68 Studies in Forestry [CHAP. iv. 



This is a subject which has hitherto been little studied, as 

 the following extract from Willkomm l may show : 



'Despite a mean winter temperature of + 17-5 R. (71-4 Fahr.) the 

 Beech in Madeira has a period of rest for 149 days, during which it 

 remains leafless ! This strange phenomenon is to be noted also in the 

 Elm, Silver Poplar, Willow, and other broad-leaved trees distributed 

 through the whole of Europe, as well as in the Apple, the Pear, and 

 other fruit-trees over all the most southern parts of Europe. They also 

 lose their foliage in autumn, and only flush anew in spring, although 

 during the whole of winter the temperature does not sink below + 5 to 

 7 R. (43^ to 47! Fahr.). And whilst in central and northern Europe 

 the buds of the above-named trees open early in spring after warm 

 winters, this is not the case in the south. This phenomenon must be 

 due to some law of nature which remains as yet unknown.' 



And in another place 2 he remarks with regard to woodland 

 trees that 



'The absolute minima during the period of vegetation are of far 

 greater importance, with regard to the thriving of trees, than the minima 

 of winter, and the average temperatures of the months and seasons.' 



Indeed he goes so far as to say that 3 



' It may be stated as a general law that the polar or northern limit of 

 any particular species of tree is determined by a certain definite isotherm of 

 July, whilst its equatorial or southern limit is determined by a certain definite 

 isotherm of January.' 



Seebohm's observations in Siberia agree with the first part of 

 that statement. For he observed that 4 



' The limit of forest growth does not coincide with the isotherms of 

 mean annual temperature, nor with the mean temperature for January ? 

 nearly so closely as it does with the mean temperature for July.' 



When once active vegetation has been stimulated by increasing 



1 Die Forstliche Flora von Deutschland und Oesterreich, 2nd edit., 1887, 

 P- 452- 



3 Op. cit., p. 364- 



3 Op. cit., p. 39. 



4 Presidential Address to Geographical Section of the British Association, 

 1893, in Royal Scot. Geog. Society's Magazine, October, 1893, p. 511. 



