382 Proceedings Portland Society Natural History 



Association-types where humus is absolutely destroyed 



In the case of fires where the soil layer is thin, as in the 

 xerophytic coniferous forests on the islands and hill-tops, 

 and after extremely severe fires in deciduous woodland, the 

 humus covering may be entirely destroyed and the bare rock 

 surface exposed. Where this occurs on steep slopes subse- 

 quent rains may wash away every vestige of the old soil and 

 particles of mineral matter as well, rendering it practically 

 impossible for vegetation to get a new start. On more level 

 surfaces a series of successions develop not unlike those al- 

 ready outlined under primary successions. There may not 

 be the same regularity in the process, for here and there 

 over an area some soil may have persisted and this will natu- 

 rally affect the nature of the succession. 



One departure from the usual course of events following 

 a fire is of frequent enough occurrence to warrant mention. 

 If either white or red pines are present nearby they soon in- 

 vade a burn and come to play an important part in its fu- 

 ture development (Figure 28). They may replace the 

 birch-popple stage from the first, but usually accompany 

 these trees for a while, eventually outstripping them and be- 

 coming dominant. Very often red oak is associated with 

 the pine in such cases. Where the soil is very shallow and 

 conditions extremely xerophytic a scrub forest of pine and 

 oak may result (Figure 29) which is seemingly permanent 

 and may constitute a physiographic climax. Occasionally 

 the oak occurs alone in such localities (Figure 30). On 

 better soils the trees develop into good sized specimens and a 

 growth of spruce and fir comes in beneath them. In such 

 situations these conifers are destined to supersede the pine 

 and oak as dominant species, although the latter do not 

 drop out entirely but remain as scattered individuals. Oc- 

 casionally the coniferous forest may be followed by one of 

 the climax type in which the pine and oak are still present. 



