16 LOCALITY AND FOREST VErTETATION. 



tlie pedunculate oak and Scotch pine tlouiisli in the low- 

 lands. 



(2.) A species which jnefers a certain altitude in one 

 locality, will descend towards the level of the sea with 

 increasing latitude, or ascend with decreasing latitude. 



(3.) At the same altitude, the more heat requiring species 

 will seek the warmer southern aspects, and the less heat 

 requiring species the cooler northern ones. 



(4.) A species, which is naturally found on a northern 

 aspect at a low elevation, will seek a southern aspect at a 

 higher altitude. 



It must not, however, be overlooked, that the actual dis- 

 tribution is affected by many other influences besides heat, 

 and that the above theories are only of an abstract nature. 



The effect of frost on the various species is intimately 

 connected with their heat requirement. Trees suffer, as a 

 rule, little from winter frosts within the region of their 

 natural distribution, but frost which occurs during the 

 growing season may do considerable damage ; especially 

 during spring (late frosts), immediately after the tender 

 leaves and shoots have been put forth, and during autumn 

 (early frosts), before the newly-formed wood has had time to 

 ripen. Many influences and circumstances contribute towards 

 the occurrence of late and early frosts. Sometimes they are 

 caused locally, especially in low lying or confined localities in 

 consequence of excessive radiation, evaporation, the descent 

 of cold air from higher localities, and absence of air currents ; 

 in other cases they are due to cold winds. The several species 

 vary much in their bearing towards late and early frosts ; in 

 a general way the following classification will hold good : — 



Most sulijtrt to suffer from laic aud carlij frosts arc : Ash, 

 acacia, sweet chestnut, beech, silver fir. 



Somewhat less : Oak, Douglas fir, Norway maple, sycamore, 

 spruce, alder. 



Least: Lime, hornbeam, elm, birch, larch, aspen, Corsican, 

 Ausli-ian, Weymouth, and Scotch pines. 



