76 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



2. The amount of water in soil is influenced. 



A portion of the atmospheric precipitations is lost to the soil of forest 

 not bare of foliage, because it is deposited on the plants and thence 

 evaporated ; this is specially true of small precipitations. In forest, 

 about fifteen per cent, of the atmospheric precipitations is thus lost, and 

 more in coniferous than in dicotylous forest. The power of soil of the 

 forest to retain the moisture that reaches it is increased, as it is protected 

 against evaporation. Snow melts more slowly, and water derived from 

 melting snow is absorbed in larger quantities by the soil. 



On the other hand, a plant-covering tends to dry the layers of soil 

 in which roots occur, and the more completely so the denser the vegetation 

 is, because plants absorb water from the soil and dissipate it by transpira- 

 tion.^ 



Soil contains less water if it be clothed with vegetation than if it 

 be bare (other conditions remaining the same). A vegetation of weeds 

 may have a very drying effect on soil. 



3. Soil covered with vegetation is less compact than bare soil. 

 This is so because descending rain can exercise no excessive mechanical 



action upon it ; moreover, animals (earthworms) play a more direct part 

 in this connexion. 



4. Light falling upon soil covered by plants is w^eakened. 



5. The action of wind is decreased among dense, and especially 

 among tall, vegetation. 



6. Air underneath a vegetable covering is changed, especially in 

 forest ; it is cooler and moister. 



The air above soil occupied by plants, particularly above forest, is also 

 cooler ; and this may perhaps lead to an increase in the deposit of dew, 

 in cloudiness, and in rainfall. It is certain that forest and dense vegeta- 

 tion in general prevents atmospheric precipitations from flowing away 

 rapidly, and thus being lost to plants and causing floods. 



7. A covering of moss requires special mention, because it differs 

 from any other vegetable covering in its effect on the amount of water 

 in soil. 



The effect varies with the species of moss concerned. Some mosses 

 (Hypnum and its allies) produce dense cushions, five to six centimetres in 

 thickness, lying loose on the soil. The stems of other mosses (Polytrichum, 

 Dicranum) are enveloped in a felt of rhizoids ; their protonemata and 

 rhizoids permeate the soil in the form of a dense felt, and promote the 

 formation of raw humus. Mosses therefore must act upon soil in divers 

 ways. But, according to Oltmanns,^ the general facts of the case are 

 as follows : 



{a) A carpet of moss acts as a sponge. The dense, low carpet, with 

 countless capillary spaces between leaves and rhizoids, absorbs capillary 

 and superficial water, but obtains little or none by suction from the soil 

 and internal conduction the internal structure is an index of this.^ 

 Consequently, living and dead carpets of moss imbibe and evaporate 

 approximately the same amount of water. 



(6) A carpet of moss does not desiccate soil. Since mosses, particu- 

 larly those forming loose-lying cushions, do not take much water from 



' See Chapter XII, p. 48. ' Oltmanns, 1885. 



* Haberlandt, 1904, Absch. vii. 



