COMMUNITY ORGANIZATION: STRATIFICATION 



477 



mixed grass ecotones, short grass-mixed 

 grass ecotones, and many more. 



By using ecotone as a term covering a 

 more or less sharply defined competitive 

 zone between two self-supporting com- 

 munities, the concept becomes reasonably 

 concise. As a synonym of biome intergrada- 

 tion, it loses its usefulness, since it could be 

 argued that we have a single area of inter- 

 gradation from pole to equator. 



When a community is destroyed, its eco- 

 tones are destroyed unless the ecotone re- 

 mains behind, adjusted to the modified 

 area once occupied by the whole com- 

 munity and developing a mixed growth 

 that persists as a self-supporting but more 

 pioneer community with much the same 

 boundaries as the one destroyed of which 

 it once formed the periphery. From 

 this it will be seen that the ecotone varies 

 in width, degree of sharpness, and stabihty. 

 It will be remembered that Weaver (1943) 

 observed the destruction of the tall grass 

 prairie and the occupation of the area by 

 mixed-grass prairie, involving many com- 

 munities and their boundaries, as a case 

 in point. Ecotones are naturally interme- 

 diate between the communities concerned 

 in their physical environment and biota. 

 Since this is essentially a zone of competi- 

 tion between communities, its instability is 

 furthermore a function of the biotic poten- 

 tials of the communities concerned; conse- 

 quently the stratification of ecotones is in- 

 termediate. 



The ecological reality of the ecotone is 

 attested by the fact that, in addition to or- 

 ganisms penetrating this boundary area 

 from both communities involved and living 

 therein for all or a regular part of their 

 fives, there are other organisms that find 

 the biotic and physical environment of the 

 ecotone more stimulating than the condi- 

 tions prevailing in either community. Such 

 organisms may be properly called ecotone 

 constituents. 



In a neglected paper, Cameron (1917), 

 discussing the ecological relations of insects 

 in English pastures near Cheshire, discusses 

 interpenetration very well: 



"Often the line of demarcation between one 

 association and another can be strictly de- 

 limited, so that species which may be intro- 

 duced into an association which is alien to 

 their habits react negatively to the prevailing 

 physical factors of their new abode and tend to 



turn back. On the other hand, where two dis- 

 tinct associations are in juxtaposition and 

 gradually merge into one another, as in the 

 case of woodland and grassland, many species 

 from both will intermingle, especially so at the 

 places of transition." 



In the parkland of central North Amer- 

 ica the Great Plains merge with the decid- 

 uous forest on the east and with decid- 

 uous forest (poplars) and coniferous 

 forest on the north. In this area of merging, 

 a narrow belt of country, in which both 

 forest and prairie intermingle, forms a 

 characteristic zone of intergradation. This 

 intergrading region was characterized by 

 Bird (1930) as having the forests stretch- 

 ing far into grassland along rivers, and by 

 groves of trees with interspersed prairie. 

 Within this grassland-deciduous forest and 

 grassland-coniferous forest border are many 

 self-supporting forest and grassland com- 

 munities and, consequently, numerous 

 grassland-deciduous forest and grassland- 

 coniferous forest ecotones. Griggs (1934) 

 studied forest edge in Alaska. Hanson 

 (1938), Pool, Weaver, and Jean (1918), 

 and Weaver and Himmel (1931) have 

 given a detailed picture of plant composi- 

 tion in prairie inclusions within the decid- 

 uous forest and the prairie forest margin. 



The subterranean stratum of the forest 

 edge in the shelterbelt zone has also been 

 examined. As wdth grassroots in the sub- 

 terranean stratum of the grassland com- 

 munity, the trees in the grassland-forest 

 ecotone have their root systems vertically 

 stratified. There are the deep-rooted trees 

 such as bur oak, western yellow pine, 

 and hackberry with roots 10 to 20 feet 

 deep; an intermediate layer of root systems 

 of green ash, American elm, and red cedar 

 at 5 to 10 feet below the surface, and finally 

 a shallow-rooted layer, including Jack 

 pine, Scotch pine, willow, and cottonwood 

 with root systems extending from 1 to 5 

 feet in depth. The subterranean stratum 

 of the forest-grassland edge is deeper and 

 more secondarily laminated than that of 

 grassland; it is less so than in the forest 

 proper. 



Both the physical environment and the 

 biota of ecotones need much study. Not 

 only is there an intermediate but developed 

 vertical stratification, but horizontal zona- 

 tion is especially clear. There is an easily 

 demonstrated horizontal gradient from 



