Notes on Forest Conditions in Northwestern Nebraska 55 



a xerophilous vegetation of which Pinus ponderosa is the most 

 conspicuous species. Large burly trees of Jwniperus virginiana 

 are not infrequent among the pines in some of the canyons. 

 Over hundreds of acres in this part of the state where the Ari- 

 karee is exposed one finds such a characteristic and striking 

 vegetation. 



Among the pioneers on such slopes, at a time when the 

 substratum is of very dry and sterile bare rock with little or 

 no organic matter, are many mat and rosette herbaceous or 

 woody perennials such as Arenaria hookeri, Paronychia jamesii, 

 Phlox douglasii, Homalobus caespitosus, Pseudocymopteris 

 tenuifoUus, Hymenopappus filifolius, and Gilia caespitosa. 

 Frequently a very young slope may show but ione or two in- 

 dividuals of some of the species, 03 on older slopes the species 

 and individuals may be of sufficient abundance to form a 

 rather conspicuous plant association. Still later a number of 

 grasses among which Calamagrostis canadensis, Eriocoma 

 cuspidata, and Stipa comata are most common, come in and 

 add their influence in preparing the soil for future invaders. 

 Yucca glauca is a rather early arrival and soon becomes a 

 prominent member of the accumulating talus flora. Such soil- 

 forming and binding species as these after years of growth 

 and extension convert the former rather unstable slope into a 

 substratum favorable to invasion by less efficient vspecies. 

 Finally pine seeds find lodging, germinate, the seedlings be- 

 come thoroughly established, and the forest cover then begins 

 to take on form. The hardy pioneer species persist for many 

 years among the developing forest trees until eventually on 

 the most thoroughly wooded slopes they are nearly all shaded 

 out, and in such places the only ground cover left is the ac- 

 cumulating layer of pine needles dotted with cones. 



The commonest aspect of this formation is that of the 

 open woodland. The tendency of Pinus ponderosa to develop 

 a park-like formation is well known, and we get here as in the 

 Black Hills and in the foothills of the Rocky Mountains the 

 characteristic open forest of this speices. The trees are of 

 medium size, rarely attaining a height of more than sixty feet. 

 The trunks are stocky with a short clear length and a di- 

 ameter rarely if ever exceeding thirty-six inches. The crown 

 is composed of short stout branches which are frequently 



