Uneven-aged stands are those in which the trees are of at least three distinct 

 age classes irregularly mixed (Society of American Foresters 1964) . Except 

 for very old stands, uneven-aged stands are distinctly irregular in height and 

 tree size. These stands are developed or maintained by relatively frequent 

 harvests made throughout the rotation age (the number of years required to 

 establish and grow timber crops to a specified condition of maturity) . The 

 distribution of diameter measurements in a balanced uneven-aged stand will 

 plot into a characteristically inverted J-shaped curve. 



Even-aged stands are those in which the difference between the oldest and the 

 youngest trees does not exceed 10 to 20 years or 25% of the length of the 

 rotation age. Trees in these stands tend to be uniform in height, but 

 frequently they cover a wide range of diameter widths. These stands usually 

 develop after the sudden removal of previous stands by logging, fire, insect 

 epidemic, or other cause. A plotting of diameter widths will usually result 

 in a normal curve. 



Management of uneven-aged stands . In uneven-aged stands mature trees are 

 removed as scattered individuals or in small groups at relatively short time 

 intervals (10 to 15 years on primary softwood sites and 20 to 25 years on 

 secondary softwood sites). The interval between cuts is based on growth 

 rates, stand conditions, and size of the intended harvest. Individual trees 

 or groups of trees are marked before cutting. The criteria used for marking 

 trees for removal are: (1) poor-risk trees (those assumed to be doomed before 

 the next harvest), (2) poor quality trees, (3) slow-growing trees, (4) trees 

 of less desirable species, (5) trees whose removal will improve spacing in the 

 residual stand, and (6) mature trees of good quality, good risk, desirable 

 species, and fast growth. The term "selection system" is applied to any 

 silvicultural program that is aimed at the creation or maintenance of uneven- 

 aged stands and that includes some form of periodic harvesting. Because 

 spruce and fir are usually able to reproduce and grow under overhead shade, 

 uneven-aged stands will develop in areas not drastically disturbed by nature 

 or people. Advantages of using the selection system of cutting in the spruce- 

 fir type are: 



1. Periodic harvests guarantee that a continuous forest cover is 

 maintained. 



2. The retention of spruce trees can favor the regeneration of this 

 species with a corresponding reduction in fir. 



3. Environmental conditions are stable so that plant and animal 

 populations do not fluctuate much. 



4. Fire hazard from slash accumulation (fallen branches and twigs) is 

 not severe. 



5. There is less chance of losing an entire stand at one time to insect 

 attack, infectious disease, or other natural catastrophies . 



6. The stands, except for the period immediately after a cut, appear 

 attractive to the esthetic-conscious public. 



Management of uneven-aged stands is complex. Because operations are conducted 

 in mixtures of different age classes logging damage to, and death of, some 

 uncut trees is difficult to prevent. Harvesting operations usually are 

 difficult and expensive in that large land areas must be covered to obtain a 

 given volume of wood. 



19-10 



