Throughout the western United States, invasions of exotic annual grasses are becoming increasingly common, often driving rapid declines in ecosystem diversity, production, and resilience. As a potential management strategy, restoration of native perennial bunchgrasses is thought to suppress these invasive species and facilitate growth of other native taxa. However, the same characteristics that confer an inhibitory effect on invasive species may also reduce bunchgrass growth and fecundity, of which seed production is a particularly strong controller of perennial grass establishment in California grasslands. To explore the relationship between invasive suppression and reproductive effort in native bunchgrasses, I established six sampling grids composed of fifty 1m 2 plots in populations of Stipa pulchra (purple needlegrass) at the McLaughlin Natural Reserve, Lower Lake, California. S. pulchra densities varied substantially within sampling grids, ranging from 0 to 85% total areal cover per meter. In each 1m 2 plot, I evaluated cover and basal area of all S. pulchra tussocks, in addition to diversity and cover of exotic and native species. To estimate bunchgrass fecundity, I counted the number of reproductive culms produced by each S. pulchra individual and collected seed from randomly chosen tussocks in plots exhibiting a range of S. pulchra cover values.
Results/Conclusions
Results indicate that exotic grass abundance was in general, negatively correlated with S. pulchra cover, though this trend varied substantially between sites. In addition, S. pulchra abundance was not associated with the increase in cover of other native species, regardless of site. Nonetheless, Shannon diversity index values peaked at intermediate S. pulchra densities. Per-individual bunchgrass reproductive effort, exhibited a linear relationship with increasing bunchgrass area. However, linear mixed effects models controlling for random effects between populations indicate that the relationship between bunchgrass basal area and reproductive output was strongly influenced by the density of conspecific individuals within 1m 2 plots—at the highest recorded values of S. pulchra cover, large bunchgrass individuals showed little reproductive culm production, irrespective of basal area. Similarly, mean seed mass in S. pulchra showed a negative relationship with increasing cover of intraspecific competitors. My data suggests that while high densities of S. pulchra are capable of greatly inhibiting exotic annual grass abundance, these populations may exhibit low rates of seed production, reducing opportunities for population growth and spatial expansion into favorable microsites.