438 APPENDIX. 



absolute amount of annual transpiration, as observed in 

 forests of mature oaks and beeches in Central Europe, may 

 amount to 50 per cent of the total annual precipitation and 

 more ; with conifers, only one-sixth to one-tenth of this. 



The percentage of rainfall, evaporated at the surface of the 

 ground, is about 40 per cent for the whole year in the open 

 field and about 1 2 per cent for the forest, and is greater under 

 deciduous than under evergreen forests. 



The evaporation from a saturated bare soil in the forest is 

 about the same as that from a water surface in the forest, 

 other conditions being the same. 



The presence of forest litter like that lying naturally in un- 

 disturbed forests hinders the evaporation from the soil to a 

 remarkable extent, since it saves seven-eighths of what would 

 otherwise be lost. 



The total quantity of moisture returned into the atmosphere 

 from a forest by transpiration and evaporation from the trees 

 and the soil is about 75 per cent of the precipitation. For 

 other forms of vegetation it is about the same or sometimes 

 larger, varying between 70 per cent and 90 per cent; in this 

 respect the forest is surpassed by the cereals and grasses, 

 while, on the other hand, the evaporation from a bare soil is 

 scarcely 30 per cent of the precipitation. 



The absolute humidity within a forest exceeds that of the 

 glades and the plains by a small quantity. The relative 

 humidity in the forest is also larger than in the glades or 

 plains by 2 per cent to 4 per cent. Forests of evergreens 

 have from two to four times the influence in increasing rela- 

 tive humidity as do forests of deciduous trees. 



The gauges in European forest stations catch from 75 to 85 

 per cent when placed under the trees, the balance represent- 

 ing that which passes through the foliage and drips to the 

 ground or runs down along the trunks of trees, or else is in- 

 tercepted and evaporated. The percentage withheld by the 

 trees, and which either evaporates from their surface or 

 trickles along the trunk to the ground, is somewhat greater 



