Selection of Mature Growth Stages of Coniferous Browse in Temperate Forests by White-tailed Deer (Odocoileus virginianus)

Mammalian herbivores in boreal areas selectively browse on mature-stage growth rather than on juvenile-stage growth of conspecific plants during winter. Such stage-dependent selection often is mediated by levels of secondary metabolites that decline as plants mature. Little is known regarding the extent to which this pattern is repeated for temperate-zone plants browsed by different mammalian species. We conducted field experiments in a temperate forest with free-ranging white-tailed deer (Odocoileus virginianus) to test whether selection of coniferous browse was influenced by a plant's maturational stage. Trials conducted during February 1990 in western Connecticut demonstrated that deer browsed a significantly greater percentage of eastern red cedar (Juniperus virginiana) collected from reproductively mature plants (mean = 72%) than from juvenile plants (mean = 16%). Trials with eastern hemlock (Tsuga canadensis) produced similar results: deer browsed a significantly greater percentage of shoots from mature trees (mean = 70%) than shoots from juvenile plants (mean = 21%). Chemical analyses revealed that crude protein levels were significantly (P < 0.05) greater in mature-stage eastern hemlock (8.2%) than in juvenile-stage growth (7.3%), but no differences existed between crude protein levels of the red cedar growth stages. Protein-precipitating phenolics were present at low levels but were 1.5 times more concentrated in mature-stage growth of eastern hemlock than in juvenile-stage growth (P < 0.05). Comparison of our results with previous research indicates that white-tailed deer exhibit stage-dependent selection of temperate plants similar to the patterns demonstrated by other species of mammals browsing on plants in boreal forests.