18 BULLETIN- 285, U. S. DEPARTMENT OF AGRICULTURE. 



But, on the other hand, the individual trees grew with extreme slow- 

 ness, especially the more tolerant. Many of the trees which ulti- 

 mately became dominant did so only after a long struggle upward 

 toward the light, during which their growth was suppressed by shade 

 almost to the point of extinction. Evidence of this struggle is found 

 when old-growth forest trees are cut, in the great and irregular varia- 

 tion in the width of the annual rings. These irregularities are not, 

 it is true, wholly due to variations in the light supply; climatic fluc- 

 tuations and the drain caused by heavy seed crops undoubtedly have 

 their effect. But the aggregations of fine rings represent chiefly the 

 periods of suppression by shade, while the wider rings represent the 

 more rapid growth under increased light. In dominant trees, there- 

 fore, the rings are apt to be narrower near the heart than elsewhere, 

 and in trees which have long been suppressed they may all be very 

 narrow. 



Most of the "intensive" trees of the northern forest retain to a 

 great age their power of recovery from moderate suppression, and 

 this is as true of the less as of the more tolerant. In consequence, a 

 graphic curve based on the growth of an individual virgin forest tree 

 is exceedingly irregular, and bears little resemblance to that of an 

 open-grown tree, in which the growth is at first slow, rapidly reaches 

 a maximum, and then gradually decreases. An average curve repre- 

 senting the growth of many forest trees is commonly almost a straight 

 line. 



It is worthy of notice that the fine rings next the bark of large, old 

 trees may be due not to insufficient light, but to the great circum- 

 ference about which the season's layer of wood must be spread. At 

 the top of the tree, where the circumference is smaller, the growth of 

 the same year will show a much wider ring on cross section. 



Tables 7, 8/ and 9 show the growth of most of the important 

 "intensive" trees of the northern hardwood forest in the Lake 

 States. They are based on decade measurements of selected, well- 

 formed, sound trees, and represent a growth slightly greater than the 

 average rate. 1 The small number of white elm trees measured (14) 

 was insufficient for thoroughly representative tables; but since the 

 trees were dominant the figures given show fairly well what may be 

 expected of vigorous white elm in unmanaged forests. The principal 

 inference from the table is that the growth rate is more or less in 

 proportion to the tolerance of the species, and that basswood is con- 

 siderably more rapid growing than any of the others. 



i The maximum and minimum figures do not indicate extremes, but only averages of maxima and 

 minima. All the measurements were separated into three equal parts, representing maxima, averages, 

 and minima, and each part was averaged (graphically) by a curve. The absolute maxima or minima can 

 be found by halving the difference between the figures given in the "maximum " or " minimum " columns 

 and those given in the "average" column for any desired year, and then increasing the average maximum 

 or decreasing the average minimum by this amount. 



