SUCCESSION 



273 



tion just described, is nicely illustrated by the balsam-spruce formation at 

 Minnehaha. Of the initial gravel slide stage, the relicts are Vagnera stellata 

 and Galium boreal e, the one modified into Vagnera leptopetala, and the 

 other into G. boreale hylocolum. The thicket stage is represented by Holo- 

 discus dumosa, greatly changed in form and branching, and in the shape 

 and structure of the leaf. The most striking relict of the aspen formation 

 is the facies itself, Populus tremtiloides. The tall slender trunks of dead 

 aspens are found in practically every balsam-spruce forest. In many places, 

 living trees are still found, with small, straggling crowns, which are vainly 

 trying to outgrow the surrounding conifers. Of the aspen undergrowth. 



r.^/-<l 



Fig. 71. Relict spruces and aspens, showing the character of the suc- 

 cession immediately preceding the burn succession now developing. 



Rosa soyii, Helianthella parryi, Frasera speciosa, Zygadenus elegans, Cas- 

 tillcia confiisa, Gentiana acuta, and Solidago orophila remain more or less 

 modified by the diffuse light. It is still a question whether the aspen stage 

 passes directly into the balsam-spruce forest, or whether a pine forest inter- 

 venes. The presence of both Pinus ponderosa and P. flexilis, which are 

 scattered more or less uniformly through the formation, furnishes strong 

 evidence for the latter view. 



The lifetime of forest and thicket stages of successions is ascertained by 

 counting the annual rings of the stumps of facies. This is a perfectly feasible 



