248 CLIMAX AND SERE 



black spruce at the northern edge of the transcontinental coniferous 

 forest, do exist. The latter may be two or three times as large as the 

 pine area. Perhaps the total area of black spruce consocies is 1,000,- 

 000 square miles, one-half of which is black spruce forming lace- 

 work, the meshes of which are composed of water and open muskeg. 

 Again the northern and southern large serai areas are only two out 

 of hundreds of types such as those on sand areas, floodplain, rock out- 

 crops, lakes, ponds, and swamps. These are all small, being frag- 

 mented in an almost unlimited manner, so that a continuous area of 

 25 square miles is out of the ordinary and facies may well be only 2.5 

 square miles, while local variations (locies) are small and perhaps a 

 quarter section (160 acres). 



The marine acjuatic communities show various sizes, but near land, 

 1/100 the size of the North American grassland is to be expected, and 

 other groupings in proportion are likely to be the rule. Fresh-water 

 communities rarely are large; as a rule they are either long narrow 

 strips, or are greatly fragmented, or both. They also possess a mini- 

 mum of decomposed vegetation. The communities of small lakes, 

 ponds, pools, swamps, marshes, bayous, and oxbows associated with 

 streams, bays, and other marginal fragments of large lakes, together 

 wdth the smaller ones about arms of the sea, are serai stages to the 

 land climaxes of the region. 



The preceding discussion shows size to be important though largely 

 relative, but on the whole communities as described herein are large. 

 They differ strikingly from the communities covering a fraction of a 

 square meter such as are often discussed by plant sociologists and 

 students of animal aggregations. The development of climax com- 

 munities in small denuded or retarded areas has received attention at 

 the expense of broader features or extensive areas of the biome, which 

 makes it unnecessary to treat the subject in detail in this connection. 



The Dynamic Nature of the Climax. AVhile climaxes may persist 

 and have persisted for thousands or even millions of years, each one 

 is the seat of dynamic processes of varying intensity and extent. The 

 outcome of these is a vast mosaic of great complexity, the under- 

 standing of which can be obtained only by the study of the processes 

 themselves. The motive forces involved are climate, topography and 

 soil, reactions and coactions of all sorts, of which those of man are 

 paramount. Cycles in climate produce a climatic succession (or cli- 

 sere) in which climaxes replace each other in their fixed geographic 

 sequence, a process best exemplified during glacial advance and re- 

 treat. In the course of geological epochs and periods, each climax 

 will leave relict areas of itself in favored situations in the two ad- 



