For the purposes of the SUSTAIN model, ``sustainability'' is defined as ``within the natural range of variation'' (NRV). The NRV concept is a currently popular approach to evaluating managed ecosystems. The primary source of NRV information for the SUSTAIN project is the modeling work of Lee Frelich. Lee ran models incorporating natural disturbance events (wind and fire) with site potential related variables. Repeatedly running these stochastic models to an equilibrium state provided a range of proportions for each land cover type and successional stage considered.