The Holocene vegetation history of Isle Royale National Park, Michigan, U.S.A.

Abstract

A vegetation history for Isle Royale National Park, Michigan, U.S.A., is reconstructed using pollen and spores extracted from two lake sediment cores. Lily Lake is on the southwestern end of the main island of Isle Royale surrounded by northern hardwoods forest. Lake Ojibway is on the northeastern end of the main island surrounded by boreal forest. Pollen and spore records were analyzed using pollen percentage diagrams, nonmetric multidimensional scaling ordination, and modern analog analysis. Squared chord distances for temporally paired subsamples from each site were calculated to determine palynological dissimilarities between the sites through time. These analyses revealed an overall vegetation history that is consistent with other regional reconstructions. High percentages of spruce pollen, indicating a cool climate, are present in the early Holocene, whereas high percentages of pine pollen, indicating a dry climate, occur in the mid-Holocene. The pollen records from the two sites diverge with increased precipitation during the late Holocene. At this time, birch-dominated forest is established near Lily Lake on till-derived soils. At Lake Ojibway, a mixed birch, pine, spruce, and fir forest is established on bedrock-derived soils. The divergence in forest composition is most pronounced within the last 500 years, and this divergence is tentatively attributed to the response of taxa on different substrates to increasing precipitation. Other possible explanations for the recent divergence include changing microclimates or disturbance regimes.