/ Prairie and Savanna in Southern Lower Michigan: History, Classification, Ecology
ï~~2008 THE MICHIGAN BOTANIST 13 TABLE 3. Plant species statistics in Michigan prairie and savanna plant communities. Mean Species % Frequent Number of Plant Community Total Species Per Stand Species Indicators Dry Sand Prairie/Oak Barrens 182 77 41.8 33 Mesic Sand Prairie 241 79 24.5 20 Oak Openings (Hillsides) 148 49 27.7 14 Oak Openings 237 65 16.8 1 Mesic Prairie 172 81 29.1 25 Wet Prairie 177 69 36.1 24 Lakeplain Wet Prairie 157 76 37.6 26 Frequent species have a presence of 50----100% among all stands of a plant community. large number of species in common with other upland prairie and savanna communities. Oak openings on hillsides was less internally homogeneous than dry sand savanna/oak barrens and wet prairies, but had a high number of indicator species. Mesic prairie and mesic sand prairie had few restricted indicators but a large number that were at low presence in other plant communities. DISCUSSION The prairie and savanna plant communities derived from field data and ordination are consistent with historical accounts, to the extent that comparisons can be made. Adding stands to the ordination and employing a finer level of analysis (e.g., ordinating only the upland prairies and savannas) may reveal finer patterns in species presence among stands. For example, Faber-Langendoen and Maycock (1987) described a wet-mesic sand prairie type in southwest Ontario which exists on Michigan's coastal lakeplains but was not revealed by the ordination. Current ordination software with greater computing power and flexibility (e.g., McCune and Mefford 1999) might also improve the ordination results. With 495 plant species recorded out of about 2500 vascular plants statewide (including non-native species), Michigan's prairie and savanna flora is large. We think it likely that these species, and others of prairie and savanna affinities, arrived at various times after glacial recession and also that some sites of appropriate parent material and physiography supported vegetation recognizable as prairie well before the mid-postglacial hypsithermal period of 6,000 to 8,000 vbp (years before present) which is widely credited with allowing prairie vegetation to become established in Michigan. Descriptions of Quaternary vegetation change published in the last 20 years (listed in Grimm and Jacobson 2004) tend to disregard the idea advanced by Gleason (1922a; see also Benninghof 1964 and Stuckey 1981) that the beginnings of the Prairie Peninsula are traceable to the early postglacial of 10,000 to 13,000 ybp (King 1981). Perhaps this neglect stemns from an over-emphasis on highly obvious fossil pollen changes of the mid-postglacial that are interpreted as a period when global mean temperature was about 1 C higher than today. At that time prairie vegetation expanded at the expense of forest. Radiocarbon dat
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