480 



THE COMMUNITY 



earthworms (Eisenia, Diplocardia, Bi- 

 mastus) found abundantly in forest soil 

 become notably fewer after burning over 

 (Pearse, 1943). 



The recuperative power of a particular 

 forest community in relation to the de- 

 structive power of the inimical agencies at 

 a given time determines its chances for 

 survival to maturity. Since there is a re- 

 ciprocal relation of the whole forest to its 

 strata, just as there is a reciprocal relation 

 between the organism and its organs, any 

 injury to the one will injure the other, and 

 furthermore community injury and its pos- 

 sible recovery directly afiFect the stratal 

 occupants initially concerned, and indirectly 

 the entire stratification, both with respect 

 to immediate state and relative maturity. 

 Injury and recuperation afiFect the total 

 biota. 



The blanketing eflFect of vegetation is 

 complex and moderates the several interact- 

 ing physical influences that compose the 

 forest climate. The light intensity is always 

 lower in forests than in the more open ad- 

 jacent communities (Allee, 1926; Park, 

 Barden, and Williams, 1940). In the 

 Chicago area at noon on a clear summer 

 day, the light intensity will be of the order 

 of 10,000 foot-candles in the open as com- 

 pared with 50 foot-candles on the shaded 

 forest floor, or roughly -5-^ of the avail- 

 able light (Park 1931; Park and Strohecker 

 1936). As this light passes down through 

 the forest canopy, each succeeding stratum 

 is potentially less illuminated (Allee, 

 1926), so that the plants are aflFected di- 

 rectly by the amount of light available for 

 photosynthesis (Coulter, Barnes, and 

 Cowles, 1911), and the animals are affected 

 directly as their photic responses are modi- 

 fied and indirectly with respect to the 

 plant food supply. There is also a differ- 

 ential change in the quality of the light 

 (Strohecker, 1938). These changes in the 

 spectrum also affect plant and animal re- 

 actions, both directly and indirectly. 



Forest temperatures are generally lower 

 in summer and higher in winter than tem- 

 peratures of adjacent areas (Williams, 

 1936) . This is a general consequence of the 

 vegetation, and especially a consequence 

 of the thick layer of vegetable mold on the 

 forest floor. Similarly, forests are cooler 

 during the heat of the day and warmer 



during the night than areas external to 

 them (Park and Strohecker, 1936). 



Relative humidity is characteristically 

 higher and evaporation rate lower within 

 the forest than in adjacent, less dense ter- 

 restrial communities (Fuller, 1911, 1914; 

 Ullrich, 1915; Williams, 1936). Similarly, 

 relative humidity is higher and evaporation 

 rate lower within forests during the night 

 than in daytime (Park, Lockett, and Myers, 

 1931; Park and Strohecker, 1936). 



Relative humidity, absolute humidity, 

 rate of evaporation, and saturation deficit 

 (Baker, 1936) affect each other in various 

 combinations (Hammond and Goslin, 1933; 

 Thomthwaite, 1940) and are affected by 

 wind velocity ( Shelf ord, 1914) and air 

 temperature. 



The weight of snails (Strandine, 1941), 

 behavior of forest floor arthropods (Lunn, 

 1939), behavior of salamanders and wood 

 frogs (Shelford, 1913a), behavior of forest 

 deer mice (Chenoweth, 1917), activity of 

 dendrophagous passalid beetles (Park, 

 1937), and aggregation of terrestrial isopods 

 (Allee, 1926b), for example, are in turn 

 modified or determined by one of these in- 

 fluences, or by several factors operating 

 together. 



These and other diverse effects of the 

 forest climate upon the contained constitu- 

 ents determine eventual selection of habitat 

 niche and, consequently, are important in 

 forest stratification, both vertical and 

 horizontal. 



Wind velocities are greatly reduced with- 

 in the forest when compared to those 

 operating simultaneously external to the 

 forest periphery. During the afoliated 

 period, deciduous forests offer tree trunk 

 and branch interference to air movement. 

 This interference is cumulative and is 

 in direct proportion to the horizontal 

 distance (separation from the ecotone), or 

 the vertical distance (separation from the 

 canopy) through which the wind must act. 

 As the season of foliation develops, there is 

 corresponding increase in the interference, 

 which reaches a plateau of maximum 

 blanketing in early summer. 



Thus there is from 13 to 20 per cent 

 more air in motion on the forest floor in 

 v\anter than in summer (Williams, 1936). 

 and the ratio of reduction of wind velocity 

 to distance from the forest ecotops is 

 similar in afoliated and in foliated forests 



