. To evaluate the relative role of these mechanisms in the effect of reindeer grazing on soil biota in northern grasslands, we subjected experimental plots in a sub-arctic alpine meadow to defoliation, fertilization (using NPK-solution), simulated trampling, and their factorial combinations once a year from 2002 to 2004 and measured the response of plants and decomposers (including microbes, nematodes, collembolans, and enchytraeids) in 2004. Trampling affected both plant and decomposer communities: the coverage of the moss Pleurozium schreberi and the sedge Carex vaginata, as well as the abundance of collembolans and enchytraeids were reduced in trampled plots. Trampling and fertilization also interacted significantly, with fertilization increasing the abundance of bacteria and bacterial-feeding and omnivorous nematodes in trampled plots only, and trampling decreasing fungal biomass in non-fertilized plots only. Defoliation had no overall effects on plants or decomposers. Nematode genera were not affected by the experimental treatments, but nematode and plant communities were significantly associated, and all decomposer biota, except collembolans, were strongly affected by the spatial heterogeneity of the study site. Our results indicate that trampling may have larger and defoliation and fertilization smaller roles than anticipated in explaining reindeer grazing effects in sub-arctic grasslands. However, even the effects of trampling seem to be outweighed by the spatial heterogeneity of decomposer abundances. This suggests that in sub-arctic grasslands spatial variation in abiotic factors can be a more important factor than grazing in controlling soil biota abundances.

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Acknowledgments

We would like to thank the Abisko field station for providing lodging and laboratory facilities for the initial processing of the soil samples. We also thank Iuliana Popovici (Institute for Biological research, Romania) for nematode identification. This study was funded by the Academy of Finland (project number 1206981). LIS also received grants from Tönningen Foundation, Envivonet, Oulun läänin talousseuran maataloussäätiö and the Ella and Georg Ehrnrooth foundation.

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Authors and Affiliations

  1. Department of Biology, University of Oulu, P.O. Box 3000, Oulu, 90014, Finland

    Louise Ilum Sørensen

  2. Department of Ecological and Environmental Sciences, University of Helsinki, Niemenkatu 73, Lahti, 15140, Finland

    Juha Mikola

  3. Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35 (YAC), Jyväskylä, 40014, Finland

    Minna-Maarit Kytöviita

  4. Department of Ecology and Environmental Science, Umeå University, Umeå, 901 87, Sweden

    Johan Olofsson

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  1. Louise Ilum Sørensen
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  2. Juha Mikola
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Correspondence to Louise Ilum Sørensen.

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LIS was involved in planning of the study, fieldwork, lab work, analysis, and wrote the article with contributions from all other authors; JM and MMK conceived the study and contributed to the data analyses; MMK further contributed to the field work and JM refined the final appearance of the text; JO started and was responsible for managing the field experiment, collecting the plant data and gave advice on statistical analysis.

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Sørensen, L.I., Mikola, J., Kytöviita, MM. et al. Trampling and Spatial Heterogeneity Explain Decomposer Abundances in a Sub-Arctic Grassland Subjected to Simulated Reindeer Grazing. Ecosystems 12, 830–842 (2009). https://doi.org/10.1007/s10021-009-9260-6

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