COMMUNITY ORGANIZATION: STRATIFICATION 



481 



(Figure 159), although actual reduction is 

 greater in the foUated period. 



Botanists have said much about the 

 mechanical eflFects of wind on tree form and 

 structure and upon the role of wind in 

 transpiration. The influence of wind upon 

 dispersal of tree species is an important 

 ecological factor, and the particular eflfect 



or OECBEASE 



Fig. 159. Horizontal gradient in wind ve- 

 locity from peripheral ecotone toward the 

 center of a forest community, in both the period 

 of afoliation and the period of foliation. (After 

 Williams. ) 



of air in motion upon local distribution and 

 establishment of trees has received less at- 

 tention. Forests tend to fade out where 

 high mountains begin to form isolated 

 peaks; above this point tree growth is found 

 only where there is local protection from 

 wind (Belyea, 1925). 



The points enumerated are relevant to 

 the consideration of stratification in the 

 community. Given the initial establishment 

 of the primary constituents, in this case the 

 dominant species of trees, a windbreak is 

 formed at the periphery. Such a primary 

 response forces the deposition of externally 

 wind-dispersed seeds at or near the pe- 

 riphery. This deposition produces an 

 ecotone rich in such plants, whereas bird- 

 dispersed seeds tend to be deposited 

 throughout the community. Further, any 

 original forest plants that are inherently 

 wind-dispersed are insured against exces- 

 sive dissemination and loss, since their seeds 



tend to drop to the floor in the relatively 

 still forest interior. 



The periphery, once stabiUzed, presents 

 a relatively brightly illuminated surface. In 

 the ensuing photosyndretic competition, the 

 ecotonal zone and forest canopy may be- 

 come densely covered with Uanas, and the 

 former tends to support numerous herbs 

 and slirubs. 



The interaction of these and other 

 physical and ecological influences produces 

 (1) a typically rich ecotone; (2) in part 

 an initiation of horizontal and vertical 

 stratifications; (3) a reenforcement of 

 horizontal and vertical stratification; (4) in- 

 crease in the kinds and amounts of foods 

 available for animals; (5) increase in the 

 kinds and amounts of shelter niches avail- 

 able for animals; and (6) a more stable, 

 moderated community climate. 



To sum up, the forest community in gen- 

 eral tends to have a relatively dark, cool, 

 moist climate with the internal strata pro- 

 tected from the full force of wind. Since 

 protoplasm has a high water content and 

 since water is vital to its continued exist- 

 ence, the loss of water by evaporation 

 from the surface of terrestrial animals 

 is physiologically uneconomical and bio- 

 logically to be avoided. 



The forest community, by its very nature, 

 solves the problem of water conservation for 

 a host of animals and plants, making un- 

 necessary for them the specific adaptations 

 to economy of water that may be essential 

 in other terrestrial environments. For these 

 and other reasons the forest community as 

 a cooperative whole is a highly evolved 

 and successful unit. 



Turning our attention from the over-all 

 interplay between cfimate and community 

 to the application of principles of strati- 

 fication to forest community organization, 

 we find that these communities typically 

 have six basic vertical lamiations: (1) sub- 

 terranean, (2) floor, (3) herbaceous, (4) 

 shrub, (5) tree, and (6) a layer of air 

 above the canopy. All six are present in 

 most forests. All are interdependent; each 

 has a particular microclimate and a par- 

 ticular biota, and is subject to, and responds 

 characteristically to, extraforest and intra- 

 forest physical and biological forces. 



The biota of the sixth stratum may difi^er 

 considerably from air strata in general, but 

 httle is known of this layer. As early as 



