Pollen Grains in Lake Sediments: Pollen Percentages in Surface Sediments from Southern Michigan

摘要

Pollen in surface sediments from five lakes in southern Michigan shows evidence of differential deposition. In most lakes, pollen from ragweed occurs in a higher ratio to tree pollen in shallow-water sediment than in deep water. Pine and certain herbs with small pollen grains follow the same pattern. One very deep lake is an exception, with the ratio of ragweed to tree pollen highest in deep-water sediment. Pollen from local aquatics and from willow, which grows along the lake shore, is also unevenly distributed, occurring in highest frequencies near the parent plants. Pollen from deciduous trees, however, occurs in similar ratios at all sampling stations within each lake. Deciduous pollen occurs in uniform ratios, also in older sediment, deposited in the early 19th century, when the landscape was still forested.Percentages of deciduous tree pollen (as percent tree pollen) were compared among lakes. Single samples were taken for this purpose from the deepest part of each lake basin. Oak pollen percentages are higher in three lakes in western Washtenaw County than in three lakes in eastern Washtenaw County. This difference reflects a similar difference in present-day vegetation: second-growth oak forests grow near the lakes in the western half of the county, while all but 5% of the area in the eastern part of the county is farmland. (The difference in the ratio of farmland to forestland in the two parts of the county is not reflected clearly in the ratio of herb pollen to tree pollen, because there is so much variation within each lake.) In 140-year-old sediment, on the other hand, tree pollen percentages in the six samples are homogeneous as shown by a chi-square test. The homogeneity in sediment deposited before the forest was cleared is surprising, because witness-tree data from presettlement time show that the frequencies of tree species in the two areas were quite different. Pollen dispersal at that time must have been effective enough, to counteract differences over distances of a few tens of kilometers in the amounts and kinds of pollen produced by the vegetation.