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2 

 Old-growth forests are typically the most complex and stable stage in forest 

 succession. Their complexity is most obvious in terms of structure, e.g., a 

 wide range in size and condition of living trees and coarse woody debris (snags 

 and down logs) and much heterogenity in stand conditions, such as the 

 occurrence of multi-layered forest canopies and well-lighted openings or forest 

 gaps. This structural variability is the most importtint factor in 

 understanding the distinctive role that these forests play as animal habitat. 

 Many animals, from insects to large animals and both terrestrial and aquatic, 

 have these varied and stable forests as critical habitat. The structural 

 diversity is also the key to the distinctive functional features of old-growth 

 forests, such as their influence on hydrologic cycles (e.g., effects on snow 

 accumulation and melt), nutrient cycling (e.g., extremely retentive of 

 nutrients), and stream sediments (e.g., very low levels of sediment production 

 relative to other forest conditions). 



The young managed forests created throu^ traditional silvicultural practices 

 are ecosystems which differ significantly from old-gpcowth forests and even from 

 the young stands which originate following some natural catastrophe, such as 

 fire and windthrow. Structurally, the young managed forests are relatively 

 simplified; for example, they typically consist of a single dense canopy layer, 

 uniformly spaced trees of similar size, and have relatively low levels of snags 

 and down logs. Such forests cannot provide the habitat needed by many 

 organisms nor can they perform some cycling functions as well as natural 

 forests. However, the managed young forests excel at growing wood fiber, a 

 function that old-growth forests do very E>oorly. Older natural forests use 

 most of their productivity for ecosystem maintenance or respiration rather than 

 for accumulation of additional wood; hence, wood volumes in old-growth stands 



