Tm. Carlsen et al., Reducing competitive suppression of a rare annual forb by restoring nativeCalifornia perennial grasslands, RESTOR ECOL, 8(1), 2000, pp. 18-29
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.