Late Holocene Climate and Vegetation Change in Central Michigan
Understanding the effects of drought and temperature change on eastern North America will be key in the context of future climate change. Perhaps the best method to understand how drought and temperature will impact this area is to study its paleoecological record. The records presented here, from Houghton Bog (44°23'4.50"N; 84°44'18.90"W) and Higgins Bog (44°28'58.90"N; 84°47'41.40"W) provide an opportunity to study the effects of drought and temperature on the interior of the Lower Peninsula of Michigan (hereafter central Michigan) during the Late Holocene (c. 4200 cal BP to present day). Pollen, charcoal, and testate amoebae records were analyzed to infer past drought and climate change events, and their effects on the vegetation assemblage. These records will add to the growing body of paleoecological data for the Lower Peninsula of Michigan. Both Higgins Bog and Houghton Bog are characterized by a mixture of boreal and deciduous forests, which is mainly dictated by the location of the forest tension zone. Early in their records, both bogs indicate that the area surrounding the bog was a temperate forest composed mostly of Acer, Betula, and Pinus, and both bogs were experiencing fairly dry conditions. At c. 2000 cal BP, both bogs record a significant change in their pollen records, which is reflected in CONISS clustering and DCA analysis. Both sites show a marked decline in temperate taxa at this time, and taxa indicative of early successional stages are abundant. The cause of this disturbance at c. 2000 cal BP is unknown, though it is unlikely to have been humans, as they were not abundant in the area (Howey 2012). After c. 2000 cal BP, temperate taxa recover, and succession continues until c. 1800 cal BP, when a drought event causes temperate taxa to once again decline. From here, a long-term succession towards a boreal taxa dominated forest continues in both sites until European settlement. Droughts and pluvials overlay this long-term succession, and cause temporary disturbances. After the c. 1800 cal BP event, temperate species decline, and boreal taxa begin to increase, suggesting that while the drought may have temporarily caused many temperate and boreal taxa to decline, the boreal taxa are able to recover and continue to increase in abundance until c. 1000 cal BP. At c. 1000 cal BP another drought event is observed in both bog records. At this point some boreal taxa decline, while others (such as Tsuga) appear only temporarily affected. This may be because of the climatic effects of the Medieval Climate Anomaly (MCA), which warmed the area from c. 1000-700 cal BP, which, based on modern day studies, may have strengthened the lake effect and increased precipitation in central Michigan (Burnett et al. 2003). Boreal taxa increase after c. 1000 cal BP, until Ambrosia spikes at c. 100 cal BP, marking European arrival. Prior studies had documented that the climate of the Late Holocene has been characterized by increasing moisture throughout the eastern United States. Within this context; however, several major droughts have been identified in Michigan, centering on 1850, 1800, 1650, 1000, 800, and 700 cal BP. (Booth et al. 2006). In Houghton and Higgins Bog, the 1800 and 1000 cal BP droughts were detected, suggesting a more regional signature for these events than the others. I found that both of these sites have remained very dry throughout their history, as indicated by the testate amoebae records from both sites. Overall, both of my study sites show little human disturbance until European arrival, although assessment of the period of greatest mound building shows that most of the mounds in central Michigan were built between the c. 1800 and c. 1000 cal BP droughts found in these bogs. Both Houghton and Higgins Bog give insight into the succession of upland forests and bogs in central Michigan, and add to the growing body of evidence of long term succession driven by climate in central Michigan, which may have implications for future climate change.