FIFTH NATIONAI, CONSERVATION CONGRESS 191 



as a river carrying sediment deposits part of it as soon as the rapidity of its flow 

 is diminished. The moisture from such clouds is intercepted by the forest in the 

 form of mist or drops of dew or crystals of hoarfrost- on the branches and foliage 

 of the trees. 



The mechanical action of the forest is especially important in the case of 

 snow. The influence of forests upon the amount of snow has been especially 

 studied in Russia, where in some places more than 30 per cent of all the precipita- 

 tion is in that form. During heavy storms the forests not only catch more snow 

 than do large open fields, from which it is blown away, but they prevent it drifting. 

 There is always more snow deposited within the forest than in nonforested areas, 

 except in depressions and protected places where snow accumulates. In regions 

 where the snowfall is heavy the amount of snow that accumulates in the forest, 

 and especially in small openings within the forest, is often so great that gauges 

 located even under the crowns of trees contain more snow than those located in 

 large open places, in spite of the fact that snow is very readily retained by the 

 branches, especially in coniferous forests, and that part of the snow thus retained 

 by the liees gets into the rain gauges only at the time of thawing and, therefore, 

 can not be accurately recorded. The results of many years of observations in 

 Russia upon the accumulation of snow in the forest and outside of it have con- 

 clusively shown that young forests, deciduous forests, and small openings within 

 the forest collect nearly twice as much snow as open fields. 



After all, it really matters very little for the final result whether the increased 

 precipitation over the forest is due to its influence upon the condensation nf vapor 

 in the air or to the mechanical action of its branches and leaves. The fact re- 

 mains that forests receive more precipitation than open fields. 



Forests in broad continental valleys enrich with moisture the prevailing air 

 currents that pass over them and thus enable larger quantities of moisture to pen- 

 etrate into the interior of the continent. The destruction of such forests, es- 

 pecially if followed by weak, herbaceous vegetation or complete baring of the 

 ground, affects the climate, not necessarily of the locality where the forests are 

 destroyed, hut of the drier regions into which the air currents flow. 



While definite observations to show the relation between the forest and the 

 climate of continents are still lacking, there are many theoretical- considerations 

 which strongly point to a distinct influence of the forest, especially upon the cli- 

 mate of large continents of a level character. 



The accompanying maps, on which the direction of the prevailing winds is 

 indicated by arrows and the mean precipitation by lines, one typical of the summer 

 period and the other of the winter, show a most intimate relation between the 

 prevailing winds and precipitation in the eastern half of the United States. A 

 high meteorological authority in this country states that the "precipitation in the 

 eastern half of the United States is from the aqueous vapor that is raised up from 

 the vast waters to the south and southeast of the continent" and that "the supply 

 is inexhaustible." 



If by this is meant that the precipitation over the eastern part of the United 

 States is derived entirely from evaporation from the Gulf of Mexico and the At- 



