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Management Situation by Revision Topic RGNF. Collectively, the vegetation zones provide the rich diversity of animal and plant life of the RGNF. Old-Growth Fort'J6t6 According to Mehl (1992), old-growth forests are unique ecosystems that are an important part of biological diversity. Old growth occurs at some point in the later stages of a stand's development. .A stand that has developed an old-growth condition has developed a diversity of functions and interactions that did not exist in earlier stages. later stages differ from earlier stages by obvious features like tree size, number of standing- and down-dead trees, and age, as well as number of canopy levels and the composition of the understory species. The age at which old growth develops and the structural attributes that characterize old growth will vary by forest-cover type, climate, site conditions, and past disturbances. However, old growth is typically distinguished from younger growth by some of the following stand attributes: -- large trees for species and site -- variation in tree sizes and spacing -- standing- and down-dead trees -- broken or deformed tops or bole and root decay indicating decadence -- multiple-canopy layers -- gaps in the tree canopy and understory patchiness A stilnd may contain some trees that meet the criteria for old growth, but the stand as a whole could lack the functions and interactions of an old-growth ecosystem. Therefore, it would not be considered old growth. Old growth encompasses both older forests dominated by fire-dependent species and forests dominated by shade-tolerant species. Different stages or qualities of old-growth will be recognizable in many forest-cover types. Sporadic, low to moderate severity disturbances are integral parts of the internal dynamics of many old-growth ecosystems. Canopy openings resulting from the death of overstory trees often produce patches of small trees, shrubs, and herbs in the understory. Frequent, low-intensity fires are important for some species to maintain dominance on a site. Old growth is not necessarily 'virgin' or . primeval.' It could develop following human disturbances. It could also develop from humans' indirect influence on the landscape by the control of fire. Table 111-6 shows the attributes used to describe old growth in the Rocky Mountain Region. The table shows the minimum criteria for the structural attributes used to determine old growth and those that add a quality attachment. Attributes with an 'X' or a numerical value are considered 'must' criteria. Those with a 'Q' are quality criteria. The quality attributes are not required for old growth, but provide for higher quality old growth if present. Any of the 'must' criteria over the minimums could also indicate a higher quality of old growth (Mehl, 1992). ( .-~ " 11I-24 ,\..,:r"'437 ,v'" It