YIELD OF EVEN-AGED HARDWOOD STANDS. 31 



regards soil moisture between those occupied by the two other types. 

 There is, however, another oak-chestnut type which is often found on 

 moist, well-drained, lowland soil, where red oak thrives and nearly 

 equals chestnut in its rate of growth. This is similar in yield to the 

 chestnut type. 



QUALITY CLASSES. 



Soil conditions are largely responsible not only for differences in 

 type but also for variations in the rate of growth within the same 

 type. The yield to be expected from a stand within a given period 

 thus depends directly upon the quality of the site. For this reason 

 three different qualities of situation, or quality classes, have been dis- 

 tinguished. The best index to the quality of a situation is the rate 

 of growth in height of the trees which occupy it. Thus the best soils 

 show the best height growth, and the poorest sites the poorest growth, 

 provided all other conditions are equal. The basis for the difference 

 in the quality classes is therefore the average height at a given age of 

 the tallest or dominant trees of the principal species in an even-aged 

 stand. In quality I, therefore, the dominant trees exceed a certain 

 minimum height, which varies, of course, with age. In quality II 

 they fall below this, but above the maximum for quality III. The 

 classification is shown graphically in figure 1, in which the quality 

 class separation is indicated with dotted curves. In the yield tables 

 which follow, the average heights of dominant trees at 5-year 

 intervals are given for each quality class. 



Quality classes are determined for each type separately, without 

 regard to corresponding qualities in other types. Since chestnut 

 almost invariably occupies the better soils, it is evident that first- 

 quality oak, not being the same in yield, can not be compared 

 with first-quality chestnut; nor third-quality oak with third-quality 

 chestnut. 



GROWING STOCK. 



In any type the number of trees per acre at a given age varies 

 widely. When the whole area is occupied by thrifty, long-stemmed 

 trees, each with a sufficient space in which to grow rapidly, the ground 

 may be said to be "normally stocked." Too many trees cause 

 crowding, which results in slender, weak, slow-growing stands; too 

 few do not fully utilize the soil and growing space, and produce limby 

 stands of relatively low value. 



Understocked stands result from scanty or irregular reproduction, 

 or from fire, grazing, disease, or too heavy thinning. The chestnut 

 blight is already a cause of understocking in the southwestern part 

 of the State. In this study only average fully stocked stands were 

 selected for measurement. No plots were measured in which the 

 crowns of the trees did not occupy approximately the total amount 



