Background/Question/Methods   Warming temperatures associated with climate change are expected to lead to more frequent and intense droughts in many mid-latitude regions. To understand and predict potential ecosystem responses to these changes, observational datasets are needed that are sufficient in temporal depth and resolution. This is particularly true for assessing the potential effects of prolonged drought, because droughts spanning decades or more have been a recurrent feature of the climate system during the Holocene, even though they have been relatively uncommon during the 20th century. To better understand the ecological effects of multidecadal-to-centennial scale drought variability we have conducted comparative studies of paleoecological and paleohydrological records from a network of peatlands and lakes in the Great Lakes region. We use these data to examine examples of dramatic drought-induced ecological changes in the late Holocene, highlighting insights provided by these past events that are relevant to understanding future ecological responses to hydroclimate change.

Results/Conclusions   Droughts variability led to dramatic ecosystem changes during the late Holocene, and some of these changes were counterintuitive. For example, comparison of paleoecological and paleoclimate records from a dense network of lakes and peatlands in the Great Lakes region reveals that yellow birch (Betula alleghaniensis), a mesic tree species, expanded rapidly across Upper Michigan and Wisconsin about 4000 years ago, during a pluvial period that followed a severe centennial-scale drought. Drought-induced disturbance followed by favorable climatic conditions likely promoted the rapid expansion. Ecological responses to Holocene droughts were characterized by complex spatial heterogeneity. For example, droughts during the Medieval Climate Anomaly triggered a widespread decline of beech (Fagus grandfolia) and other forest changes in the Great Lakes region. However, land-surface modification of hydroclimatic conditions resulted in spatially structured climate variations and vegetation changes, with vegetation changes most strongly expressed in areas of pronounced drought. Responses to late Holocene drought variability not only involved changes in community composition, but also triggered the abrupt and permanent transformation of some ecosystems. For example, multidecadal drought caused the rapid expansion of floating peatlands within kettlehole ecosystems, probably leading to a cascade of physical and biogeochemical changes in remnant lake systems. Ecological responses to climate changes of the coming decades are likely to include similar surprises, spatial heterogeneity, and ecosystem state-shifts. Retrospective studies can help identify critical climatic thresholds for ecosystem transformation, and when coupled with observational, experimental, and modeling efforts, can help guide ecosystem management in the face of an uncertain future.