152 REPORT OF THE COMMISSIONER OF AGRICULTURE. 



subsoil, where tlie water accumulates and reappears elsewhere as 

 springs; the less inclined the surface the more water penetrates, as- 

 sisted by the deep and far-reaching ramifications of the roots and the 

 permanent vegetable cover. While this beneficial action is especially 

 noticeable in the mountainous regions, the forest of the plains also 

 acts as a water reservoir; which will appear from the observed fact 

 that in deforested localities the ground-water level has sunk and 

 aridity is increasing. 



Under the forest cover of the mountains the melting of snows is re- 

 tarded, and thus the flow of streams is more gradual and continued 

 evenly through a longer period. While the large floods are proba- 

 bly to a great extent due to cosmic causes, their aggravation through 

 deforestation at the headwaters of streams cannot be denied; and that 

 local floods, especially the ravages of mountain streams, can be obvi- 

 ated by proper forestry has been proved practically in France and 

 the Tyrol within the last thirty or forty years. 



The mechanical action of forest belts in breaking the force of winds, 

 alleviating the effects of hot and cold blasts upon crops, and in ame- 

 liorating the severity of the climate of a neighboring district is well 

 known to every prairie settler. 



CLIMATIC INFLUENCE, • 



The influence claimed for forest areas upon the local climate of a 

 neighboring region must be considered as mainly due to a difference 

 of insolation and consequent difference of temperature and evapora- 

 tion over the forest and the open field. This influence, therefore, is 

 appreciable only when sufficiently large and dense forest areas alter- 

 nate with open grounds. 



In consequence of the difference between the temperature of the 

 forest and that of the surrounding region, local currents of air are 

 established, so that the forest acts like a large sheet of water as a 

 starting-point for diverging local winds. 



The cooler and generally moister air over the forest promotes the 

 condensation of the lower layer of clouds and the condensible strata 

 of air, so that, while the forest may not positively cause rain to fall, 

 yet it does not at least prevent it, as the heated bare ground or field 

 often does. 



The climatic influence of the forest upon its neighborhood may be 

 said to consist in the communication of its own climatic characteris- 

 tics, i. €., shorter range of thermometrical extremes and more perma- 

 nent moisture in its atmosphere. These characteristics are more pro- 

 nounced in summer than in winter; their degree is proportionate to 

 the extent and density of the forest, and their communication to the 

 surroundings is graduated by the distance from the forest. The for- 

 est, in short, is a regulator of climatic, as it is of hydrologic extremes. 



Our present knowledge of forest influences, based on experiment, 

 observations, and logical inferences, allows us to summarize the fol- 

 lowing facts: 



A. — Effects of deforestation on the climate. 



(a) On the climate of the deforested area. 



(1) Extremes of temperature of air as well as of soil are aggravated. 

 {p) The average moisture of the air is lowered. 

 (3) Whether the moisture of the soil after deforestation will be 

 greater or less depends on the nature of the soil. 



