Increasing vole numbers cause more lethal damage to saplings in tree monocultures than in mixed stands |
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Authors: | Sonja Gilbert Jocelyn Martel Tero Klemola Kai Norrdahl |
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Institution: | 1. Section of Ecology, Department of Biology, University of Turku, 20014 Turku, Finland;2. Environmental and Health Studies Program, Department of Multidisciplinary Studies, Glendon College, York University, 2275 Bayview Avenue, Toronto M4N 3M6, Ontario, Canada |
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Abstract: | Overall, mammalian herbivores are more harmful in mixed plantations than in monocultures, but the effect of herbivore abundance has not been experimentally tested in this context. It has been proposed that there is a critical threshold density where herbivore pressure spreads from preferred plants to everything edible, leading to non-linear density effects on low-quality plants. We experimentally investigated whether survival of an unpalatable plant is similarly related to herbivore density in both monocultures and mixed stands. This we did by establishing monocultures of unpalatable black alder (Alnus glutinosa) and mixed stands of black alder and five more palatable tree species in enclosures, where Microtus voles were introduced and their abundances monitored.The effect of stand diversity tended to depend on vole abundance. Vole damage of tree saplings did not differ between monocultures and mixed stands, but at higher vole abundances attacks had a stronger effect on sapling survival in the monocultures. Sapling survival showed a significant drop in the monocultures at peak abundance of approximately 300 voles ha?1. In monocultures herbivores do not have alternatives and therefore are forced to become deadlier consumers. |
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