Evaluate the broad ecosystem-level impacts of disruption of pre-European fire regimes associated with fire suppression, land-use change, and long-term ecosystem dynamics. FRID measures the current era (1906 to present) fire frequency patterns against reference conditions that occurred during the pre-European Settlement period. It reflects modern-era difference in fire frequency and potential ecological instability that this divergence might cause. FRID can exist both as a positive (more fire than reference conditions) and negative (less fire than reference conditions) values, and carries some inference regarding ecosystem stability and dynamics of vegetation under current fire management policy.