Hill: Penobscot Vegetation 339 



Trees 

 Picea rubra Betula alba var. papyrifera 



Coniferous forest association-type 



As indicated above, isolated trees may be present on the 

 heath-mat and rarely in crevices. As the depth of the soil 

 and its water-holding capacity increase these trees spread 

 gradually, often reproducing by layering, until gradually a 

 coniferous forest develops. This does not occur uniformly, 

 owing to local differences in physiography, so that even in a 

 fullv established forest there may be areas in which succes- 

 sion may not have proceeded beyond the heath-mat or even 

 the rock surface stage. 



This first coniferous forest to appear naturally grows in 

 the rockiest places, where there is comparatively little soil 

 present, only the thin layer of duff derived from the old 

 heath-mat and the decaying needles. Often this layer can be 

 lifted like a blanket from the surface of the rock. The for- 

 est is almost completely dominated by the red spruce (Fig- 

 ure 7), with a little white spruce and fir balsam, especially 

 near the edges or bordering on open places. The trees grow 

 very closely together with their branches interlacing, and 

 the ground beneath them is nearly devoid of vegetation 

 (Figure 8). This latter feature is characteristic of young 

 coniferous woods elsewhere and has been attributed to vari- 

 ous factors, such as insufficient light, dryness of the soil ow- 

 ing to the interception of moisture by the crowns, or the 

 burial of the seeds under the covering of dead needles. Not 

 only is there no herbaceous or shrubby vegetation, but the 

 lower limbs of the trees themselves are dead or dying and us- 

 ually festooned with the beard moss Usnea barbata. The 

 trees of the stand are in general even-aged and the only 

 young growth to be noted are occasional spindling spruces, 

 which have germinated in the mineral soil, but have been 

 suppressed by the dense shade. This type of forest is es- 



