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 
