Documenting forest stand invasion: fossil stomata and pollen in forest hollows

Stomata and pollen from forest hollow sediments in the Great Lakes were evaluated for their ability to document stand-scale forest invasion. In surface sediments of 19 forest hollows, stomate presence is related most closely to tree presence within 10-20 m for eastern hemlock (Tsuga canadensis (L.) Carr.), pine (Pinus spp. L.), balsam fir (Abies balsamea (L.) Mill.), and northern white cedar (Thuja occidentalis L.). Although the abundance of hemlock pollen in surface sediments is positively related to the abundance of hemlock trees within 100 m, it does not consistently reflect the presence of hemlock trees within this distance. Pollen and stomata preserved in forest hollow sediments from northwestern Wisconsin were used to document two stages of stand invasion by eastern hemlock. First, hemlock stomata initially appear approximately 2400 (14)C years BP, representing the initial colonization of the forest stand. Hemlock pollen also first appears at this time but in such low abundance that the presence of hemlock trees at the site cannot be known with certainty. Second, hemlock pollen percentages increase sharply around 350 (14)C years BP, approximately 2000 years following initial colonization, and represent an expansion of the local hemlock population not reflected by stomatal and needle abundance.