RELATIONS AMONG SPECIES 



363 



lichen 



moss herb shrub tree 



Figure 19.8 The typical stoges of xerarch succession. 



climax 



climax of the area might be grassland, but the stream- 

 side vegetation could be serai and a woodland. The 

 climax, then, would come when the streamside wood- 

 land was transformed to a grassland. This complica- 

 tion of "normal" succession would take place because 

 xerarch succession is modified by the presence of ex- 

 cess water. Fifth, stages in addition to those that 

 might be considered typical can occur. For example, 

 a spike moss (Selaginetla) stage commonly follows 

 a moss stage in central coastal California. Finally, 

 certain species found in late serai stages may remain 

 in the climax. For example, a serai shrub may enter 

 the shrub layer in a woodland climax. 



There are two main reasons why serai stages tend 

 to proceed in an orderly change of vegetation forms. 

 First, the sequences in serai stages are in direct re- 

 lationship to plants' needing a better environment. 

 By "better," it is meant that later stages require a 

 more fully developed soil, more moisture, a reduction 

 in temperature extremes, and a generally more stable 

 environment. Second, an existing serai stage creates 

 the better environment necessary for the next stage. 

 This phenomenon might actually start prior to the 

 invasion of a rock by the pioneer lichen stage. The 

 various physical factors of erosion may have to modify 

 a rock to some extent before lichens can invade. The 

 lichens further erode the rock, primarily by acid 

 production, and many of the eroded rock particles 

 are held by the lichens. This accumulation of rock 

 particles is the start of soil formation. After suf- 

 ficient particles accumulate, the proper conditions of 

 moisture and other factors exist for the invasion of 

 mosses. The mosses further the process of soil for- 

 mation and general modification of their own en- 



vironment until herbs can invade and replace them. 

 Further serai stages continue this process of 

 creating conditions for the next stage to invade. 

 Finally, a serai stage gives rise to the climax, the 

 stage featuring dynamic equilibrium. 



HYDRARCH SUCCESSION 



Succession in water tends to follow a set sequence 

 in standing waters. This sequence was diagramed 

 in f^igure 17.13 (see p. 320). The primary feature in 

 this series of changes is alteration of an aquatic en- 

 vironment to a land one by the filling in of a body of 

 water. This major tendency progresses in a gradient 

 from the water to the land. Recall that the central and 

 deepest part of the water may have a bottom free of 

 vegetation. In suitable adjacent, shallower depths the 

 primary serai stage is found. This submerged stage 

 has plants with thin, dissected, and/or linear leaves, 

 and these underwater plants are at depths that are 

 mostly determined by available light. Closer to 

 the shore are plants whose leaves float upon the water 

 surface. This floating stage generally excludes fur- 

 ther shoreward occurrence of the submerged stage 

 by creating shade; however, floaters are attached to 

 the bottom and so are excluded from the deeper 

 waters inhabited by the submergents. The next serai 

 stage, the emergent stage, is limited to shallower 

 waters where such plants as cattails, sedges, rushes, 

 and reeds crowd out the floaters. Emergents usually 

 grow very close to one another, and for this reason 

 they regularly hold a great deal of sediments. The 

 sediments, in the form of silt and remains of dead 

 emergents, accumulate to the point where an herb 



