39^ The Trees of Great Britain and Ireland 



Average precipitation for ten years 2 1 . 70 inches. 



Rain or snow fell on 76 days; 91 days were cloudy; 49 days were partially 

 cloudy ; and the sky was clear on 149 days. 



The above figures show that the climate is an extreme one, the winter season 

 being cold and severe and lasting five months, while in summer a high temperature 

 is often reached. 



The western larch grows usually mixed with other conifers ; and the number 

 of accompanying species and the proportions of the admixture are very variable, 

 being dependent on the climate and altitude, and on the quantity of moisture in the 

 soil. Douglas fir is the most common companion of the larch, and Pinus ponderosa 

 steps in where the soil is dry. Engelmann's spruce and Abies lasiocarpa descend 

 into the larch forests, but never constitute any large element of it. Pinus monticola, 

 Tsuga albertiana, and Abies grandis are often met with in small quantity at low 

 altitudes in the larch forests of Montana ; farther west, in the Priest River Forest 

 Reserve, Pinus monticola is more abundant than the larch itself between 2400 and 

 4800 feet. Thuya plicata, in regions with a moist climate, forms a notable part of 

 certain larch stands, often to the exclusion of the other species which usually 

 accompany the larch. 



The following notes on a few of the larch forests visited by me will illustrate 

 some different types in Montana. 



Near Missoula, in Pattie Canon, which is a very dry valley at 3500 feet eleva- 

 tion in a rather arid climate, the larch only grows on the cool northern aspect, and is 

 mixed with Douglas fir and Pinus ponderosa. An acre contained, of trees over a 

 foot in diameter, twenty larches, four firs, and three pines. An average good larch tree 

 measured 143 feet by 9 feet 7 inches ; and a tree which we cut down, 14 inches in 

 diameter, showed 211 annual rings, the sapwood being i^ inch in thickness and 

 containing thirty-one rings. 



On the southern end of Lake Macdonald, at 3500 feet altitude in a humid 

 climate, I saw a fine stand composed almost exclusively of larch and Thuya plicata. 

 The soil was glacial clay, very deep, and covered with a thick layer of humus. The 



