Microclimates 117 



that, if the precipitation comes in short showers, the rain may never 

 reach the soil. The plant cover also tends to reduce the water con- 

 tent of the deeper layers of the soil as a consequence of its transpira- 

 tion, but at the same time it adds moisture to the air. On the other 

 hand, the presence of forest vegetation favors the reduction of evap- 

 oration at levels near the ground because it lowers the temperature, 

 slows down the wind, and sometimes causes increased condensation. 



The amount of water present at any point in a land habitat and the 

 rates of gain and loss are thus seen to be the result of the equilibrium 

 of processes both climatic and biological. The interrelations of the 

 water factor and the vegetation are frequently very complicated and 

 are the subject of elaborate studies beyond the scope of this book. 

 For further information the reader is referred to such treatments as 

 Daubenmire (1947, Ch. 3). In addition to presenting the general 

 picture, the present discussion reveals the interdependency of vegeta- 

 tion and moisture as another outstanding example of the organism and 

 the environment acting as a reciprocating system. 



Microclimates 



In the foregoing discussion of the variation in water loss under 

 various conditions of humidity, temperature, and wind the values 

 given were often those characteristic of extensive areas. Very fre- 

 quently, however, the values of these factors in the immediate sur- 

 roundings of the organism differ markedly from the regional values. 

 In other words, the immediate climate of the organism is often sharply 

 different from the average climate of the region as reported by stand- 

 ard meteorological records. The realization of this fact has led to 

 the development by ecologists of the concept of the microclimate. In 

 considering the effect of climatic influences on plants and animals it is 

 absolutely essential to measure the environmental factors as they ac- 

 tually reach the organism. The effective climate, upon which the 

 life of the organism depends, is its microclimate. The characteristics 

 of the microclimate may, or may not, be similar to the regional climate 

 (Geiger, 1950). 



Local variations in climatic factors of concern to many organisms 

 may involve distances of several meters in the surroundings, but the 

 microclimates of forms living in the soil, in rock crevices, under the 

 bark of trees, or in similar confined habitats often have dimensions 

 measured in fractions of a centimeter. The concept of microclimates 

 is particularly important in relation to moisture because relative 

 humidity varies widely within short distances in any irregular habitat. 



