AIR REACTIONS. 95 



Weber found the annual rainfall near Nancy to be 4 inches greater at a 

 forest station than in one situated in a denuded area. Observations by 

 Muttrich of the effect of forestation upon the rainfall of the Liineberg heath 

 showed that the precipitation increased steadily during a 7 years' period, and 

 finally exceeded that of adjoining areas. Similar results were obtained after 

 a plantation had been made in the steppes of southern Russia, where the 

 average rainfall from 1893 to 1897 was 17.9 mches in the steppe and 22.2 inches 

 in the newly established forest. Blanford found that the new forest growth 

 in a protected area in British India had a decisive effect upon the rainfall, 

 increasing it from 2 to 12 inches at various stations. Fautrat has made obser- 

 vations which not only show that the rainfall above tree-tops is greater than 

 in the open, but also that it is appreciably greater above coniferous than above 

 broad-leaved forests. These were confirmed by the rainfall recorded imder 

 broad-leaved and coniferous canopies. In 1876 the soil under the former 

 received 16.7 inches and that under the latter only 11 inches. 



Ney determined the amount of dew and frost condensed by leaves in north- 

 ern latitudes to be as much as 0.4 to 0.8 inch per year. On the Pacific coast 

 of North America and in tropical regions the condensation must be very much 

 greater. There are no conclusive observations as to the height at which 

 the cooling effect of a forest is felt, but Zon (219) cites the statement of Renard 

 that this has repeatedly been noticed at an elevation of 5,000 feet during bal- 

 loon ascensions. 



R. von Hohnel, in the study of oak forests in Austria from 1878 to 1880, 

 found that an acre of oak forest 115 years old absorbed from 2,200 to 2,600 

 gallons of water per day. This corresponds to a rainfall of 3 to 4 inches per 

 month, or a rainfall of 17.7 inches for a vegetation period of 5 months. Zon 

 cites also the experiments of Otozky to the effect that forest, on accoimt of its 

 excessive transpiration, loses more water than grassland or a bare area. He 

 concludes that the transpiration of forests has a critical effect upon the rainfall 

 of continents, since the amount of water consumed by a forest is nearly equal 

 to the total annual precipitation. Briickner concludes that the vapor evap- 

 orated from the peripheral areas of continents, i. e., the 79 per cent of land sur- 

 face which drains directly towards the ocean, is able to supply seven-ninths of 

 the precipitation over such areas. From the balance-sheet of water circula- 

 tion over the earth's surface, Zon reaches the conclusion that 20 per cent of the 

 vapor comes from evaporation on land, that only 7 per cent of the evaporation 

 from the ocean reaches the land as rainfall, and that 78 per cent of all the pre- 

 cipitation over the peripheral land area is furnished by this area itself. While 

 his conclusions are in accord with the facts so far as known, it is evident that 

 their acceptance is impossible -without much more exact study of evaporation 

 and transpiration, as well as of the rainfall of many regions. 



(21) Reaction upon aerial organisms. — ^As in the case of soil organisms, this 

 may be the direct consequence of the presence of the host-plant or matrix, or 

 it may be the indirect result of the reaction upon the air factors. As a rule, 

 the two effects are correlated, the presence or the success of the parasitic or 

 saprophytic organism being affected by the conditions as well as controlled 

 by the host-plant or matrix. This reaction is characteristic of communities 

 with a dominant canopy, such as forest and thicket, but obtains in some 

 degree in all vegetation. It is most obvious in the .development of lichen 



