ECOLOGY. 353 



by grazing and is controlled only secondarily by climate. This is 

 similarly true of the great mesquite savannahs which occur from cen- 

 tral Texas to western Arizona. While their origin is possible in the 

 first place because of wet phases of the climatic cycle, their spread and 

 persistence is due to overgrazing. 



Natural Parks, by F. E. Clements and E. S. Clements. 



During the summers of 1909 and 1910, a comprehensive survey was 

 made of the natural parks of Colorado, with especial reference to their 

 causes and origin. This problem has again been taken up in the last 

 few years in other mountain regions. These results agree completely 

 with those obtained earlier, and the findings are now being brought to- 

 gether. For the most part, natural parks are grassland areas of a half- 

 mile to several miles in extent, surrounded by forest or woodland. 

 Sometimes this term is also applied to savannah, in which the trees are 

 merely scattered in the grassland. The grassland concerned is usually 

 subclimax meadow, found in the montane and subalpine zones, though 

 it may be an association of the grassland formation, especially in the 

 case of yellow pine or lodge-pole pine parks. The best parks studied 

 are those formed by Engelmann spruce and alpine fir, and by lodge- 

 pole pine, though yellow pine is sometimes a good second. All trees 

 at the upper or lower tree limit, such as foxtail pine, pinyon, aspen, etc., 

 may form parks, but these are less common and less characteristic. 

 Moreover, while all natural parks in the usual sense are carpeted with 

 grass, it is difficult to distinguish these from parks containing sagebrush 

 or scrub. 



All natural parks are consequences of succession. The forest mass or 

 edge is the climax, or sometimes the subclimax, as in the case of lodge- 

 pole pine. The grass center, or the scrub center, is the stage preceding 

 as a rule. Probably the greatest number of natural parks are due to 

 fire, a secondary succession developing on the burned area and persist- 

 ing for a long time in the subclimax or grass stage. This grass cover 

 may persist for several hundred years or longer, largely owing to the 

 advantage the grasses have in competition with the tree seedlings. 

 The fine parks in the subalpine forest of the Uncompahgre Plateau in 

 southwestern Colorado and those of lodge-pole pine in the Big Horn 

 Mountains of Wyoming are burn parks. A very large number of parks 

 also arise as a consequence of primary succession in lakes and ponds. 

 These are very frequent, but usually of much smaller extent. A great 

 many of them remain for a long time in the sedge-swamp stage and 

 hence do not appear to be parks at all. The most striking parks of this 

 type are those which have given their name to three great intermoun- 

 tain regions in Colorado, namely. South Park, Middle Park, and 

 North Park. These are so extensive that all stages of primary suc- 

 cession occur in them, though by far the greatest area is covered with 



