Phenolics/Nitrogen Ratios in the Blueberry Vaccinium myrtillus in Relation to Temperature and Microtine Density in Finnish Lapland

The blueberry Vaccinium myrtillus, a deciduous dwarf shrub, is the most important food item of the grey-sided vole Clethrionomys rufocanus. These rodents show pronounced population cycles in northern Fennoscandia. We studied the phenolics/ nitrogen ratios in the blueberry during one microtine cycle, from 1979 to 1984. Chemical analyses were made on leaves and stems. Both seasonal and multiannual variation was studied. In the course of the growing season, total nitrogen declined and total phenolics increased. Differences among years were significant. Nitrogen concentration was low in warm summers and higher in cold summers. This seemed to reflect a dilution of similar leaf nitrogen mass by increased amounts of structural tissue in a warm year favourable for photosynthesis. The trends in total phenolic were the opposite. The role of temperature was confirmed by greenhouse experiments in the field; the concentrations of phenolics were significantly higher and nitrogen lower in plants from greenhouses with higher temperature. Temperature also explained the altitudinal variation. High densities of Clethrionomys rufocanus did not induce high phenolics/nitrogen ratios. In contrast, phenolics had a high positive correlation and nitrogen a negative correlation with the density of C. rufocanus in the following year. However, due to the short study period (6 years) we cannot firmly conclude whether these correlations are merely accidental or reflect real patterns.