290 



INFLUENCE OF WOODLANDS UPON STREAMS. 



raoistening influence of a grove may extend. It must depend upon 

 many circumstances, and especially upon the slope of the surface and 

 the direction of the winds. The effect is often apparent to the eye 

 from the freshness of the herbage in adjacent fields for many rods m 

 width. 



In noticing the influence of forests upon the humidity of the air 

 within them, it is found that the coniferte differ greatly from the broad- 

 ieaved deciduous species, the former being drier than the latter; and 

 that where a forest-growth has changed from the latter to the former, 

 the same soil will be drier than before. Some have ascribed this to a 

 difference in the amount of rain retained or of evaporation given off by 

 the foliage; but a more probable reason may be found in the fact that 

 the soil in woods of the coniferous class is more exposed to the sun,' and 

 that the surface-evaporation is therefore more active. The mat of dead 

 foliage is more pervious, allowing the air to permeate and evaporation 

 to go on. The tops of the trees of this class are less uniform in height, 

 and are therefore more exposed to the winds, which also tends to more 

 evaporation. 



It is found by careful observation, that different kinds of timbers pos- 

 sess very unequal powers for the absorption of water from the soil dur- 

 ing the active season of vegetation, and that it is generally in inverse 

 j>roportion to the density of the wood. We know that the leaf-surface 

 of a tree is in direct relation with the amount of wood formed, and 

 that the evaporation from leaves is greater when their tisues are lax 

 and moist, implying a larger amount of water in circulation and greater 

 absorption from the roots. The greater drying-powers of the resinous 

 species has been attributed to the demands made by the latter in the 

 formation of resin. The abundance in which this secretion is formed by 

 the maritime pine growing in damp soil behind the littoral dunes of the 

 coast, has been noticed as proportional to its power of drying the soil.^ 



The tall grass and other herbaceous covering of the western prairies, 

 in a state of nature, answered in some degree instead of forests in 

 maintaining the humidity of the climate. The summer rains were cer- 

 tainly more uniform and abundant than now, and the streams were 

 higher in summer. But as pasturage and cultivation have removed 

 this protection, we are everywhere beginning to feel the consequences, 

 and nothing short of establishing some shelter from the sun and drying 

 winds, will save the country from increasing aridity. The excessive 

 rains and consequent damage from floods, and delays to agricultural 

 operations during the early part of the summer of 1877, throughout 



'The leaves of tho Eucali/jHus, of AustvaVia, in their mature state, iin-sent their edges 

 to the sky, and therefore cast but little shade. Should this tree hereafter be exten- 

 sively grown in California or elsewhere in the United States, we would doubtless lind 

 the soil in such woods much drier than in woodlands of other kinds. 



-The following table shows the drying-power of different kinds of trees, as cal- 

 culated by M. Burger : 



Quantity of water required to supphj, daily, a cubic meter of standing tiniber Iciiveen April 

 15 and October 15. 



