Variation in nutritional quality of plants for deer in relation to sunny versus shady environments

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Variations in nutritional quality of natural forages for black-tailed deer (Odocoileus hemionus) was studied in the summer and winter in southeast Alaska. Freeze-dried samples of 17 summer forages collected in early July and 10 winter forages collected in February from three replicate sites each of shady forest understory and open, sunny habitat were analyzed and compared for their concentrations of digestible protein, digestible dry matter, and digestible energy. Data from each forage, ranging from forb and shrub leaves to woody twigs, were analyzed separately in a single-factor analysis of variance experimental design. Only five to seven of the summer forages different significantly (P < 0.05) in values from sun and shade treatments: digestible protein was lower while digestible energy were higher in sun-grown than shade-grown forages. Differences in winter forages were fewer and inconsistent. However, despite general trends in patterns across all forages, variation both among and within forages was high at all scales of sampling: subsamples within a site, replicate sites of habitat types, and habitat types. Light intensity plays a major role in determining plant chemistry of some species, but the high degree of variation in plant response makes underlying patterns of variation especially important.