20 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



but not always. With most species, both height-growth and 

 diameter-growth are greater per tree in the open wood. Although 

 not yet generally accepted, this has been experimentally proved. 

 That dense planting forces trees to greater height-growth is a 

 fallacy. Mutual shelter from exposure, afforded by the growing 

 of trees in company as opposed to growing isolated trees in the 

 open, is quite another matter and does influence height-growth, 

 simply by reducing the exposure. It is very remarkable, in close- 

 grown woods of some species, how the dominant trees put on 

 girth while the suppressed trees practically stand still. In a 

 Douglas wood trees were girthed in 19 19 and again in 1920, 

 and the increase in the smaller trees was quite immeasurable. 

 On the other hand, some of the dominant trees had put on 

 more than an inch in girth. 



5. The root-formation is much greater in the more open 

 woods, owing to the greater amount of ground space available. 



As a direct result of this : — 



6. Thinning is more difficult in the closely-grown woods, and 

 damage by wind and snow more frequent. This is a serious 

 consideration in this country. For species like spruce and 

 Douglas fir sufficient root-room makes all the difference, and 

 many examples show that they are wind-firm with sufficient 

 space, but in extreme danger when densely grown. Over- 

 crowding of roots is also pathologically important, and certainly 

 encourages root-fungi. This is very noticeable in pine woods 

 at the present time. 



7. The carrying out of cleaning and thinning operations is 

 more difficult in the closely-grown wood. This becomes a very 

 serious problem with Douglas fir and Sitka spruce. 



8. The greater amount of dead branches and brushwood in 

 the closely-grown wood increases the danger from fire. 



9. It is claimed that in the open wood aeration of the soil 

 is better, and that the soil changes go on more rapidly, with 

 a beneficial effect on the growth. This affords an excellent 

 example of how difficult it is to distinguish between the effects 

 of thinning and those of original planting distance. In the 

 period of formation it has been urged that the accumulation 

 of humus is an advantage, yet here it is now stated that the 

 aeration of the soil is better and that the soil changes go on 

 more rapidly. This, also, is claimed as an advantage. The 

 two statements require to be reconciled. In the early life of 



