19 



Big Hole distribution: The Big Hole Study Area is in the center of the known distribution of 

 this species in Montana, represented by 18 occurrences which are all on BLM lands, including 

 those 14 new occurrences documented during the course of this study (Figure 8). 



HABITAT: In the center of its range, it occupies stony crests, screes, and talus in montane and 

 subalpine settings and is strongly or obligately calciphile. However, in the north disjunct areas of its 

 global distribution, it occupies gravelly ridgetops and barren ridges in the sagebrush belt (Bameby 

 1989). 



The elevation range of the species in Montana extends from ca. 4600 to 6800 ft., from the edge of the 

 Tobacco Root Mountains along the lower Jefferson River valley to the open montane slopes of the 

 Higliland Mountains. The settings are most often on ridgetops, but the species may also be found over 

 the full range of slope positions, often appearing as an outlier of hilltop population centers. The 

 substrates are all calcareous but are not all derived from limestone. Parent materials include alluvium, 

 calcareous argillite, and gabbro (volcanic). 



In Montana, broad-keeled milkvetch is most often found on thin-soil slopes and ridgetops o\erlying 

 Madison Group limestone as it outcrops in sagebrush foothills and montane elevations (Appendix D-2). 

 The dominant species is either Cercocarpus ledifolius ox Agropyron spicatum. The species is not 

 restricted to this type of habitat, however. In fact, a wide variety of elevations, substrates, and plant 

 associations were documented in this study as compared with previous habitat characterizations. 



Local dominants of the sparsely- vegetated settings include: 



Agropyron spicatum h.t. 



Artemisia tridentata/Agropyron spicatum h.t. 



Stipa comata - Bouteloua gracilis h.t. 



The following is a representative list of associated species compiled fi-om this species' habitat range: 



Agropyron spicatum 

 Artemisia frigida 

 Artemisia nova 

 Artemisia tridentata 

 Astragalus adsurgens 

 Astragalus crassicarpus 

 Asti-agalus purshii 

 Aster scopulorum 

 Bouteloua gracilis 

 Erigeron compositus 

 Erigeron tM>eedyi 

 Eriogonum cespitosum 

 Eriogonum ovalifolium 



