Ecotone 



411 



(Fig. 11.4). In this transition zone of tension the outposts of each 

 community are maintaining themselves in environments that are in- 

 creasingly unfavorable. The tension may result chiefly from a strug- 

 gle with physical conditions or from a direct competition between 

 certain species in each commimity. At the border between a shrub 

 community and a marsh, for instance, the shrubs may compete 

 directly with marsh reeds for light, nutrients, or other necessities of 



li(,. 11.4, A distinct fct)tone dominated by smooth sumac (Rhus glabra) and 

 goldenrod ( Solidago ) between forest and grass communities in eastern Massachu- 

 setts. 



life in such a way that one type of plant gives way completely to the 

 other. In such a situation and in areas where controlling physical 

 factors change rapidly the transition between communities is abrupt 

 and the ecotone is correspondingly narrow. In other circumstances 

 the two communities may interdigitate to a considerable extent. At 

 the edge of a forest individual trees may pioneer into a scrub com- 

 munity, and the scrub species will invade the margin of the forest as 

 far as they are able to survive. 



When one community gives way only gradually to the other com- 

 munity, a wide ecotone results. The transition zones— sometimes 

 more than 100 km in width— between major continental communities 

 are regarded by some as ecotones (Pitelka, 1941), but others restrict 

 the term to areas of smaller scale. Strictly speaking a transition 



