- Sitka black-tailed deer (Odocoileus hemionus sitkensis Sm.) prefer understory forages growing beneath a forest canopy despite a greater abundance of the same plant species in forest clearings. This research examined
responses of the deciduous shrub - blueberry (Vaccinium ovalifolium), to test the hypotheses that 1) forage is less nutritious and less palatable when grown in clearings than in forest understories, and that 2) changes in the plant carbon/nitrogen (C/N) ratio in response to light and nitrogen supply determine forage nutritional quality.
Responses to irradiance and nitrogen supply were examined with respect to plant physiology, morphology, biochemistry and nutritional quality in three phases : 1) under controlled conditions in a growth room, 2) with manipulations in a field experiment and 3) along natural gradients of light and nitrogen in the native forests.
The results were highly consistent from the growth room to the field. Light strongly affected plant physiological responses, including photosynthesis, relative growth rates and growth efficiency, whereas nitrogen had little effect. In regression analyses, leaf morphological properties,
including specific leaf weight and leaf succulence, were the best predictors of relative growth rates (R2=.67). Irrespective of nitrogen supply, the biochemical properties of sun leaves included higher concentrations of starch, nonstructural carbohydrates and % lignin + cutin in the
cell wall, but lower concentrations of structural polymers, total nitrogen, free amino acids, and ratio of free amino acids : total N, compared to shade leaves. Sun leaves also were slightly higher in digestible energy, much lower in digestible nitrogen and presumably less palatable due to
higher tannin concentrations, compared to shade leaves. Tannins were directly correlated to specific leaf weight (R2=.89). Regression equations based upon specific leaf
weight, leaf succulence and leaf structural polymers accurately predicted field values for digestible nitrogen (R2=.91) and digestible energy (R2=.96) in foliage.
Nutritional properties of blueberry forage grown under variable irradiance in the natural stands matched predictions based upon results from the growth room and field. Compared to even-aged stands and oldgrowth, leaves of plants grown in clear-cuts were similar in digestible energy, much lower in digestible nitrogen, and presumably less palatable due to higher tannin concentrations.