FOREST COMMUNITIES 441 



rat, among rodents. In the main the supply is decreasing, although a 

 closed season and protection of females and young are preventing 

 extermination. 



Just as in the tropics, these forests spread into the temperate grass- 

 lands along river courses, and these gallery forests, or parklands, are 

 particularly rich in animal life. The outer edge of such forests ad- 

 vances or retreats as climatic or biotic conditions favor or retard the 

 growth of trees. Animals influence such changes more than is generally 

 appreciated. Thus in England forest margins advance where rabbits 

 are carefully excluded while neighboring unprotected regions are sta- 

 tionary, and in Manitoba in the poplar parkland region, after a year 

 of unusual rabbit abundance, the majority of the small trees along the 

 woodland margin are killed by rabbits. 55 



At the climatic limits of forests, whether in the far north or in 

 mountains, the trees become dwarfed to mere bushy thickets, which 

 may, however, extend for miles. In polar regions such forests consist 

 of scrub spruce, dwarfed birches, willows, and alders, no higher than 

 a man, and frequently so light in stand that one can readily make 

 his way through. Here arboreal creatures are absent; even the dis- 

 tinctively arboreal birds such as the woodpeckers are no longer found. 

 Such woody growths, however, serve as nesting sites for a large bird 

 population; the birch siskin is as characteristic of the dwarf birch 

 forests of Iceland as is the crossbill of the conifers. 56 



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