The age structure, both on the ramet and the genet level of boreal willow populations (Salix myrsinifolia-phylicifolia) was studied in relation to fluctuations in populations of voles (Clethrionomys glareolus and Microtus agrestis). Willow ramets were collected from four different areas in the coastal part of Sweden and Finland at the Bothnian Bay. In each area we selected one mainland and one island locality. The ramets, as well as the scars left by bark-eating voles on the ramets, were aged. We found in general a high correlation between ramet ages and peaks in vole cycles (based on frequencies of scars).
From a sample of 79 willow clones where all ramets were aged, more than half of the ramets were produced the summer following a vole peak. Field surveys of seedling survival showed that successful establishment is mainly confined to disturbed ground (vole run-ways, etc.) in connection with vole peak years. It is concluded that the age structure both at the genet and the ramet level in boreal willow populations are strongly influenced by the fluctuations in vole populations. The consequences of vole herbivory are manifold, e.g. periodic damage of willows may delay senescence of individual genets and delay the rate of succession due to increased opportunities for willow recruitment.