be considered in developing an index of "old-growthness" and for defining old-growth forests: Structural features: Uneven or multi-aged stand structure, or several identifiable age cohorts Average age of dominant species approaching half the maximum longevity for species (about 150+ years for most shade-tolerant trees) Some old trees at close to their maximum longevity (ages of 300+ years) Presence of standing dead and dying trees in various stages of decay Fallen, coarse woody debris Natural regeneration of dominant tree species within canopy gaps or on decaying logs Compositional features: Long-lived, shade-tolerant tree species associations (e.g., sugar maple, American beech, yellow birch, red spruce, eastern hemlock, white pine) Process features: Characterized by small-scale disturbances creating gaps in the forest canopy A long natural rotation for catastrophic or stand-replacing disturbance (e.g., a period greater than the maximum longevity of the dominant tree species) Minimal evidence of human disturbance Final stages of stand development before a relatively steady state is reached Importance Old-growth forests often contain rich communities of plants and animals within the habitat due to the long period of forest stability. These varied and sometimes rare species may depend on the unique environmental conditions created by these forests. Old-growth forests serve as a reservoir for species, which cannot thrive or easily regenerate in younger forests, so they can be used as a baseline for research. Plant species that are native to old-growth forests may someday prove to be invaluable towards curing various human ailments, as has been realized in numerous plants in tropical rainforests. Old-growth forests also store large amounts of carbon above and below the ground (either as humus, or in wet soils as peat). They collectively represent a very significant store of carbon. Destruction of these forests releases this carbon as greenhouse gases, and may increase the risk of global climate change. Although old-growth forests therefore serve as a global carbon dioxide sink, they are not protected by international treaties, because it is generally thought that aging forests cease to accumulate carbon. However, in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive; old-growth forests accumulate carbon for centuries and contain large quantities of it. Ecosystem services Old-growth forests provide ecosystem services that may be far more important to society than their use as a source of raw materials. These services include making breathable air, making