Refers to a forested area with trees that are more than 40 inches in circumference. These trees are often hundreds of years old, but size, not age, is more important in the old growth classification.
Old forests often containing several canopy layers, variety in tree sizes and species, trees at least 180 to 220 years old, and standing and dead woody material.
Old-growth or ancient forests are forest lands that have not been harvested or in any other way altered by people. Of the estimated 2 million acres of coast redwood forest shortly after the arrival of the Europeans, less than 4% are now uncut. An old-growth stand is usually well past the age of maturity as defined by the culmination of mean annual increment. It often exhibits characteristics of decadence. Example: low growth rates, dead and dying trees, snags, and down woody material. The stands usually contain large-diameter trees relative to species and site potential, multi-layered canopies, a range in tree-diameter sizes, and the presence of understory vegetation. Some documents specify a lower limit on size; for example Michael Barbour et al. in Coast Redwood: A Natural and Cultural History state that old-growth redwoods exceed 200 feet high and 40 inches in diameter.