This book discusses the historic range of variation (HRV) in the types, frequencies, severities and scales of natural disturbances, and explores how they create heterogeneous structure within upland hardwood forests of the Central Hardwood Region (CHR). The book was written in response to a 2012 forest planning rule which requires that national forests to be managed to sustain 'ecological integrity' and within the 'natural range of variation' of natural disturbances and vegetation structure. Synthesizing information on HRV of natural disturbance types, and their impacts on forest structure, has been identified as a top need.