56 INITIAL CAUSES. 



Bare areas due to drouth. — The action of drouth in destroying vegetation 

 and producing areas for colonization is largely confined to semiarid and arid 

 regions. In humid regions it is neither frequent nor critical, while in desert 

 regions it is the climax condition to which vegetation has adapted itself fully 

 or nearly so. The usual effect is to produce a change in existing vegetation, 

 but in regions like the Great Plains it sometimes destroys vegetation completely. 

 As a rule, the destruction operates upon cultivated fields, simply freeing the 

 area somewhat earlier for the usual development of a ruderal stage. It also 

 occurs occasionally in tree plantations, with somewhat similar results. In 

 native vegetation the complete destruction of a community is rare. When it 

 does occur it is nearly always in lowland communities which have followed 

 streams far beyond their climatic region. Ruderal and subruderal commu- 

 nities which pioneer in disturbed soils are the most frequent sufferers. In 

 desert regions, which are characterized by communities of summer and winter 

 annuals, the destruction of the latter by drouth before the vegetative season 

 is over must occur occasionally. It has no significance for succession, however, 

 as it is wholly periodic. 



Bare areas due to wind. — The direct action of the wind upon vegetation is 

 seen only in so-called "wind-throws" in forests. While areas in which trees 

 have been blown down by the wind are frequent in some regions, they are 

 local and of small extent. They are most apt to occur in pure stands of such 

 trees as balsam, spruce, and lodgepole pine. "Wind-throws" are frequent in 

 mountain regions where the soil is moist and shallow. The action of the wind 

 affects only the tree layer, in addition to tearing up the soil as a consequence 

 of uprooting the trees. It is supplemented by evaporation, which destroys 

 the shade species by augmenting their transpiration greatly at a time when the 

 holard is being constantly diminished by the drying out of the soil. As a 

 consequence, "wind-throws" often become completely denuded of vegetation. 

 In the case of completely closed forests, such as mature forests of the lodgepole 

 pine {Pinus murrayana) and Engelmann spruce (Picea engelmannii) , the fall 

 of the trees amounts to denudation, since occasional saprophytes are often 

 the only flowering plants left (plate 9 c). 



Bare areas due to snow, hail, and frost. — ^Bare areas due largely to snow 

 are restricted to alpine and polar regions, where they occur usually in a zone 

 between the area always covered with snow and that in which the snow dis- 

 appears each summer. An abnormal fall or unusual drifting will cause the 

 snow to remain in places regularly exposed each summer. After a winter of 

 less precipitation or a summer of greater heat, the drifts or fields will melt, 

 leaving a bare area for invasion. This frequently happens in the denser por- 

 tions of conifer forests, as well as in and around the outposts above the timber- 

 line. In such cases the resulting development has to do chiefly with the 

 undergrowth. 



The effect of frost in producing bare areas by destroying the plant popula- 

 tion is almost negligible. Its action is confined almost wholly to cultivated 

 areas, such as orchards, fields, and gardens. In such places only the first 

 pioneers of a ruderal population can appear, except m rare cases where the 

 area is abandoned because of the frost. Communities of ruderal annuals are 

 sometimes destroyed by frost, but this delays the usual course of succession 

 for but a year at most. Native vegetation may be changed by the action of 



