Hill: Penobscot Vegetation 34?? 



b. Association-types of sedimentary rocks 

 The succession of vegetation described above, while par- 

 ticularly characteristic of granite, applies also to other ig- 

 neous rocks. The succession on schists and other sedimen- 

 tary rocks, however, differs in certain respects. 



On a schist the crustose lichens are usually different spe- 

 cies from those occurring on granite, such forms as Rhizo- 

 carpon Oederi, Lecidia platycarpa and Leeanora cinerea 

 being conspicuous. The foliose and fruticose lichens and 

 most of the herbaceous crevice plants, on the other hand, 

 are those characteristic of the igneous rocks. More varia- 

 tions appear, however, among the wood} 7 forms. Associ- 

 ated with such ever present species as the huckleberry (Gay- 

 lussacia baccata), the sheep laurel (Kalmia angustifolia), 

 red spruce and Pyrus mclanocarpa, in the crevices and on 

 the heath-mat, are the broom crowberry (Corema Con- 

 radii), gray birch (Betula popidifolia) and several species 

 of pine. Of these latter the white pine (Figure 10) is the 

 most frequent, but the red pine (Pinus resinosa) and the 

 pitch pine (Pinus rigida ) are not uncommon. The Corema 

 is particularly striking in such localities (Figure 11), oc- 

 curring in dense cushion-like patches scattered over the sur- 

 face of the mat. These clumps originate in a crevice and 

 spread slowly, sometimes attaining a diameter of several 

 yards, but they never occupy the whole area of the mat. 

 Gaylussacia has a somewhat similar tendency to form 

 clumps, while Kalmia, Pyrus, and the trees occur as scat- 

 tered individuals. Reproduction of the pines is abundant 

 and in course of time the heath-mat is replaced by an open 

 forest, the most xerophytic in the region. 



This forest may be an almost pure stand of pitch pine, 

 particularly where it occurs in the more exposed situations, 

 as on Isle au Haut, or it may comprise a mixed growth of 

 pitch pine and red pine. More often still these two species 

 are associated with the white pine and also occasional speci- 

 mens of the red spruce, red oak, and gray birch (Figure 



