COMMUNITY ORGANIZATION: STRATIFICATION 



479 



and each stratum may be said to have its 

 own microclimate. This being so, each 

 stratum has its own stratal population of 

 resident organisms, in addition to organ- 

 isms that may regularly or irregularly 

 visit the stratum for food, temporary 

 stratal residents— that is, those species hav- 

 ing their principal habitat niche or home 

 within a given stratum, which utilize 

 the stratum for periodic physiological re- 

 cuperation—are often highly adjusted to the 

 structural peculiarities of the stratum, its 

 particular microclimate, or the available 

 food (Chap. 27). The more perfect their 

 structural and functional adjustment, the 

 less able are they to live in other strata. 

 Such organisms are stratal indices. 



Finally, as a consequence of this strati- 

 fication, the forest community has a large 

 intracommunity surface in proportion to its 

 volume, and the active inhabitants of the 

 forest (the animals, as distinct from the 

 more passive inhabitants or plants) have a 

 greatly increased variety of food and a 

 more moderated climate than are to be 

 found in adjacent communities. Let us ex- 

 amine these three general ideas of forest 

 climate, forest stratification, and forest 

 animal life in relation to the whole com- 

 munity and to the several communities 

 which take part in the formation of forest 

 ecotones. 



The chief reason for the more moderated 

 forest community climate is the blanketing 

 effect of its dominant species, the trees. Th6 

 degree of moderation depends upon man-' 

 factors. In north temperate deciduous 

 forests, with well-marked seasonal cycles 

 of vernal foliation and autumnal afoliation, 

 such as an oak-hickory or a beech-sugar 

 maple community, there is less moderation 

 in winter, when vegetation is largely bare of 

 leaves, than in late spring and summer, 

 when the trees are in leaf. 



Evergreen forests are an exception, since 

 their leaves are shed more or less con- 

 tinuallv and insure more or less moderation 

 all the time. One such type is the tropical 

 rain forest, which exhibits some instability 

 in quality, if not in the volume of vege- 

 tation, depending upon the periodic 

 flowering, fniiting, and leafing out of domi- 

 nant trees in relation to numerous in- 

 fluences, often including regional wet and 

 dry periods. 



Another type of evergreen community is 



that of the coniferous forest, where the 

 annual temperature cycle is marked, in 

 sharp contrast to the evergreen rain forest. 

 In coniferous forests the snowfall is heavy 

 in winter, and snow tends to bank upon 

 vegetation, moderating the forest climate 

 (Park, 1931). 



Another factor affecting forest climate is 

 the degree of maturity of the community. 

 This may be a relative maturity in relation 

 to its exact position in the serai sequence 

 (cf. Chap. 29), or it may be an actual 

 maturity in terms of its particular life 

 history. Both concepts are affected by the 

 state of community health. Within recent 

 times such forest hazards as fire (either 

 through physical agency such as lightning, 

 or as a result of man's activity), unnatural 

 flooding (often as a result of modification 

 of the watershed by man), diseases of 

 epidemic proportions (caused chiefly b) 

 bacteria, viruses, and fungi), overgrazing, 

 exposure through clearing for crop planting, 

 and lumbering, retard, change the serai 

 position, or destroy the community. The 

 total result depends upon the toleration of 

 the primary constituents of the community, 

 the trees, and the extent to which floor 

 litter has been removed, with exposure of 

 top soil to leaching and erosion. 



Burning over an experimental pine stand 

 was found to be less detrimental to the 

 animals of the subterranean and floor strata 

 than the complete removal of litter (Pearse. 

 1943), since the latter changed the soil 

 habitat so completely that dependent 

 animals died or decreased sisnificantlv. 



Two or more forest hazards mav operate 

 jointly or consecutively. The once extensive 

 Southern Appalachian spruce forest com- 

 munities have been depleted chiefly as a re- 

 sult of fire following destructive logging. 

 The forest communitv was usuallv wholly 

 destroved (Korstian, 1937), since the trees 

 not cut down were burned out. and the 

 surface soil was ignited bevond recover)' 

 except bv long-time processes. 



In addition to obvious destruction of 

 vegetation after a severe forest fire, there 

 are numerous more subtle reactions. De- 

 struction of floor litter not onlv removes the 

 floor stratum, but exposes the subterranean 

 stratum to ignition and erosion, or in a 

 small conflagration serves to decrease the 

 stability of the soil microclimate. Those 



