88 



7- Ventilation of Land Habitats 



The preceding account of the temperature, humidity, and evapo- 

 rating conditions in various habitats forms a necessary basis for an un- 

 derstanding of the processes of ventilation or atmospheric change in 

 land habitats. The differences in pressure due to the different densi- 

 ties of cool and warm air and to the friction and retardation of mov- 

 ing air currents, determine to an important degree the composition 

 of the air in many habitats. In such an unstable medium as air, 

 changes take place very rapidly through diffusion, and through this 

 constant process of adjustment there is a tendency to level off all local 

 differences. These are naturally best preserved where diffusion cur- 

 rents are least developed — in the most stagnant or stable atmospheric 

 conditions ; therefore any factor which retards an air current and pro- 

 duces eddies, or slow diffusion, will favor local differentiation of 

 the air. 



We have seen that any vegetable cover retards air currents, so that 

 the air within the vegetation becomes different from the faster moving 

 air above it. The accumulation of humidity at different levels above 

 the soil within the vegetation, clearly shows this. The denser the vege- 

 tation the more completely are the lower strata shut off and, to a cor- 

 responding degree, stagnant and subject to the local conditions. Two 

 factors have an important influence upon these conditions : the charac- 

 ter of the cover itself, and the character of the substratum. If both 

 of these are mineral rather than organic, in general comparatively 

 little local influence is to be expected, although in some localities CO2 

 escapes from the earth and on account of its density may linger in de- 

 pressions and thus kill animals (Mearns '03). Generally, however, 

 the organic materials are of most importance both as a cover and as a 

 substratum, and are often the source of carbon dioxide. Living vege- 

 tation may also add oxygen to such stagnant air, but the main source of 

 it is the free air itself. The forest litter, on account of its imperfect 

 stage of decay, consumes oxygen and gives off carbon dioxide ; in the 

 humus below it, shut off even more from free access to air, the carbon 

 dioxide is relatively more abundant and the oxygen relatively less so 

 or absent; and in the deeper mineral soil the amount of carbon 

 dioxide is relatively less on account of the absence of organic debris, 

 and a small amount of oxygen is present. 



The aeration of the soil is influenced to a large degree by its poros- 

 ity; the looser it is, the freer the circulation. Buckingham ('04) has 

 shown that "the speed of diffusion of air and carbonic acid through 

 these soils was not greatly dependent upon texture and structure, but 

 was determined in the main by the porosity of the soil. . . . the 



