HEAT. 15 



spring is retarded. This may be beneficial in preventing 

 damage b}' spring frosts, limiting the formation of inferior 

 spring wood, etc. ; on the other hand it shortens the growing 

 season, and delays the sprouting of seeds in spring.* 



Heat is a most powerful agency in the distribution of plants 

 on the earth ; the species change with increasing latitude, 

 elevation and other influences which govern the temperature. 

 This applies to forest trees as well as other plants. 



Attempts have been made to ascertain the absolute sum of 

 heat required annually by the more important forest trees, 

 which have been enumerated in the introduction of this part, 

 but so far the available data are not of much practical use 

 in sylviculture. Moreover, it is beyond doubt, that mean 

 temperatures are much less important to forest trees than 

 the extremes of temperature which occur in a particular 

 locality, more especially during the growing season. 



Something more definite is known of the relative heat 

 requirements of the several species. According to Gayer this 

 is as follows : — 



It is (jreatest in : Common elm, sweet chestnut, pedun- 

 culate oak. 



Someichat smaller in : Sessile oak, Austrian pine, silver fir, 

 beech, Weymouth pine, lime, Scotch pine. 



Less again in : Norway maple, birch, sycamore, ash, alder, 

 hornbeam, spruce. 



Smallest in : Larch, Cembran jjine, mountain pine. 



The different degrees of heat requirement produce many 

 phenomena of interest to the forester, of which the following 

 may be mentioned : — 



(1.) On the same latitude the several species, if left to 

 natural selection, are found at different elevations. While 

 the Cembran pine finds full development near the ujjper 

 limit of tree vegetation (up to 7,000 feet in the Alps), the 

 larch and next the spruce prefer a somewhat lower zone ; 

 lower again appear beech, silver fir and sessile oak, wliile 



* For further details on these questions, see Vohime I. 



