• Hayslett, Jr., Homer T.
  Colby College
  
• Solomon, Dale S.
  Forest Service USDA
  

The foliage on a balsam fir tree consists of needles of varying age: some that have developed last year and are now 1 year old' some that developed 2 years ago and are now 2 years old; and so on. The fir tree can reatin its needles approximately 8 to 12 years. Each age class of foliage has a different photosynthetic efficiency or capacity to produce food and energy needs. The annual volume increment on the bold (stem growth) of the tree depends not only on how much foliage is present but also on how that foliage is distributed among the age classes. For example, if the current foliage is destroyed for several sequential years by the spruce budworm, the tree will continue to grow but at a reduced rate--the reduction will occur first in the upper part of the bole and later over all of the bole.