PLANT SUCCESSION 223 



of the vegetation becomes more effective. The final changes, after 

 tree dominance, are again very slow. Changes of currents or drain- 

 age in the lake and wind throw or fire on the rock may disrupt 

 either of the trends and result in secondary succession. The result 

 of succession in both habitats is, however, a gradual change in the 

 direction of habitat conditions that are relatively mesic for the 

 climate of the region and a community adapted to such conditions. 



RATE OF SUCCESSION 



If succession is to be recognized as universal and occurring in all 

 habitats, it becomes necessary to ignore time to some extent. A 

 mesic habitat in a given climate will obviously produce a forest 

 much more quickly than a xeric one, especially if the initial habi- 

 tat is bare rock. Yet the potential ultimate communities of the two 

 sites are the same, for all successions in a climatic area progress 

 toward communities of mesophytes. Two habitats of apparently 

 similar characteristics might support the same successional se- 

 quence, but progress of the successions might be at different rates 

 because of the type of soil and the difference in its response to 

 reaction. Or, if seed sources were not equally available to both 

 sites, one might develop more rapidly than another. This could 

 result from an oversupply of seed, producing overstocking of cer- 

 tain species and consequent delay in development of the next stage 

 because of competition; on the other hand, poor seed sources or a 

 series of poor seed years might materially delay the initiation of a 

 community that otherwise could have started. This should make it 

 clear that the rate of succession is extremely variable. Pioneer 

 stages of primary succession are commonly very slow because 

 they can progress only with soil development. An extreme exam- 

 ple is probably that of succession on bare rock, which must wait 

 not only upon soil development but also upon the disintegration 

 of the rock for soil formation. In contrast, the pioneer stages of sec- 

 ondary succession, especially on abandoned fields, are remarkably 

 rapid, for often the dominants change every year for several years. 



STABILIZATION AND CLIMAX 



All successional trends lead toward relative mesophytism within 

 a climatic area. This explains why related successions parallel each 



