Rydberg: Phytogeographical notes 681 



the season may have also another effect on the timber line, i. e., 

 the seeds would not have time to ripen. This may be of great 

 importance in accounting for the arctic timber line, but it can 

 have very little influence on the alpine timber line, for most of the 

 conifers that reach the timber line have winged seeds, which are 

 easily carried above the line of maturing seed and then can 

 germinate there. 



Late frost 



Another factor which has been given as having effect on the 

 timber line is late frost in the spring, killing the new sprouts. As 

 the conifers, which are most affected, do not readily produce a 

 second crop of shoots the same season, the forests after a few 

 repeated frosts will soon be killed. In such a way large districts 

 of pine forest were destroyed in Montana a few years ago. 



Strong desiccating winds 

 One of the most important factors is strong wind. This factor 

 has been much underestimated in earlier times, but later writers 

 on the phytogeography of the arctic regions have recognized it 

 more and more. In my belief it is one of the most important 

 factors in the Rockies. The trees at the timber line and especially 

 those few isolated stragglers above the real forest line show marked 

 effects from the wind. The trees are not only low, stunted, 

 gnarled, ragged, with enormously elongated lower branches often 

 spreading on the ground, but conspicuously one-sided, telling at 

 the glance the direction of the prevailing winds. But the me- 

 chanical influence of the wind is not the most important, however. 

 Of greatest importance are its desiccating effects, especially in the 

 winter. This effect of the wind has been recognized even in 

 arctic regions, but it must be taken into consideration still more 

 in the mountains. The timber line is much lower on the north 

 side of the Alps than on the south side. This is due not only to 

 the difference in temperature (for the difference in altitude should 

 not be so great), but still more to the desiccating northern winds. 

 In some places in Montana these winds are northerly, but in 

 southeastern Colorado and southern Utah they are from the south- 

 west, and it is on this side of the mountains that the timber line is 

 the lowest. In the Abajo Mountains of southeastern Utah, for 



