Cervid Exclusion Alters Boreal Forest Properties with Little Cascading Impacts on Soils
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Large herbivores are capable of modifying entire ecosystems with a combination of direct (for example browsing/grazing, trampling, defecation) and indirect (for example affecting plant species composition that then alters soil properties) effects. With many ungulate populations increasing across the northern hemisphere it is important to develop a general theory for how these animals can be expected to impact their habitats. Here we present the results of an 8-year experimental exclusion of moose (Alces alces) from 15 recent boreal forest clear-cut sites in Central Norway. We used standard univariate techniques to describe the treatment effect on multiple forest and soil properties and combined this with a multivariate Bayesian network structure learning approach to objectively assess the potential mechanistic pathways for indirect effects on soils and soil fertility. We found that excluding moose had predictable direct effects, such as increasing the ratio of deciduous to coniferous tree biomass and the canopy cover and decreasing soil bulk density and temperature. However, we found no treatment effects on any measures of soil processes or quality (decomposition, nitrogen availability, C/N ratio, pH, nutrient stocks), and furthermore, we found only limited evidence that the direct effects had cascading (indirect) effects on soils. These findings oppose the commonly held belief that moose exclusion will increase soil fertility, but still highlights the strong ability of moose to directly modify forested ecosystems.
KeywordsAlces alces cervid boreal forest herbivory Norway Bayesian network nitrogen availability tea bag index carbon stocks
We wish to thank Marc Daverdin for helping with the fieldwork and with database management, Winta Berhie Gebreyohanis for helping with fieldwork and the calibration of the biomass models and Marte Fandrem for helping with fieldwork. Then we wish to thank the numerous landowners who let us use their forests for this long-term experiment. Finally, we are grateful to the two anonymous reviewers who gave insightful and critical feedback that helped us to improve this paper. The establishment of the experimental design and the field work was funded by the Research Council of Norway Environment 2015 programme (Project 184036), the Norwegian Environment Agency and Nord- and Sør-Trøndelag County Administration.
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