ECOLOGICAL SUCCESSION OF MOSSES 201 



cans (Web.) Lindb. Above this zone is another in which the 

 foliose lichens are the most conspicuous plants. Grimmia and 

 Orthotrichum have now lost their dominance, being eliminated 

 in the competition with the larger lichens and Hedwigia, which is 

 here the commonest moss. In the third lichen zone, dominated 

 by fruticose forms, especially the large cladonias, mosses are 

 infrequent, being unable to compete with the comparatively 

 tall and wide spreading lichens in the contest for space and light. 

 Rhacomitrium canescens (Timm.) Brid. var. ericoides (Web.) 

 Schimp. sometimes accompanies the cladonias, making the mat 

 more coherent than where it is absent. Hedwigia persists to 

 some extent, and the other two pioneer species occur in areas of 

 incomplete invasion. 



In the Crevice Subsuccession mosses are of little importance, 

 Polytrichum juniperinum Willd. and P. piliferum Schreb. being 

 the commonest species occurring as crevice plants. In the late 

 stages of the subsuccession Thuidium abietinum (L.) B. & S. 

 often appears. In sheltered crevices, particularly upon the steep 

 northwestward facing cliffs, occurs a rather numerous group of 

 mosses, of which the commonest is Swartzia montana (Lamk.) 

 Lindb. Others are Bartramia pomiformis (L.) Hedw., Bryum 

 intermedium, Encalypta ciliata (Hedw.) Hoff., E. procera Bruch., 

 Hylocomium triquetrum (L.) B. & S., Neckera pennata (L.) Hedw., 

 Pogonatum alpinum (L.) Roehl., Polytrichum commune L., Tortula 

 ruralis (L.) Ehrh. Because of the vertically of the surfaces, 

 such habitats remain much the same for a long time, and one 

 frequently finds small cliffs in the fully developed climax forest 

 bearing a moss flora almost identical to that listed above. 



In the Rock Pool Subsuccession mosses are frequently absent, 

 but in certain pools they are the exclusive mat formers. Several 

 depressions were seen in which were growing luxuriant masses of 

 Climacium americanum Brid. In one locality a mat of Polytri- 

 chum commune and Aulacomnium palustre (L.) Schwaegr., one- 

 third of a meter thick, bound together with roots and rhizomes of 

 various plants, was growing out into a rock pool. The area 

 under the control of the moss was about one square meter — 

 one-fifth of the total. Upon the surface of the mat grew cladonias 



