STREAM-DEPOSITED PLANT MACRO-REMAINS ARE GOOD INDICATORS OF PAST AND PRESENT UPLAND FLORAS IN THE MIDWEST

Stream-deposited plant macrofossil assemblages tend to be dominated by lowland wetland and riparian taxa growing along valley bottoms and slopes. Therefore, fluvial assemblages are generally assumed to be poor indicators of plant communities occupying upland settings. This assumption is probably true for fossil assemblages deposited by large rivers with wide floodplains supporting extensive areas of lowland vegetation. However, Quaternary plant macrofossil assemblages preserved in the alluvium of small, upland streams in the Midwest contain abundant evidence of upland vegetation, such as prairie and oak woodland. To understand how diversity of stream-deposited plant macrofossils relates to diversity of upland and lowland floras, we used a modern analogue approach. We chose a small, upland stream environment in the prairie/deciduous forest region of southeastern Minnesota, as an analogue for paleoenvironments generating fossil assemblages with upland elements. Several depositional environments in Prairie Creek including point bar, pool, and debris trap deposits were sampled for plant remains. Diversity of present-day vegetation in the drainage area was estimated using existing species lists and quadrat surveys. Wetland, riparian, mesic forest, dry forest, and prairie communities were identified. Comparison of species found in modern stream deposits to those present in the standing vegetation revealed that ~19 % of present species diversity is represented in the sediment record. Plant taxa from lowland riparian and mesic forest habitats are most highly represented in all of our samples. Taxa common in drier upland environments like prairie and oak woodland are present in many of our samples, especially those with high specimen abundance. Fossil assemblages formed in small stream environments potentially sample both lowland and upland vegetation because small streams typically have narrow riparian corridors with upland vegetation growing near the stream channel. Past regional vegetation interpretations are typically based on pollen records, but upland components present in stream-deposited macrofossil assemblages are potentially valuable regional paleoenvironmental indicators, because upland taxa typically characterize regional floras and climates better than lowland edaphic taxa.