1920] TAYLOR—SUCCESSION OF MOSSES 465 



out the mosses, until the latter are found only on decayed sticks 

 or bases of trees. A few relics from the pine association occur 

 here and there. On some slopes and in ravines where herbaceous 

 forms have not taken full possession, mosses are more common. 

 As previously mentioned, these are somewhat xerophytic species 

 which appeared only rarely in the earlier succession, together with 

 some relics from the former association. It is possible that the 

 roots of the herbaceous plants, because of the need for moisture, 

 rob the surface soil of its water and thus make it more difficult for 

 mosses to secure a sufficient supply. Competition, therefore, can 

 be said to be the great limiting factor on the more mesophytic 

 slopes; while low humidity and high evaporation seem to be 

 more important on those facing the south, where neither mosses 

 nor herbaceous plants are very abundant. Sand laden winds are 

 not of much importance unless the area is near a rejuvenating 

 dune. In the older stages of the oak succession the forest becomes 

 more mesophytic. There is less evaporation and higher humidity, 

 with entire lack of covering by sand. Humus has now accumulated 

 to a degree necessary for the growth of many more species of seed 

 plants. Apparently these have become so successful as to cause 

 almost total elimination of the mosses, which have contributed 

 to their own extinction by adding to the humus content. Only in 

 exposed paths or roads, on decaying logs, or sometimes on tree 

 bases, do the mosses continue to exist at all. Old logs are rare in 

 these woods, and only bases of old trees are favorable habitats, so 

 that in the advanced oak association in this region the moss flora is 

 often almost confined to a few species which spring up in paths or 

 tracks left by the feet of animals. 



We may summarize the causal factors for presence or absence 

 of mosses in the dune succession as follows. Mosses are excluded 

 from the flora of the beach and foredune by great exposure to 

 desiccation and to covering by sand. Xerophytic species may 

 appear on the cottonwood dune, but are prevented from becoming 

 conspicuous by these same two factors. Mosses suddenly become 

 abundant in the pine dunes, ' their growth being favored by high 

 humidity and low evaporation during spring and fall, a result 

 largely of the shade cast by the pines and juniper. Competition 



