The Impact of Controlled Grazing by Clethrionomys rufocanus on Experimental Guilds of Boreal Forest Floor Herbs

Abstract

Thirty plots representing herb-rich middle boreal forest floor vegetation were established in a greenhouse in January 1984. During the spring, the vegetation of the plots grew under simulated summer conditions and was exposed to different intensities of grazing by the vole Clethrionomys rufocanus. Survival of marked shoots was followed and the plots were harvested 10 May. Moderate grazing intensity had practically no impact upon the vegetation. High grazing intensities resulted in decimation of populations of small, broad-leaved herbs with soft leaf texture. Under very high grazing intensity, all herbs with umbrella shape (leaves attached apically to erect shoots) suffered heavily, regardless of toxicity. The results have following implications: (1) Serious food depletion by C. rufocanus during the summer in luxuriant habitats requires population densities which exceed observed maxima about ten times. (2) Toxic substances are of limited value as defense against voles, whereas shoot morphology is crucially important. (3) Differences between the grazing tolerance of umbrella and rosette herbs are great enough to have consequences for the structure of relatively unproductive plant communities where moderate vole densities suffice to create high grazing pressure. In particular, vole grazing may prevent temperate spring herbs, adapted to a cool and short growing season, from invading northern forest and tundra.