18 JOURNAL OF l\)Ri;STRV 



and only ten trees per acre attained a height of six feet. Evoi on this 

 plot the great density of the birch did not prevent the persistence of 

 the white pine beneath. Its chief etlfect appeared to be to arrest the 

 growth of the pine. Without exception, however, the foHage was 

 short and sparse and the trees lacked vigor. 



The average height growth of the pine during the past four years 

 was as follows: 



Year Height growth in inches 



I915 2.3+ 



1916 2.7+ 



I917 2.2-(- 



1918 9+ 



On this plot, 7 per cent of the pine, mostly in the upper height 

 classes, died the growing season following the removal of the over- 

 wood, although the season was exceptionally favorable. From the 

 data derived from Plot IV, which is discussed later in this article, it 

 is believed the poor condition of the pine on Plot III and the loss 

 after the removal of the overwood are not due so much to the density 

 of the birch, but to the much denser foliaged red maple intermixed 

 with it. 



The above studies suggest that gray birch at 24 years of age, and 

 probably at all ages, is never so dense that zvhite pine in southern New 

 Hampshire i<.'ill not survive beneath it. The growth of the pine, how- 

 ever, is governed by the density of the birch. When the birch is 

 moderately open, even with as many as 600 to 800 trees per acre up 

 to six inches in diameter and 28 feet tall, the pine as a whole is not 

 held back essentially in height growth, and the chief advantage in re- 

 moving the birch is to prevent the whipping of the pine tops by the 

 birch. As the density of the birch increases up to the maximum for 

 the species, there appears to be no essential reduction in the number of 

 pine beneath due to the birch, but there is a progressive falling off in 

 height growth with increase in density, and it becomes necessary to 

 remove the birch in order to secure adequate growth in the pine to 

 justify its management as a pine forest. 



What is the cause of the falling off in height growth in an under- 

 story of white pine when under gray birch, and why is there so little 

 loss in the number of pine per unit of area even under the densest 

 stands? Is the poor growth due to the effect of the birch in with- 

 holding light, or is it due to root competition for soil nutrients and 

 moisture? 



A carefully selected sample plot was laid out in a very dense stand 

 of 18-year-old gray birch growing on a moist, loamy sand. On this 



