Palaeoecological evidence of state shifts between forest and barrens on a Michigan sand plain,USA

Abstract

Recent disturbances have induced state shifts in many ecosystems. It has been proposed that human actions degrade system resilience, thus allowing state changes to occur more easily. Unless system dynamics in the absence of human influence are understood, however, we can not address two fundamental questions: are human actions producing dynamics that were previously absent in those systems? Or, are human actions simply replacing natural drivers of change in systems susceptible to state shifts? I use fossil pollen and charcoal to document Holocene vegetation and disturbance dynamics on a sand plain in Michigan to evaluate a recent anthropogenic state shift from forest to a barrens plant community. A 10 500-yr pollen record shows that a community similar to the modern barrens was present three times between 9000 and 5000 yr BP. These periods coincide with distinct peaks in the charcoal record, linking the barrens vegetation community and fire. The barrens vegetation community is thus a rare component of the Kingston Plain system, associated with both natural and anthropogenic disturbances. However, with no evidence of barrens vegetation during the last 5000 years, it appears that climate is also an important driver of vegetation change on the Kingston Plain. Ancient barrens occurred when annual precipitation was lower than current climate, and summer temperatures were warmer than current climate. Projections of decreased soil moisture and warmer temperatures in the Great Lakes region during the next century may thus imply decreased resilience of Great Lakes forests to future disturbances.