Spatial and temporal heterogeneity in disturbance regimes regimes result in variation in the size and age structure of live trees in the forest, as well as shaping the distribution of dead material (Spies, 1997). By attempting to mimic this heterogeneity (often referred to as the "natural range of variation") and by leaving an appropriate number of trees, snags, and logs as legacies, silviculture can move towards a more natural basis.