69 



b. Interspecific: Gr indel ia howel 1 i i 

 thrives in greatest numbers in 

 disturbed, weedy habitats. The species 

 occurs less frequently in native 

 grassland habitats, and the plants in 

 these sites are more widely scattered. 

 These observations suggest that G. 

 howel 1 i i has poor competitive ability in 

 climax, closed grassland communities. 



3. Toxic and allelopathic interactions: 



Serious infestations of Centaurea macu losa 

 Lam. (Spotted knapweed) are occurring 

 throughout west-central Montana. There is 

 evidence that Centaurea spp . produce 

 allelopathic substances which inhibit plant 

 growth (Fletcher and Renney , 1963). Though 

 G. howe 1 1 i i frequently occurs in weedy 

 habitats associated with C. macu losa , the 

 potential impacts of the latter on natural 

 communities were most noticeable at one site 

 (Lost Horse Creek (027)). The area is a 

 P j nus ponderosa / Festuca scabre 1 la habitat 

 type in which C. macu losa is replacing native 

 species. Though 200-250 individuals of G. 

 howel 1 i i were found here, the plants were 

 noticeably smaller and appeared quite 

 feeble, most likely due to the competition 

 from C. maculosa . The replacement of native 

 vegetation in western Montana by this 

 agressive species will continue to be a 

 serious problem in the future. Though not 

 currently threatened rangewide by this 

 species, populations of G. howe 1 1 i i should 

 be periodically field-checked to assess any 

 increasing replacement which may take place. 



Hybr idization. 

 1 . 



Naturally occurring: Evidence of hybrid 

 contact and i nt rogr ess l on with the sympatric 

 Gr i nde 1 i a squar r osa was found at the 

 following four sites: 



Ho 1 land Lake (001 ) 

 Spring Creek Lake (032) 

 Blackfoot Roadside (035) 

 Barber Creek ( 05 1 ) 



Gr i ndel i a howel 1 i i has glandular, sometimes 

 villous stems, while G. squar rosa has 

 glabrous stems. At the sites listed, 

 individuals referable to both species were 

 found; in addition, plants intermediate with 



