18 Restoration Ecology Vol. 8 No. 1, pp. 18–29 MARCH 2000 © 2000 Society for Ecological Restoration Reducing Competitive Suppression of a Rare Annual Forb by Restoring Native California Perennial Grasslands Tina M. Carlsen 1 John W. Menke 2 Bruce M. Pavlik 3 Abstract Populations of the rare annual forb Amsinckia grandi- flora may be declining because of competitive sup- pression by exotic annual grasses, and may perform better in a matrix of native perennial bunchgrasses. We conducted a field competition experiment in which Amsinckia seedlings were transplanted into forty 0.64-m 2 experimental plots of exotic annual grassland or re- stored perennial grassland. The perennial grassland plots were restored using mature 3 cm-diameter plants of the native perennial bunchgrass Poa secunda planted in three densities. The exotic annual grassland plots were established in four densities through manual re- moval of existing plants. Both grass types reduced soil water potential with increasing biomass, but this reduction was not significantly different between grass types. Both grass types significantly reduced the production of Amsinckia inflorescences. At low and intermediate densities (dry biomass per unit area of 20–80 g/m 2 ), the exotic annual grasses reduced Amsin- ckia inflorescence number to a greater extent than did Poa , although at high densities (  90 g/m 2 ) both grass types reduced the number of Amsinckia inflores- cences to the same extent. The response of Amsinckia inflorescence number to Poa biomass was linear, whereas the same response to the annual grass bio- mass is logarithmic, and appeared to be related to graminoid cover. This may be because of the different growth forms exhibited by the two grass types. Re- sults of this research suggest that restored native pe- rennial grasslands at intermediate densities have a high habitat value for the potential establishment of the native annual A. grandiflora . Key words: Amsinckia grandiflora , Poa secunda , rare plants, native plants, exotic plants, interspecific plant competition, light competition, water competition, grassland restoration. Introduction T he original native bunchgrass communities of Cali- fornia have undergone a major transformation since the arrival of the first Europeans (Baker 1989). Grasslands occur in several California plant communities. They are the primary component in the valley grassland commu- nity, and form the understory in oak woodland (also known as foothill woodland) and other cismontane wood- land communities of the California Floristic Province (Bar- bour & Major 1990). During exploration and intensive set- tlement, annual grasses native to the Mediterranean area were either intentionally introduced as cereal or forage crops, or inadvertently introduced through impurities in crop seed and in packing material (Murphy & Ehrlich 1989). By the 1850s, European plants had largely displaced native perennial grassland species, particularly in the val- ley grassland and oak woodland communities, resulting in the California winter annual grasslands of today. In recent years, the decline of locally restricted native plant species has been observed within California winter annual grasslands. Many such taxa are now of conserva- tion concern, and some have been formally listed as threatened and endangered. In their inventory of rare and endangered vascular plants of California (Skinner & Pavlik 1994), the California Native Plant Society (CNPS) listed 454 taxa found in valley grassland, foothill wood- land, and cismontane woodland communities. Annual plants comprised 48% of this total (216 taxa). Competition with annual exotic grasses has been shown to reduce some measure of fecundity in native species (Guerrant 1992; Gordon & Rice 1993). Conse- quently, research on the role of annual exotic grasses in the decline of native species (both annual and peren- nial) may help determine the mechanisms of the de- cline, and provide recommendations on maintaining native plant diversity in annual grasslands. Amsinckia grandiflora (Gray) Kleeb. ex Greene (Boragi- naceae) (large-flowered fiddleneck) is a rare forb native 1 Environmental Restoration Division, Lawrence Livermore National Laboratory, Livermore, CA 94551, U.S.A. 2 Department of Agronomy and Range Science, University of California, Davis, CA 95616, U.S.A. 3 Department of Biology, Mills College, Oakland, CA 94613, U.S.A.