THE ROLE OF ASPEN IN REFORESTATION 253 



ing the i)reseiice of aspen. Because of the reaction of the aspen 

 stands upon the habitat, absolute uniformity cannot be obtained. 

 The accumulation of leaf litter under the aspen adds humus to 

 the soil. The herbaceous growth is invariably more luxuriant 

 under the aspen than in the openings. In the former situations 

 broad-leaved, more or less succulent types prevail, while in the 

 latter there is a predominance of grasses. Minor variations in 

 the original soil composition are sometimes unavoidable. 



It may be argued that the fact that aspen occurs only on cer- 

 tain areas is in itself an indication of important differences in 

 soil conditions, in other words, that aspen has occupied only the 

 areas where certain soil conditions exist and that therefore we 

 should expect to find a difference in the growth of coniferous 

 trees in the aspen and in the open. This argument is answered 

 by the presence of aspen root suckers throughout many of the 

 openings, showing that they were once occupied by aspen. 

 Grazing is and has for many years been responsible for keeping 

 down these suckers which if undestroyed would develop into 

 trees, thus obliterating many of the present openings. The 

 area used in this experiment has been fenced and as a result 

 aspen is springing up in all of the openings (see fig. 1). Grazing 

 animals avoid the aspen thickets to a great extent and congregate 

 in the openings where more desirable forage exists, with the i-esult 

 that the latter are closely grazed while the former are but lightly 

 grazed. This is undoubtedly a factor to be considered in explain- 

 ing the more common occurrence of coniferous reproduction in 

 the aspen thickets than in openings. 



The effect of aspen upon the establishment and growth of 

 planted Douglas fir has been studied by comparing the percentage 

 of loss and injury in aspen thickets and in openings, supplemented 

 by a study of physical factors. Of the latter, moisture conditions 

 were considered by far the most important. Soil moisture, 

 evaporation, relative humidity, and wind movement were meas- 

 ured in the two situations at different times. A comparison of 

 other physical factors, while of scientific interest, was not consid- 

 ered essential to this study. 



The results of planting were observed on two plots under 



