Analysis of hemlock pollen size in Holocene lake sediments from New England

We explored the middle-Holocene decline of Tsuga canadensis by measuring the diameters of pollen grains in two lake-sediment cores from New England. We hypothesized that a drop in pollen size at the time of the decline followed by an increase in pollen diameters as Tsuga recovered during the late Holocene might indicate reduced abundance of Tsuga in the vicinity of the lake during the decline, as smaller pollen grains travel farther than larger ones. To provide context for this hypothesis, we also measured the diameters of Tsuga pollen grains in the surface sediments of sites spanning the modern-day gradient of Tsuga in New England. Both fossil records exhibited a reduction in pollen size during the interval of the middle-Holocene decline, with diameters similar to those observed in the upper sediments of those sites, yet larger than Tsuga pollen grains in the surface sediments of coastal sites beyond the modern range of Tsuga. This pattern suggests that Tsuga persisted in scattered, low-density populations during the middle Holocene, as it has remained on the landscape since European settlement.