Paleoenvironmental reconstruction of a Middle Miocene forest from the western Canadian Arctic

Abundant fossil plant remains are preserved in deposits of Middle Miocene age of the Ballast Brook Formation on Banks Island, Northwest Territories, Canada. Intact seed cones, logs, and stumps are preserved in situ as mummified remains and present an opportunity to reconstruct the composition, structure, and productivity of a Pinaceae-dominated forest that once grew north of the Arctic Circle (paleolatitude ca. 74°N). We mapped and measured 78 tree stumps exposed in three dimensions on a 0.12 ha peat layer. An analysis of the wood anatomy and seed cones indicates that a haploxylon pine dominated this lowland forest, although species of both Picea and Glyptostrobus were growing in the swamp as well. Stump diameters ranged from 7 cm to 108 cm (average = 42 cm). We utilized allometric relationships to predict tree heights based on the stump diameters in the fossil forest. Our results indicate that the average tree height of this Miocene forest may have been as large as 21 m. We used stump diameter data and predicted tree height to calculate the parabolic stem volume and stem biomass for the exposed area of fossil forest. Stem biomass (assuming an average wood density of 410 kg m− 3) may have been as great as 259 Mg ha− 1 or as little at 202 Mg ha− 1. The annual ring width of the stem wood sampled in the field was 1.26 mm for Pinus, 1.20 mm for Picea and 0.21 mm for Glyptostrobus. Based on these growth rates, our estimate of biomass sequestered aboveground as wood is at most 3.8 Mg ha− 1 y− 1. Based on our estimates, these lowland forest communities were of moderate biomass and productivity typical of modern cool temperate forests in North America.