42 SEEDING AND PLANTING 



factors is responsible for the crop, we do know that the crop is 

 a result of their combined effect. When a given site has already 

 produced a crop of trees, it is reasonably safe to assume that 

 under normal conditions all of the site factors have found due 

 expression in its development. 1 It is for this reason that the 

 crop itself is the best indicator of what the site is capable of 

 producing. It is the safest guide for the determination of the 

 quality of the site or its yield capacity. For instance, if an acre 

 of land has produced 30,000 board feet of white pine in 50 years, 

 the annual yield capacity is 600 board feet. If, in another case, 

 an acre of land under similar treatment produces but 20,000 board 

 feet in 50 years, its annual yield capacity is but 400 board feet. 

 The difference in the site factors which combine to form quality 

 of site is expressed in the difference in yield. 



In order to determine the quality of the site from the condition 

 of the crop, the history of its development regarding the follow- 

 ing should be known : 



a. Whether the crop upon which our assessment is based has" 

 developed under normal conditions. 



b. Whether there has been a change in the site factors during 

 the development of the crop. 



c. Whether the age of the crop is sufficient to indicate that the 

 site factors have been fully expressed in its development. 



Where the crop has grown under unusual or abnormal con- 

 ditions that have affected its health or development, such as 

 damage from fire, fungi, insects, grazing, removal of litter, or 

 faulty treatment, it does not express the yield capacity of the 

 site, and allowance must be made for the injuries to which it 

 has been subjected. In some cases the site factors, particularly 

 the soil factors, undergo rather marked changes toward either im- 

 provement or deterioration during the development of the crop 

 upon which the assessment is based. Allowances must be made 

 for the change in site factors because the stand itself is no longer 

 indicative of yield capacity. Where the assessment is based upon 

 immature stands, it is necessary to know that the site factors are 

 already fully expressed in the crop. This is necessary because 

 many species make good growth and appear thrifty in their 

 juvenile stage, even on sites unsuited to them, but later fail or 

 fall off in growth and become unsatisfactory. Therefore, in all 

 1 Schlich, Wm.: Manual of forestry, vol. II, p. 51. Londoo, 1910. 



