Bark consumption of voles in relation to snow cover, population density and grazing impact

The voles Microtus agrestis and Clethrionomys glareolus consume more bark in north than in south Sweden in spite of lower availability in the former area due to snow‐pressed ground. Field experiments with genetically homogeneous aspen sticks demonstrated that this pattern reflects differences in animal behaviour and not differences in plant palatability. Within a region consumption in snow‐pressed areas was usually less than in subnivean spaces or runways. The amount of bark consumption was generally correlated with population density and amount of grazing impact. Great differences in bark consumption between north Swedish areas with high peak vole populations and south‐central Swedish areas with moderate peak numbers imply that nutrition is of different importance at population declines in different regions. The ability of voles to move into snow‐pressed areas and locate woody sticks in the snow suggests that control of ground cover will not protect tree seedlings at reforestation.