The average height growth of the 166 poplar trees employed in table VI, is 10.8 inches in a year, and the average increase in diameter is one inch in six years. It will be noted that it takes approximately 30 years' growth to produce a poplar tree five inches in diameter, breast high, the lowest diameter at which trees are cut for pulp-wood. Further, it takes 13 years, on the average, to produce a tree one inch in diameter, 26 years for a tree three inches in diameter and 36 years for a tree six inches in diameter. Growth studies were made on a few trees of larger diameter than those given above, but they were on a much better quality of soil and so were not included. Basing the statement upon growth studies of poplar from other sources, one may say that it does not reach its most rapid growth in volume until about the fiftieth year. Since none of the trees studied had reached that age, it is assumed, in forecasting the yield at the end of the next 30 years, that the trees will grow at the rates stated above.
Applying the rate of growth indicated above to the average number per acre of poplar trees of the various diameter classes on the areas burned once, as given in table 1B (pages 176-177), and assuming that all were to live, it would be found that, at the end of the next 30 years, the diameters, number of trees and their contents in cubic feet would be as indicated in the table below :
NUMBER OP POPLAR TREES PER ACRE AND VOLUME TO BE EXPECTED ON THE AVERAGE ACRE AFTER THE NEXT 30 YEARS ON THE AREAS BURNED ONCE, ASSUMING ALL TREES SURVIVED