192 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY, 



observations by Wiesner showed that lodgepole pine, at an 

 elevation of 6400 feet, required a minimum light intensity of 

 16 of the total daylight; at an elevation of 8500 feet the 

 intensity required sinks to i/6"4 and even to i 6-9'. 



Thus the behaviour of trees in their extension toward higher 

 latitudes and higher altitudes is not the same. This is due to 

 the fact that the intensity of both direct and diffused light 

 decreases with the increase in latitude, and the light limit of 

 a species is reached when the intensity of the total daylight 

 becomes equal to the tree's minimum light requirement. With 

 increase in altitude, however, diffused light decreases, but direct 

 sunlight increases. With an increase in the intensity of direct 

 sunlight, even though there be a decrease in the diffused light 

 and a lower temperature, the light requirements of a species 

 remain constant or even become less at higher altitudes. 



3. Soil !)ioistii7-e. — Tolerance of trees is emphatically in- 

 fluenced by moisture conditions in the soil, as well as by its 

 quality. Fricke (1904) clearly demonstrated by a series of 

 convincing experiments that deficient moisture in the soil, 

 brought about by competition of the roots of older trees, may 

 cause the death of young growth under the shelter of mother 

 trees. His experiments were made in a Scots pine stand on 

 poor, dry, sandy soil, on which, according to all authorities, the 

 light requirements are greatest. In a stand from 70 to 100 

 years old, with a crown density of 07, there were isolated 

 groups of suppressed young pines, among which were no old 

 trees. The young pines were 10 years old and but little over 

 a foot and a half in height. Ditches 10 inches deep were cut 

 around these groups, and in this way all the roots extending 

 from neighbouring old trees were cut through to the depth of 

 the ditches. The relative amount of light received by these 

 groups of undergrowth was not affected by the experiment, 

 since not a single tree was cut down or trimmed. 



The ditches were made in the spring. In the first summer the 

 needles that appeared on the little pines within the isolated 

 groups had doubled the length of the preceding summer, the 

 terminal shoots became longer, and this thrifty growth continued 

 up to the time the results of the experiments were described 

 (1904), while the same undergrowth outside the areas surrounded 

 by the ditches preserved the same su[)pressed character. The 

 old trees, whose superficial roots were cut through, apparently 



