RELATION OF GRAY BIRCH TO WIIITF, PINE 19 



plot, 52 by 52 feet in area, out of 317 hardwood trees growing thereon, 

 all but 16 were gray birch. They all originated from seed and were 

 uniformly distributed over the plot. For the most part they were tall 

 and straight, with the canopy 18 to 28 feet above the ground. The 

 understory of pines was entirely below the birch canopy. Reduced to 

 an acre basis, there were 4,816 birch stems per acre from one to six 

 inches in diameter and approximately 200 small stems of poplar and 

 black cherry. The white pine beneath this remarkably dense stand of 

 18-year-old gray birch were 432 per acre, of which approximately 25 

 per cent were over five feet in height. The high quality of the site, 

 particularly the better soil and moisture conditions, permitted a fair 

 grozcth in the pine, even under the densest gray birch. Measurements 

 taken from a large number of trees showed the average height growth 

 during the past four years to be as follows : 



Year Height growth in inches 



1915 6.7-\- 



1916 6.4+ 



1917 6.4-1- 



1918 5.5+ 



Plot I\' represents the maximum density of 18-year-old gray birch. 

 Measurements were made at noonday, August i, of the chemical light 

 intensity under the average canopy of this plot. The measurements 

 were made with the Clements photometer by exposing it while walking 

 back and forth across the plot in two directions at six-foot intervals. 

 The average chemical light intensity under this maximum density of 

 gray birch canopy was 1/9 of full light. Only a short distance away 

 white pine seedlings were abundant and growing well under an over- 

 wood of pine, where the average chemical light intensity measured the 

 same day was 1/14 of full light, while pine reproduction did not en- 

 tirely disappear until the light was reduced to from 1/25 to 1/35 of 

 full light. It zi'ould appear from the above that even under the densest 

 gray birch there is adequate light for the persistence and growth of 

 white pine. Why, then, is the growth so greatly checked in the pine 

 when under dense gray birch, particularly upon upland dry soils? It 

 appears to be primarily, if not entirely, due to root competition for 

 moisture and nutrients. On Plot l\\ with more than 4,800 birch 

 stems per acre, forming the overwood and evenly distributed, every 

 square foot of the surface soil is occupied by the birch roots, and the 

 understory of smaller pine has but little chance for adequate soil 

 moisture and nutrients to sustain growth. 



In the spring of 191 7 a large area of almost pure gray birch, 27, 

 years old and varying in density, was underplanted with white pine 



