NATURALISATION OF FOREST TREES. 225 



a slightly warmer climate having a rather longer growing season 

 than those of its native habitat. 



Moisture. 



The second chief factor of climate is moisture. The average 

 annual rainfall by itself is of little value when comparing climates, 

 but what are of importance are the range of annual rainfall, the 

 minimum rainfall in any year, the season of rainfall, and the nature 

 of the rainfall. It is only when these are more or less similar 

 for two climates that the average annual rainfall indicates cor- 

 rectly their relative wetness. Consequently the statement which 

 is sometimes made, that forest growth is found where the mean 

 annual rainfall is 20 inches or more, is only an unreliable 

 generalisation. 



The range of annual rainfall is expressed by the ratio of the 

 lowest to the highest annual rainfall. In humid regions the 

 ratio is usually about 2:3; while in desert regions it is sometimes 

 1 : 8. The ratio in most places on the high veld of South Africa 

 seems to be about 1:2. 



The minimum rainfall in any one year is probably of greater 

 consequence in the distribution of species than the mean annual 

 rainfall ; and so, even where the latter may be fairly high, the 

 growth of a species may be impossible if the minimum rainfall 

 is very low. Some species, however, show a remarkable capacity 

 for enduring such years of extreme drought. For instance, on 

 the table-land of Arizona records in the middle of the forests 

 of Pinus ponderosa with an average annual rainfall of about 20 

 inches, show a minimum rainfall of only 7 inches. 



The nature of the rainfall especially as to its frequency, inclusion 

 of mist and so on, is of great consequence in determining the 

 moistness of a climate, and figures of rainfall should always be 

 accompanied by the number of days on which the rain falls. 



Closely connected with this are the relative humidity of the 

 atmosphere and the rate of evaporation. The former of these is 

 probably quite sufficient by itself to limit the range of some species 

 owing to its influence on the rate of transpiration, (as for instance 

 silver fir, hemlock and probably the Yellow-w^oods), and abundant 

 soil moisture with such species would not compensate for a low 

 atmospheric humidity, for the latter would induce an excessive 

 rate of transpiration from the leaves, which the trees is physio- 

 logically unable to endure. Other species, however, are decidedly 

 adaptable in this respect, if given sufficient soil moisture. 



It has already been mentioned that Professor Mayr states 

 that forest growth requires an average relative humidity during 

 the growing season of not less than 50%. This seems only to have 

 been based on observations on the edge of the prairies in Eastern 

 America, and cannot be taken as a world-wide rule. Probably 

 the figure would be a good deal lower on the border of some 

 forests such as the Eucalyptus forests in the interior of Australia. 

 At Bloemfontein the average relative humidity at 8.30 a.m. for 

 the seven months from September to March is 61%, and at Lindley 

 it is 70%. 



