1 



Abstract 



At three sites in southwest Montana, Arab is fecunda 

 (Brassicaceae) , a rare regional endemic, occurred more often than 

 expected in cryptogamic soil crust than in bare soil. In two of 

 four transects, small, young plants were underrepresented, and 

 older, reproductive plants were overrepresented in crusts. In 

 the other two transects, the distribution of life stage classes 

 was independent of substrate. Measures of fecundity were 

 independent of substrate in all four transects. These results 

 demonstrate that the presence of soil crust is beneficial to 

 populations of A^ fecunda at some sites and suggest that this 

 relationship is more likely due to increased survival of older 

 plants rather than enhanced recruitment. 



Introduction 



Cryptogamic soil crusts composed primarily of lichens, 

 bryophytes and algae are common in arid and semi-arid regions of 

 western North America. Soil crusts are often important 

 contributors to nitrogen fixation (Snyder and Wullstein 1973, 

 Rychert and Skujins 1974) and have been shown to reduce soil 

 erosion (Booth 1941, Bailey et al . 1973). Soil crusts also 

 improve water penetration into the soil (Loope and Gifford 1972, 

 Brotherson and Rushforth 1983) and may increase soil water 



