Effects of competition on resource availability and growth of a California bunchgrass

In California, little is known about the sensitivity of native bunchgrasses to competition or to changes in resource availability. We investigated the effect of nonnative annual vegetation on resource availability and growth of a native bunchgrass, Nassella pulchra, in a pair of factorial field experiments that incorporated effects of both interspecific and intraspecific competition as well as variation in soil depth. Plots of differing target densities and neighborhoods were used to assess changes in aboveground (light) and belowground (water) resource availability over multiple seasons in two sites with differing soil depth. N. pulchra grown without interspecific competitors grew larger and produced more culms at all planting densities compared to plants in plots with interspecific competitors. Intraspecific competition significantly influenced growth only in the absence of interspecific competition. Reproductive effort, as measured by flowering culm production, was more sensitive than vegetative growth to both forms of competition. Light availability and variability at the soil surface was greatly reduced by the nonnative annual neighborhood. As expected, soil moisture was rapidly depleted by annuals to 30 cm in all plots. In deep-soil plots, soil moisture was reduced at 60-150 cm depths only when annual vegetation was removed, and depletion was correlated with N. pulchra basal area. This result suggests that the interspecific neighborhood reduced root growth in N. pulchra and its subsequent ability to use deep moisture. Within California's inland grasslands, nonnative annual vegetation has changed seasonal patterns of resource availability. We conclude that (1) increased competition for light during the spring, when growth of annuals is most rapid, suppresses growth and reproduction of N. pulchra; (2) by suppressing bunchgrass growth, annual grasses reduce access to belowground resources by competitive interference; and (3) the loss of perennial grasses in California grasslands and the general dominance by nonnative annual species results in the relative underutilization of deep soil resources. These conclusions suggest that the dominance of California grasslands by nonnative annual vegetation has shifted the primary limiting resource from soil moisture to light and the timing of resource limitation from summer to winter and spring.