1920] TAYLOR—SUCCESSION OF MOSSES 463 



In the pine dune there is a much greater difference in the effect 

 of the first two factors, moving sand and evaporation. It is here 

 and in the mesophytic transition regions that the third factor 

 enters into the causal conditions. According to the results of the 

 evaporation work done by Fuller, the pine dune shows the lowest 

 evaporation rate to be found among the tree associations of the 

 dune series, other than the climax forest. It is still more significant 

 that the rate is lower during the early summer and late fall, 

 the most vital part of the season for mosses. The debris upon 

 the ground aids in the absorption of moisture during rains. The 

 moisture as it slowly escapes from the soil is confined near the 

 surface by the close canopy of the juniper, and also by the dense 

 overhead covering of pines. All of this leads to a high degree of 

 humidity during spring and autumn, the seasons of greatest rain- 

 fall, not found elsewhere in the dune associations. In midsummer 

 evaporation may surpass that of the oak dune (fig. 2), but the 

 mosses by that time have passed their period of vegetative growth, 

 and in many cases the production of sporophytes also. The matur- 

 ing of sporophytes in other species, such as Thuidium, is carried on 

 late in the season when humidity again rises. The fact that we 

 find T. delicatulum as the dominant species under the juniper 

 indicates decidedly mesophytic conditions, for except as a relic 

 this species usually occurs only in moist habitats. Another reason 

 for its dominance seems to be its ability to endure shade. Either 

 there is no competition with other plants under the juniper or 

 such plants have been crowded out, while Thuidium thrives best 

 when well shaded. Other plants become competitive beyond the 

 juniper where herbaceous vegetation, including several typically 

 northern species, becomes more frequent. Thuidium less often 

 covers extensive areas, and seed plants may even be found germinat- 

 ing on the mosses. In places more favored by light the mosses are 

 likely to lose out altogether or be forced to take refuge on sticks 

 or bases of trees. Another factor which seems worthy of consider- 

 ation is that Thuidium grows directly on the slightly decayed 

 needles of the conifers. These probably produce a chemical 

 condition of the soil which effectively eliminates many other 

 plants. While the pines also shed their needles, there is much 



