AFFORESTATION IN THE UNITED KINGDOM 617 



Thus there are at present 3,071,047 acres, or only 4 per cent, 

 of the United Kingdom, devoted to the growth of forest, while 

 there are 15,343,670 acres, or 20 per cent., mountain and heath 

 land used for grazing. This latter figure is exclusive of 

 3i537»i72 acres of deer "forest" or land devoted exclusively 

 to sport, and 1,836,859 acres of barren mountain land, turf, bog 

 and marsh in Ireland. Only 3 per cent, of the area of forest 

 is the property of the State. 



Varying views are expressed by writers on forestry for the 

 United Kingdom as to the suitability of the climate, and it is 

 not uncommonly made out that advocates of afforestation sup- 

 press facts as to the limitation of the extension of forests due 

 to climate. In the circumstances it seems advisable to state 

 concisely the limitations now deduced by Prof. Mayr from 

 extensive investigations. Climate, that is to say, the external 

 influences on the earth, its plant growth and animal life, is 

 subject to gradual changes with which changes in vegetation 

 correspond ; its chief factors are temperature, moisture, wind 

 and light. 



Temperature. — Comparison of temperature in different regions 

 definitely proves that extremes do not determine the existence 

 of forest, but that it depends on a certain irreducible minimum 

 temperature during the vegetative period. This period can 

 hardly be less than six weeks, and is roughly four months at 

 50° N. latitude. Further examination of the variations from 

 the minimum temperature over large areas have enabled Prof. 

 Mayr to prove that it is chiefly on these that the natural dis- 

 tribution of species depends. The minimum temperature for 

 four months is found to be 10° C, and to correspond in the 

 northern hemisphere with a mean annual temperature of 

 about 3° C. Temperatures so high during the period as to 

 preclude the possibility of tree growth are nowhere known 

 to exist. 



The mean temperature of the months of May, June, July and 

 August is about 12° C. in the United Kingdom, and although 

 of course aspect, slope and the presence of water may con- 

 siderably modify the altitude at which the vegetative minimum 

 for trees is reached, 1,500 feet above sea level is the average 

 in the extreme north of the United Kingdom (60° N. Lat.) and 

 at 3,500 feet in the south (50° N. Lat.). 



Moisture. — Insufficient moisture may preclude the growth 



