4 

 Germination occurs in the fall or the following spring. Fall 

 emerging plants overwinter as a rosette and may flower during 

 their first summer. Spring-germinating plants generally bloom 

 their second summer (Watson and Renney 1974) . 



In Ravalli County nearly all Arabis fecunda habitat has been 

 invaded by Centaurea maculosa . Classical competition theory 

 predicts that species with similar ecological reguirements should 

 compete most strongly (Hardin 1960) . The morphological 

 similarity between these two species suggests that C^_ maculosa 

 may be interfering with A^. fecunda at these sites. The purpose 

 of our study was to examine the effects of Centaurea maculosa on 

 populations of Arabis fecunda over a period of three years. We 

 employed technigues of demographic monitoring (Menges 1986) in 

 order to determine which life stages are being most affected and 

 which mechanisms are most important. 



Study Areas 



We conducted our study at Birch Creek and Charley's Gulch, 

 in Ravalli County. Both sites are on gentle southwest-facing 

 slopes, the Birch Creek site at 14 3 m and the Charley's Gulch 

 site at 1525 m. At Hamilton, ca. 8 km southwest and 300 m lower, 

 mean temperatures for July and January are 19.4° and -3.8° C 

 respectively, and mean annual precipitation is 32 cm. Soils are 

 sandy loams derived from calcareous parent material (Lesica 



