Restoration of a Coastal California Grassland after Invasion by Nitrogen-Fixing Shrubs, French Broom and Scotch Broom

Abstract

We studied the effect of two years of sawdust addition on the growth of native perennial grasses in a site where we manually removed two introduced nitrogen-fixing shrubs, French broom (Genista monspessulana) and Scotch broom (Cytisus scoparius) in central coastal California. We tested the hypothesis that sawdust addition to soil would reduce plant-available nitrogen levels, and thereby decrease the competitive effect of fast-growing exotic annual grasses on slower-growing native perennial grasses. Sawdust did not alter the competitive interaction between annuals and native perennials: native perennial grass seedlings were greatly suppressed by the presence of annuals and this was not changed by sawdust addition. In the absence of competition with annuals, we observed both direct stimulation and suppression by sawdust on native species. Annual grass biomass did not respond to sawdust addition. Nitrogen:carbon ratios of leaf tissue were similar across all species, suggesting that both annuals and perennials responded similarly to depletion of soil resources. Our results confirm that although it is possible to use sawdust to reduce nitrogen availability in broom-invaded soils, it is difficult to target the species that will benefit.