Differences in vole preference, secondary chemistry and nutrient levels between naturally regenerated and planted Norway spruce seedlings.

Field voles (Microtus agrestis) cause severe damage to young Norway spruce (Picea abies) plantations during wintertime in Fennoscandia. We experimentally investigated vole preference for winter-dormant, naturally regenerated seedlings; spring-planted seedlings; or autumn-planted seedlings; and how preference corresponds with seedling chemistry. Voles showed the highest preference for autumn-planted seedlings and the second highest for spring-planted seedlings, while naturally regenerated seedlings were avoided. The stems of the autumn-planted seedlings contained higher concentrations of nitrogen and piperidine alkaloids and lower concentrations of stilbenes than did the other groups. In addition to differences between naturally regenerated and planted seedlings, we investigated seasonal differences in naturally regenerated P. abies needle and bark secondary chemistry. While piperidine alkaloid concentrations did not vary with season, the soluble non-tannin phenolics of needles and the condensed tannins of bark were lower in May than in November or January. At the time of planting, the concentration of bark piperidine alkaloids was higher in autumn-planted than in spring-planted seedlings. We detected two alkaloids not previously found in P. abies, 2-methyl-6-propyl-1,6-piperideine and a tentatively identified pinidine-isomer. Our results demonstrate that vole choice of spruce seedlings is promoted by high nitrogen and low stilbene content, both associated with seedlings planted late in the season. As vole damage is linked to seedling chemistry, damage potentially could be mitigated by advancing planting or by manipulating plant chemistry in nurseries.