Feltleaf Willow‐Snowshoe Hare Interactions: Plant Carbon/Nutrient Balance and Floodplain Succession

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Abstract
This experiment was designed to determine if winter browsing by snowshoe hares is a potentially important control over primary succession on floodplains of Alaskan rivers. In a 2 x 2 x 2 factorial experiment, I increased nutrient availability by NPK fertilization, decreased light intensity with shade cloth, and clipped winter—dormant feltleaf willow (Salix alaxensis) to stimulate severe snowshoe hare browsing. After 3 yr of fertilization, 1 yr of clipping, and 1 yr of shading, the 240 experimental plants were measured for height growth and current—year twig production, and their winter—dormant twigs were bioassayed for palatability to snowshoe hares and analyzed chemically. With one exception, both fertilization and shading increased the palatability of twigs in comparison to twigs of control plants. Moreover, shade had a comparatively strong effect on the palatability of stump sprouts of clipped plants. Both fertilization and shade generally reduced the concentrations of soluble carbohydrate and phenolics in winter—dormant twigs of unclipped plants. However, only shade significantly affected the concentrations of soluble carbohydrate and phenolics in twigs of clipped plants. These results indicate (1) that the palatability and chemical defenses of winter—dormant S. alaxensis are sensitive to plant carbon/nutrient balance and (2) that the effects of soil fertility and light intensity on plant carbon/nutrient balance vary with respect to browsing history. Although severe clipping increased production of current—year twig biomass it greatly reduced plant height. These results imply that severe browsing of winter—dormant willow growing on floodplains of interior Alaskan rivers, such as occurs at some points during snowshoe hare cycles, may result in reduced height growth of willow. I suggest that such a reduction in height growth may result (1) in shading of floodplain willow by alder and (2) in a shade—caused increase in the palatability of willow stump sprouts. Furthermore, I suggest that this increase in the palatability of shaded willow may be a partial control over the shift from willow dominance to alder dominance that occurs in the late shrub stage of primary succession on floodplains of interior Alaskan rivers.