Reducing competitive suppression of a rare annual forb by restoring nativeCalifornia perennial grasslands

Populations of the rare annual forb Amsinckia grandiflora may be declining because of competitive suppression by exotic annual grasses, and may perfor m 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 restored perennial grassland. The perennial grassland plots were restored using matu re 3 cm-diameter plants of the native perennial bunchgrass Poa secunda plan ted in three densities. The exotic annual grassland plots were established in four densities through manual removal of existing plants. Both grass typ es reduced soil water potential with increasing biomass, but this reduction was not significantly different between grass types. Both grass types sign ificantly reduced the production of Amsinckia inflorescences. At low and in termediate densities (dry biomass per unit area of 20-80 g/m(2)), the exoti c annual grasses reduced Amsinckia inflorescence number to a greater extent than did Poa, although at high densities (>90 g/m(2)) both grass types red uced the number of Amsinckia inflorescences to the same extent. The respons e of Amsinckia inflorescence number to Poa biomass was linear, whereas the same response to the annual grass biomass is logarithmic, and appeared to b e related to graminoid cover. This may be because of the different growth f orms exhibited by the two grass types. Results of this research suggest tha t restored native perennial grasslands at intermediate densities have a hig h habitat value for the potential establishment of the native annual A. gra ndiflora.