Interactions between snowshoe hare herbivory and white spruce establishment on the Tanana River floodplain, Alaska

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Snowshoe hares are a dominant herbivore of the boreal forest, and are known to affect vegetation dynamics during peak phases of their population cycle. We investigated the effect of snowshoe hare herbivory on the establishment and growth of white spruce seedlings at the Bonanza Creek Long Term Ecological Research (BNZ-LTER) site near Fairbanks, Alaska. White spruce individuals were measured inside browse exclosures and in adjacent control plots. We measured changes in the height to diameter and height to age ratios of white spruce in relation to browse history. Individuals were also measured at 13 additional sites in order to reconstruct the age structure for young white spruce along the Tanana River at BNZ-LTER. We used linear regression to compare the patterns in white spruce age structure to historical snowshoe hare abundance. We found that exposure to browsing results in an average 3 to 4 cm year^-1 decrease in the height to age ratio of spruce seedlings. Our best model included the interaction between hare abundance and white spruce seed production, along with flood history, and explained 40% of the pattern in age structure for young white spruce. This interaction highlights the role played by snowshoe hares in the successional dynamics of the boreal forest. When hares are abundant, spruce establishment is reduced. Once established, sustained browsing by hares can suppress seedlings and increase the time required for them to become dominant members of the forest canopy.