CLIMATE 61 



In connection with this reference to light it should be re- 

 membered that on the parallels of latitude extending through 

 this region the period of daily illumination during the summer 

 is relatively great. On the longest days of the year, actual 

 darkness hardly amounts to more than 6 hours of the 24. That 

 the amount of carbon assimilated stands in direct ratio to the 

 duration of daylight during the growing season is, of course, 

 obvious. 



While the fundamental importance of light in the economy 

 of plant life is widely known, not so general is the knowledge 

 of the relation of light to other climatic factors in the growth 

 of plants, or of the peculiarities, with reference to light, of the 

 particular forest species. In discussing the forest conditions 

 of a region it is pertinent to point out that the amount of light 

 required by a tree is increased with the lowering of the tem- 

 perature. In warmer latitudes, therefore, a tree may endure a 

 crtain amount of shade, while in more northern regions it re- 

 quires full exposure to sunlight. A difference of this sort may 

 be seen in the western yellow pine, which in Montana and else- 

 where in the northern Rocky Mountains is very intolerant of 

 shade ; in Arizona, however, this tree is much less sensitive, and, 

 like many other plants of that region, actually prospers better 

 for a time under a moderate shade. This is, however, not wholly 

 a matter of temperature difference, since the sunlight is more 

 intense in latitudes toward the equator, and, as all plants are 

 attuned to a certain range of light intensity, any excess of light 

 is avoided wherever possible. Whether the difference of be- 

 havior is due to a difference of light intensity or of temperature, 

 or both, it finds expression in the appearance of the forest, 

 which must be more open in the cooler latitudes, in order to 

 provide sufficient illumination. This opening of the .stand may 

 give an opportunity for the entrance of other species less sensi- 

 tive to shade, giving rise to a forest of mixed species, and even 

 resulting in the end, perhaps, in the total elimination of the 

 intolerant ones. 



