38 HORTICULTURAL SOCIETY OF NEW YORK. 



ter cold and strong winds is extreme. In such cases, the dry- 

 ing-out at low temperatures, and not the actual cold, seems to 

 be the cause of winter-killing. Additional evidence of this is 

 furnished by the alpine spruces and pines. These extend 500 

 to 600 feet higher where they occur in shallow ravines and de- 

 pressfons with a higher water content, particularly on the ex- 

 posed north and northwest slopes. This same phenomenon is 

 even more marked on the Continental Divide, where the al- 

 pine thickets carry the spruces to outposts far above timber 

 line proper by increasing the water content of the soil, decreas- 

 ing the depth of frost, and protecting the young trees from the 

 drying effects of the constant winds of winter. The upper tim- 

 ber line on mountains is accordingly a line of winter-killing. Its 

 direct cause is not the extreme cold of winter, but the excessive 

 water loss at a time when the water supply is chiefly in the 

 form of ice, and hence non-available. 



The feeling that the climatic habits of plants are fixed is 

 widespread though it appears to rest upon no definite founda- 

 tion. It has led not infrequently to the statement that woody 

 plants, and trees in particular, cannot be acclimatized. The evi- 

 dence from the mountain forests of Colorado is distinctly in 

 favor of the conclusion that trees and other woody plants can 

 be acclimatized. Indeed, it only requires reciprocal planting at 

 various altitudes to constitute final proof. The spruces (Picea 

 engelmannii and P. parryana), the firs (Abies concolor and A. 

 lasiocarpa), the yellow pine (Pinus scopulorum), lodgepole pine 

 (P. murrayana) and limber pine (P. flexilis), and even the 

 Douglas spruce (Pseudotsuga mucronata), have repeatedly been 

 found growing vigorously in such divergent local climates, 

 measured in terms of water content, humidity, temperature and 

 soil, that the conclusion is unavoidable that they adjust them- 

 selves readily to new climatic conditions. For a time the ap- 

 parent distribution of the aspen well within the forest zone of 

 the mountains was a puzzling exception, though an extra- 

 regional outpost was known at 4,000 feet in western Nebraska. 

 This difficulty was finally cleared up by finding that the aspen 

 occasionally reaches the lower limit of Pinus scopulorum at the 

 base of the foothills. It was also found this summer growing 



