158 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



The climate of both the Eastern region and the Rocky 

 Mountains region is an extreme one, a so-called continental 

 climate, distinguished by great cold in winter, an abrupt change 

 from winter to spring, with an absence of spring frosts, followed 

 by a sunny, warm summer and autumn. The climate of the 

 Pacific Coast region and that of the British Isles and the 

 western border of Europe generally may be called insular, or 

 maritime, being characterised by the absence of great cold in 

 winter, an erratic transition from winter to spring, with great 

 liability to spring frosts, and no great heat in summer, while 

 dull, cloudy days and fogs occur at all times of the year. The 

 trees which live in a continental climate are rarely adapted 

 to thrive in an insular climate. The great distinction between 

 the regions of North America is well shown at the Arnold 

 Arboretum,^ near Boston (U.S.A.), where the Douglas fir, 

 Abies grandis and Thuya plicata from the shores of the Puget 

 Sound refuse to grow, while individuals of all these species from 

 Idaho and Colorado make thriving specimens. 



The trees in the Pacific Coast region are remarkable for their 

 great height and vigour of growth, resulting in an amazing 

 volume of timber per acre, and constituting a unique type of 

 forest. No complete explanation of this phenomenon has been 

 offered, but it is evident that the trees along the coast are not 

 checked in growth by any xerophytic conditions. The evapora- 

 tion from their foliage is small, being hindered by the humid 

 atmosphere which surrounds them. The heavy rainfall and 

 the splendid condition of the soil, which is usually deep, 

 nearly always heavily shaded, and invariably covered with the 

 humus of a thousand years, provide an immense supply of 

 moisture. Possibly the dull, cloudy light which prevails is an 

 optimum one for the chlorophyll function. The combination of 

 all these factors enables timber to be built up at an extraordinary 

 rate by the trees in this region. 



It is fortunate for us that when introduced into our congenial 

 climate, these wonderful trees preserve their vigour. It may be 

 noted that the Pacific Coast region has a climate singularly 

 like that of the extreme west of Europe, Alaska corresponding 

 roughly to Norway, British Columbia to Scotland, Washington 

 to Ireland, Oregon to Western France, and California to 

 Portugal. These climatic subdivisions are important in regard 



* Cf. Arnold Arboretum Bulletin Pop. Inf., No. 60 (1914). 



