Assessing the impact of changes in climate and megaherbivory on late-glacial vegetation communities in northern Indiana

Background/Question/Methods

No-analog plant communities are common features of late-glacial pollen assemblages in eastern North America. Previously, these have been explained as climatically forced phenomena, caused by more-seasonal-than-present climates at the end of the Pleistocene. Searches for a cause, however, are complicated by nearly coeval rapid climatic warming, the arrival of the first humans, the extinction of 33 genera of large mammals, and peaked vegetation dissimilarity. Biotic controls such as herbivory have typically not been tested, largely because uncertainties in the dating of mammalian fossils and lake sediments have hindered the establishment of precise lead-lag relationships. Megaherbivores are an important component of many landscapes, and can affect vegetation directly through grazing and disturbance, or indirectly by modifying competition or fire regimes. In this study, we used spores from the coprophilous fungus Sporormiella, which persist in lake sediments, as a proxy for megafaunal population abundance. Sediment cores from Appleman and Spicer Lakes in northern Indiana were analyzed for changes in pollen abundance, vegetation dissimilarity from present, charcoal, and Sporormiella to test hypotheses about the vegetation response to megaherbivore removal locally and its environmental context. 

Results/Conclusions

At Appleman Lake, Sporormiella declined to very low levels (<4% of the upland pollen sum) around 13,000 years BP  and did not recover during the Holocene. This decline occurs slightly earlier than the most-recent dated megafaunal fossils in the region, suggesting that Sporormiella is a qualitative proxy for declines in population abundance, rather than the presence of the last surviving individuals. Sporormiella decreased before a major vegetation transition from Picea to Pinus, suggesting that climate-driven habitat change was not the main driver of the extinction at this site. Immediately after the decline in spores, pollen assemblages reached peak dissimilarity due to a rise in Ostrya/Carpinus and Fraxinus pollen fractions; this may indicate the release of palatable hardwoods following the relaxation of herbivory pressure. Charcoal data indicates few or no fires prior to the Sporormiella decline, moderately increased fire frequency immediately following the extinction, and a large increase at the end of the no-analog interval and coeval with the transition to an Quercus-Ulmus-Carya complex. Data is currently being collected from nearby Spicer Lake, to test whether the temporal relationships observed at Appleman are repeated on the landscape scale.