Origin of Biomes and Succession 273 



(1959) expressed the opinion that fire is important in estabhshing 

 or perpetuating birch, poplar, jack pine, white pine, red pine, and 

 lodgepole pine in Canada, that much of the forested area in Canada 

 is subject to severe hghtning storms, and that on numerous oc- 

 casions scores or hundreds of Hghtning fires might be burning at 

 the same time. During the summer of 1940, about 18 forest fires 

 were caused by hghtning during a single summer night's electrical 

 storm in Glacier National Park, Montana. Lightning fires have been 

 observed repeatedly in the American short grass prairie (Shantz, 

 1956). It seems highly probable, therefore, that fire has been a 

 constant and possibly the most important factor in providing suit- 

 able areas for the maintenance of many subclimax communities. 



The other factors in this category need little comment. Each may 

 be especially important in a particular region. Landslides, for ex- 

 ample, are common in mountainous terrain, and shifting stream 

 beds with subsequent oxbow and sand bar formation are common 

 in wide, flat river valleys. Drought is frequently most important 

 if associated with high winds, and this combination is most ef- 

 fective in areas of loose soil or low rainfall. Weaver (1954) (Fig. 

 121) cited many cases in which this combination has destroyed 

 large areas of the dominant Andropogon grasses in the tall grass 

 prairies of central United States and describes the subclimax com- 

 munities which became established in their place. Wind alone is 

 especially effective in forested areas of predominantly shallow- 

 rooted species, but in the violent form of tornadoes or hurricanes 

 it will level vegetation of any type. 



Volcanoes produce a variety of effects. Their extrusions on snow 

 banks may produce mud flows which denude large acreages just 

 as a flood will, as has happened in Mt. Lassen National Park, Cali- 

 fornia. The ash and lava flows kill vegetation and produce bare 

 areas which may not be colonized until they are considerably 

 weathered. The meager biota of the lava hills of the Pinacate lava 

 cap in northwestern Mexico attests to this. This lava cap originated 

 sometime in the Pleistocene and, although it has in patches weath- 

 ered to black soil, it is inhabited by only a small proportion of the 

 flora and fauna found in the neighboring desert (Dice and Blos- 

 som, 1937; Smith and Hensley, 1958). 



The sum total of these short-term events accounts for the con- 

 tinuity through time of a series of different areas suited for the 

 establishment of new local communities of early, short-lived stages 

 of succession. These mechanisms account for the occurrence of a 

 large number of subclimax communities. 



