The Causes of Timber-line on Mountains. 



173 



In order to cover the general question as well as possible 

 we may take up in succession some of the phenomena in North 

 America with all of the above factors in view. In the White 

 Mountains of New Hampshire and the Adirondacks of New 

 York, timber lines occur with some regularity at about 1,500 

 meters. Relatively few of the peaks surpass this altitude, 

 and owing to their isolated position they are much exposed to 

 wind action. The vegetation of their higher altitudes bears 

 every evidence of the severe effects of this factor. As one passes 

 up through the sub-alpine zone the trees become more and more 

 reduced in height, and associate thickly together. At timber 

 line the forest appears as a thicket of stunted trees (Abies 

 balsamea, Picea sp.), sometimes, as on Mt. Tahawus, (often 

 called Marcy) so dense with interlocking branches that one is 

 forced to use the axe in order to advance. Farther up the out- 

 posts of the dwarf forest are found in little hollows or beside 

 sheltering rocks. At the last we find prostrate dwarfs, strangest 

 monstrosities of arboreal vegetation. 



Everywhere in the alpine zone the healthy shoots extend 

 to a certain contour line. Those that pass beyond 

 the intangible limit are moribund or already dead. A sectional 

 view of such a dwarf forest is shown in Fig. 1. The trees grow 

 to a height roughly corresponding to that of the rock which acts 

 as a windbreak. Their form is characteristically that 

 of wind cripples. In both their arrangement and form 

 we see the unmistakable results of this factor. Air currents 

 increase rapidly in velocity with increasing distance from the 

 ground. Doubtless they bring about the results seen largely 

 by dry killing in cold weather. Sand blast action may also play 

 apart. Mechanical pressure of the snow beds leaves its impress 

 on the dwarf trees, especially the prostrate ones, but it is wind 

 which chiefly governs the form and distribution of the trees. 

 It is probably not too much to say that the timber lines of the 

 Adirondacks and White Mountains express a wind relation. 

 Harvey* believes that on Mt. Katahdin, in Maine, the vegetation 

 of which is very similar to that of the mountains just mentioned, 

 the timber line is not a climatic but an edaphic one; that, in 



♦Harvey. Le Roy Harrier. A study of the physiographic ecology of Mt. Katahdin 

 Maine The Univ. of Maine Studies, No. 5, Dec. 1903. 



