INVASION AND SUCCESSION 



279 



tent. Such soils usually contain considerable organic matter 

 and a large number of dormant seeds. The successions upon 

 them consist of relatively few stages, and are usually mesophytic 

 in character. Secondary successions may arise in eroded, flooded, 

 or drained soils; they may be caused by subsidence or landslides, 

 or by animals or man. 



299. Succession in eroded soils. Habitats are ordinarily 

 eroded by the action of water. Sands and gravels are readily 

 worn away, owing to their lack of cohesion, while loam and clay 

 are easily eroded only on slopes. In the former the extreme 

 porosity and slight capillarity of the soil result in a low water 

 content. In the finer soils the water content is also low, on 

 account of the excessive run-off. The action of wind in eroding 

 soils which bear vegetation is not general. It is found to some 



Fig. 114. Secondary succession in a Hooded area. The plants of the alpine 

 meadow have been drowned out, and the soil entirely occupied by one 

 species, Bistorta bistortoiiles. 



extent in dunes, and is frequent in sandhills, where it produces 

 blowouts. 



The early stages of successions in erodetl soils are usually com- 

 posed of xerophytes. In loose soils these are forms capable of bind- 

 ing the soil particles together, thus preventing wash and increasing 

 the accumulation of fine particles, especially of organic matter. 

 In compact soils the pioneers not only decrease erosion, but also 

 increase the water content by retarding run-off. 



