FOR WHOLE TREES. 35 



Form. 



By the "form" of a tree is meant the shape of the bole, or stem. 

 Trees of the same species and of different diameters breasthigh neces- 

 sarily do not have stems of the same shape. It may be that a large 

 tree will have a very small taper in the lower third, a slightly greater 

 one in the middle third and an exceedingly large taper in the top third. 

 Or it may be that the taper will be rather uniform at all points along 

 the stem. Trees with increasing taper, such as cited in the first case, 

 have boles which approach very closely the shape of a paraboloid of 

 revolution, where trees with a practically uniform taper approach more 

 closely the shape of a cone. Young trees, as a rule, are of the latter 

 form. Old trees are likely to be filled out more than the young, and 

 therefore approach more closely the shape of a paraboloid. 



Each layer of wood formed during the growth of a tree is not uniform 

 in thickness at different heights from the ground. The wood of each 

 annual layer as a rule is a little thicker in the top third of the stem 

 than it is in the bottom third. This is very noticeable in trees which 

 are nearing maturity. The annual layers formed by western yellow 

 pine trees at this stage of life are a great deal thinner at the diameter 

 breasthigh than in the top third of the bole. Such trees are "filling 

 out ' ' ; their bark is also shedding faster than it is formed, and it is very 

 possible for the diameter breasthigh measurement to hold constant or 

 even decrease while the volume is gradually increasing. Fullness of 

 bole or form is usually expressed in terms of * * form factors. ' ' 



Form factors. 



A form factor for a given tree may be the volume of that tree in 

 cubic feet divided by the volume of a cylinder, with dimensions equal 

 to the breasthigh measurement and height of the tree. If the factor for 

 a certain tree is .333, it means that the volume is equal to one-third of 

 the cylinder to which the tree is compared. If the factor is .500, the 

 tree has half the volume of the cylinder, and so on. 



Tables 11 and 12, page 138, show form factors for the merchantable 

 portions of both immature and mature yellow pine trees. It will be 

 observed that the factors worked out for immature trees are larger for 

 the small trees than for the large ones. This is due to the effect of 

 "butt swelling." Since for small trees the breasthigh measurement, 

 which is taken at four and one-half feet above the ground in all cases, 

 is well above the enlarged portion of the butt. In large trees the v effect 

 of "butt swelling" enters in quite noticeably, thus giving a large breast- 

 high measurement and therefore larger cylinder for comparison. This 

 reduces the form factor. It will be seen in Table 12, page 139, that the 

 form factors for mature trees of different sizes are practically constant, 

 and that the values in all cases are greater than for the immature, 

 which shows that the former are filled out more than the latter and 

 therefore contain more wood for a given diameter measurement. 



