From: Carlson, Paul J., March 1995: An assessment of the old-growth forest resource on national forest system lands in the Chattooga River watershed. Chattooga Ecosystem Demonstration Management Project, USDA Forest Service- Region 8, 114 p.
“Class A old-growth stands are those where no significant signs of human disturbance to the forest canopy or understory could be determined, and which had forest canopies dominated by old trees, generally over 150 years of age. 150 years was considered an appropriate “coarse filter” age for old-growth candidacy as this corresponds to a period (1844) when logging was limited near to early settlement sites.
Class B old-growth stands included two different types of conditions: a) where the canopy is dominated by old-growth trees, but there exist signs of significant past human disturbance to the forest canopy or understory, (generally a half century of longer ago); b) where no sign of past human disturbance could be confirmed, but the forest canopy is dominated by younger forest. Stands heavily impacted by American chestnut blight are included in the first condition type, and younger stands (often 80-150 years) possibly affected by natural fire or wind disturbance events are included in the second.
Class C old-growth stands are areas with obvious past human disturbance histories (sometimes quite recent) but which include appreciable mature to old trees in the canopy. These were mostly mapped as good candidates for old-growth restoration when found in parts of the landscape with very little old-growth, or when in fairly unique sites or forest types.”