AIR REACTIONS 93 



Reaction upon Climate. From the preceding account of air reac- 

 tions, it is evident that plant communities exert definite effects upon 

 climate, especially in terms of water relations. These find expression 

 in the processes of precipitation, condensation, and interception, which 

 are consequences of transpiration, cooling, and mechanical action, and 

 hence most marked in the forest. The long debate over the effect of 

 forestation on rainfall has led to a decision in favor of the affirmative, 

 though the amount of increase is still a moot question to be decided 

 only by organized experiments on a much larger scale. One of the 

 basic problems to be settled is the relative transpiration of forest, 

 scrub, grassland, and crops of various sorts. Thus, a possible excep- 

 tion to the rule that forest increases precipitation may be found in 

 the replacement of xerophytic forest and scrub by grass and crops in 

 Australia, which was followed by a rainfall increase of 3 per cent 

 (Quayle, 1922). AVhile the water loss of a deciduous forest is so great 

 as to support the assumption of Briickner that 78 per cent of the pre- 

 cipitation over the continents is derived from this source, the evidence 

 is still too general to warrant its acceptance as more than a plausible 

 working hypothesis (cf. Briickner, 1905; Zon, 1913; cf. Brooks, 1928). 



Reaction by condensation is partly mechanical and in part due to 

 the lowering of temperature in some degree. It produces striking 

 results in regions much subject to fog and fine mistlike rains, the in- 

 creased precipitation often amounting to twofold or greater. Again 

 the effect is most pronounced in forest and decreases with reduced 

 height and spread in scrub and herbaceous vegetation. Dew belongs 

 in the same general category, but it is much less significant (Marloth, 

 1903, 1905; Phillips, 1926). 



Reaction by interception is wholly mechanical in nature and may 

 be exerted by animals as well as plants, though to a much smaller 

 degree. It is produced by all plants, but is of little import in open 

 communities of forbs. It increases with size and density, reaching a 

 maximum in forests, where it may amount to as much as 25 to 50 per 

 cent of a particular rain, bearing an inverse relation to the intensity 

 of the latter (de Forest, 1923; Phillips, 1926; Zon, 1929:24; Brooks, 

 1928). 



Reactions Produced by Man. As a superdominant, man may exert 

 all the reactions caused by animals and nearly all those due to plants, 

 working more or less directly through his own activities. When his 

 innumerable coactions enter the scene, he becomes also a superinfluent, 

 with the reactions of plant and animal as well as of the entire commu- 

 nity at his command. Hence, he is unsurpassed in the variety and 

 intensity of his reactions, though many of these are local and inter- 



