94 REACTIONS. 



parent trees without changing the light values. In the first summer the 

 growth of the seedlings within the area much exceeded that of those outside, 

 while a totally new and vigorous herbaceous layer developed. He also deter- 

 mined the holard of soils with and without living roots, and found the latter 

 to contain 2 to 6 times as much water. This emphasizes the influence of water- 

 content in the later stages of succession and the degree to which competition 

 can modify it. It also makes it plain that the more obvious effects of light in 

 these same stages must be checked by the quantitative study of the water 

 relations. 



(19) Reaction upon humidity, temperature and wind. — These three factors 

 are necesBarily linked together because of their direct effect upon the plant 

 through transpiration and the indirect effect through the evaporation of soil- 

 moisture. The plant community reacts directly upon each factor, and these 

 act upon each other, but the response of the plant is controlled by humidity. 

 The reaction of a sparse pioneer population is more or less negligible, but the 

 increasing density and height of the individuals bring about a measurable 

 result, which becomes significant in most closed associations, especially those 

 of shrubs and trees. In layered forests the j-eaction is greatopt in the ground 

 layer or beneath it, where it consists"of herbs. Himiidi'fy is directly increased 

 by transpiration, but the effect is cumulative because the moisture-laden air 

 is not carried away. The heat rays are absorbed or reflected, and the lower 

 temperature that results causes an increase in relative humidity. The capac- 

 ity of the air for moisture is correspondingly decreased and both transpiration 

 from the plants and evaporation from the soil-surface are reduced. The final 

 effect is to make the water-content more efficient and thus essentially to 

 increase it. The general effect of the reaction is the same as that of increasing 

 humus, and the two are indistinguishable as a rule. The reduced evaporation 

 from the surface soil, and perhaps from the seedlings as well, is a critical factor 

 in the ecesis of many seedlings, especially those of trees. 

 ..(20) Reaction upon local climate. — Plant commvmities react upon the air 

 above them by transpiration and by lowering the temperature. As a conse- 

 quence, they receive more soil-moisture as dew and rain than do bare areas. 

 This reaction of vegetation is measurable only in the case of forest and scrub, 

 but probably occurs in some degree in all vegetation, particularly in the 

 formation of dew. The effect of wooded areas upon rainfall has long been 

 a subject of controversy, but the evidence in favor of a positive reaction is now 

 available from so many sources that it seems conclusive. Zon (1912 : 205) has 

 made the most recent summary of the evidence that forests increase rainfall. 

 At Nancy the average increase in forested areas for 33 years was 23 per cent, 

 while in Germany and India it was computed to be 12 per cent. A four years' 

 experiment to check out the possible error due to faulty instruments yielded 

 an excess of 6 per cent for the forest. Observations in the north of Germany 

 indicate that the influence of forest increases rapidly with the altitude. At 

 elevations less than 300 feet the effect was negligible, while at altitudes of 

 2,000 to 3,000 feet it ranged from 19 per cent to 84 per cent. Denuded moun- 

 tains often fail to cause moisture-laden winds to precipitate their moisture, 

 as Angot has shown to be the case in Spain. A similar influence is often exerted 

 by the hot, dry gravel ridges about Pike's Peak upon the local showers in mid- 

 simuner. 



