Stevens Pond: A postglacial pollen diagram from a small Typha Swamp in Northwestern Minnesota, interpreted from pollen indicators and surface samples

The pollen assemblages of a core in the coniferhardwood formation in northwestern Minnesota are compared with the floristics of the recent vegetation in the region. Percentage levels of the main tree components have been compared first with those from recent surface samples taken at the same short distance from various types of upland forests and second with the regional values of the pollen rain in this area (McAndrews 1966). To that end all the data were recalculated on the basis of special pollen sums. The regional diagram of Stevens Pond shows basically the same assemblage zones as established by McAndrews but without the late-glacial Picea-Populus assemblage zone. The pollen in the following Pinus-Pteridium assemblage zone has been interpreted as derived from a pine forest. During the midpostglacial expansion of the prairie eastwards the regional vegetation must have been a Quercus savanna, locally with prairie. Corylus reaches relatively high percentages in this zone. Among the prairie elements especially the occurrence of Lilium philadelphicum may be noted. In the next zone the pollen diagram shows a rise of the curves of mesic elements. In spite of this the comparison with recent surface samples indicates a xerophytic Quercus forest rather than a mesophytic deciduous forest. In the following Pinus assemblage zone pine was present along the margin of Stevens Pond and is therefore overrepresented in the diagram. In the uppermost zone the pollen curves show the effect of logging of the forest about 1900. Pollen of cultivated and introduced plants appear in this zone. Many local pollen types were found, on account of local overrepresentation. This made it possible to compare the local Stevens Pond sequence with the composition of recent lowland vegetation types. The pollen sequence was similar to a large extent to the recent pattern of lake filling, starting with a eutrophic vegetation of Typha latifolia and Salix in the prairie period and leading to a Larix forest and then to a mesotrophic Picea mariana forest, the present edaphic climax on peaty soils. There is a delay, however, in the introduction of acidophilous species, the Larix forest being without Sphagnum and Ericaceae. This is explained by assuming an influence of the vegetation of the surrounding slopes upon the local vegetation. About 1900 the bog forest was destroyed by logging operations and replaced by the present Typha latifolia mat.