Mature growth-phase internodes of Alaska paper birch (Betula resinifera) are preferred by the snowshoe hare (Lepus americanus) over juvenile growth-phase internodes due to the low food value of the latter. While the mature over juvenile preferencec cannot be explained by the levels of inorganic nutrients or gross chemical fractions (resins or phenols), it can be explained by the striking differences in secondary metabolites of the two growth phases. The principle compound which renders the juvenile phase internodes unpalatable is papyriferic acid, a triterpene which is a demonstrated feeding deterrent to snowshoe hares and which is present in juvenile internodes at concentrations 25 times greater than those in mature internodes.

Evidence from experimental feeding trials and chemical analysis showed that, when severe winter browsing of adult—form Alaska feltleaf willow (Salix alaxensis) causes it to revert to juvenile—form stump sprouts, the twigs are unpalatable and of lower nutritional quality to the hares than are twigs of adult—form S. alaxensis plants. Further feeding choice trials demonstrated that the palatability of winter—dormant S. alaxensis to snowshoe hares is a function of the age of the sprout and its chemistry rather than the accessibility (height) or morphology of the twigs. The low nutritional quality of juvenile sprouts is partially caused by decreased dry matter and nitrogen digestibility, which may be related to increased lignin and phenolic content of the twigs. This browsing—induced resistance to future winter browsing by hares relaxes within 3 yr and within the height range of twigs normally available to snowshoe hares in winter. These results are compatible with hypothesis that the inducation and relaxation of resistance to browsing in the preferred winter food of snowshoe hares is a cause of the well—known 10—yr snowshoe hare cycle.

Pinosylvin methyl ether (PME), a toxic phenol, is a potent deterrent to showshoe hare feeding on green alder. Concentrations of PME found in green alder parts can account for the low palatability of winter-dormant foliar buds and staminate catkins but cannot affect internode palatability. The lack of a PME-related defense system in internodes suggests that green alder has at least a two-level defense system: defense of growth stages and defense of parts within growth stages.