88 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



is considerable proof that tunnels and mounds have a beneficial action 

 in promoting absorption (cf. Grinnell, 1923, 1933). 



Reaction by Decreasing the Water Content. As a direct reaction, 

 plants diminish the water content by absorption and transpiration. 

 The transpiration of a deciduous forest during the summer may be 

 greater than that from a free water surface, and it may be greater still 

 from streamside trees, such as alder, poplar, and willow. Water loss 

 also attains great proportions in the case of most herbaceous crops 

 and amounts to a large total in true and mixed prairie, in spite of the 

 more or less xeric nature of the grass dominants. Probably the highest 

 transpiration in relation to surface is found in emersed water plants 

 and especially those of reed swamps, all of which have permanently 

 open stomata. The reaction is particularly significant in such serai 

 areas, as it materially hastens the drying-out and successful movement 

 set on foot by the shallowing action of plant remains. 



Animals obviously reduce the water level and remove suspended 

 matter in ponds and playas by drinking, but this is of slight impor- 

 tance except in arid regions, where it is usually secondary to drying 

 by evaporation. They may likewise exert an indirect effect by pack- 

 ing the soil and increasing runoff, and this reaction is turned to advan- 

 tage in the range country for the puddling of earthen reservoirs or 

 "tanks" to insure the retention of impounded waters. 



Reaction by Returning Plant Nutrients. This reaction is a re- 

 curring one incident upon the annual fall of leaves and the death and 

 decomposition of plants or their parts. A portion of the nutrients 

 may be returned by more or less immediate leaching, but the major 

 part must be unlocked by the coactions that produce decay. The share 

 which animals have in this is unknown. However, the significant fact 

 is that in nature the material returned to the soil corresponds more or 

 less closely to the amount withdrawn, and a fairly definite balance of 

 nutrients is maintained. In crop communities, this balance is dis- 

 turbed to the extent that the individuals are removed, a headed crop 

 of grain contrasted strikingly, in this respect, with one of sugar beets. 



A severe fire and particularly a complete burn permits the much 

 more rapid incorporation of mineral salts in the soil. Under favoring 

 rainfall, the bulk of these may be returned the first season, as is indi- 

 cated by the exceptional growth made by annuals after a fire. An 

 increase in nutrient content is likewise well known to result from the 

 activity of legumes and other nitrifying organisms, but this is properly 

 considered among coactions. 



Reaction by Decreasing Plant Nutrients. A progressive or per- 

 manent reduction in soil nutrients, as a consequence of their utilization 



