210 CLIMAX FORMATIONS OF NORTH AMERICA. 



shrubs succeed them, the most important being Juniperus horizontalis, J. 

 communis var. depressa, and Ardostaphylus uva-ursi. These spread over the 

 rocks from the crevices in all directions, and weaving among the cladonias 

 and plants of the rock pools bring about the formation of a firm mat. The 

 crevice vegetation is of extreme importance because of its rapidity of develop- 

 ment and its preeminent part in the formation of the heath mat. Forest 

 establishment is accomplished much sooner where crevices are abundant in the 

 rock than where they are scarce. 



"The rock pool subsiuxession goes through its development in depressions 

 where water stands at least a part of the time. These become gradually 

 filled after the manner of the bog succession, and the vegetation later becomes 

 an element in the formation of the heath mat. 



"The heath mat results from the coalescence of the vegetation developed 

 through the instrumentality of the three subsuccessions named above. 



"The climax forest often follows immediately after the formation of the 

 heath mat, the trees obtaining their first foothold in the crevices. A relatively 

 xerophytic forest stage characterized by Pinus banksiana and Picea mariana 

 sometimes intervenes, in which Pinus is the pioneer and Picea remains for 

 some time after the climax trees have attained dominance. 



"Telescopiag of stages is pronounced throughout the series, so that pioneer 

 and climax forms, with those of all intermediate stages, are frequently found 

 occupying a single limited area. 



"The effect of special conditions upon the rock shore succession is expressed 

 in the two following laws: 



"1. The lower limit of possible forest extension is determined approximately 

 by the upper limit of effective wave and ice work, the lake level remaining 

 constant. 



"2. The extent to which the forestable territory has been occupied at the 

 present day depends upon the rapidity of invasion, which is governed by the 

 character of the rock, the angle of slope, and the degree of exposure to winds. 



"The operation of these laws results in the production of three phases of 

 rock shore vegetation, characterized respectively by {A) climax forest to the 

 water's edge, (B) a zone of incomplete invasion, (C) abrupt transition from 

 bare rock shore to climax forest. 



"During the early subaerial history of Isle Royale the rock shore succession 

 may have differed from that of the present day. A study of Gull Islands indi- 

 cates that birds may have been important agents in determining the composi- 

 tion of the primitive rock shore flora of the island. 



"The beaches on account of their sheltered location usually bear the climax 

 forest down to its hmit of possible extension. Low shrubs of various kinds 

 are the most important pioneers in the beach succession, and larger ones, 

 especially Alnus crispa, intervene before the establishment of the climax 

 type." 



Nichols (1914 : 169) has described the serai development on the uplands of 

 Connecticut: 



The pioneer stage of crustose lichens consists of Buellia petraea and Lecanora 

 cinerea, followed by Physda tribacea. The chief dominant of the f oliose stage is 

 Parmelia conspersa, but Dermatocarpon, Umbilicaria and Parmelia caperata 

 are sometimes equally abundant, and the fruticose Stereocaulon not infre- 

 quently follows the crustose Hchens. The mosses of this stage are Grimmia 

 olneyi and Hedwigia ciliaia. In the rock crevices Cladonia, Polytrichum, etc., 

 are the pioneers. Ferns and perennial herbs soon follow these. As the soil 



