Effects of Competition on Resource Availability and Growth of a California Bunchgrass

Abstract

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.

Competitive impacts and responses of an invasive weed: dependencies on nitrogen and phosphorus availability

“…The presence of C. diffusa in neighborhoods had little impact on the growth and survival of the four focal species or on any of the soil processes we measured. Although strong competitive impacts of other invasive exotic species have been measured under conditions where density is high (D'Antonio and Mahall 1991;D'Antonio et al 1998;Dyer and Rice 1999;Hager 2004), biomass rather than species-specific effects predominately explained the competitive effects measured at lower fielddensities in this study. In related work, we found few species-specific effects of C. diffusa growing in monospecific patches on nutrient cycling rates, nitrogen or phosphorus levels, or microbial biomass (LeJeune 2002).…”

Resource pulses, species interactions, and diversity maintenance in arid and semi-arid environments

“…In the upper soil layers, however, large evaporational water losses decrease the contribution of shallow-rooted plants to soil moisture depletion (Cable 1969;Noy-Meir 1973;Dyer and Rice 1999), therefore decreasing the link between uptake and resource shortage (Kadmon and Shmida 1990;Gebauer et al 2002, Sher et al 2004. As a result, the effectiveness of temporal resource partitioning by the storage effect would be lower in shallow-rooted species, because covariance between environment and competition would be less pronounced.…”

“…As a result, the effectiveness of temporal resource partitioning by the storage effect would be lower in shallow-rooted species, because covariance between environment and competition would be less pronounced. Conversely, the storage effect should be stronger for plants rooted in soil layers where plant use of water is the dominant mode of depletion (Cable 1969;Dyer and Rice 1999).…”

Summer water use by California coastal prairie grasses: fog, drought, and community composition

“…Our findings are consistent with those of Dawson and Pate (1996), working in Banksia woodlands, and Williams and Ehleringer (2000), in pinyon-juniper forests, that plants in arid and semiarid ecosystems rely on shallow root water uptake where precipitation is relatively predictable. Coastal prairie grassland species, therefore, may contrast with those in interior California grasslands, where perennial grasses are generally assumed to access water >0.5 m below the soil surface during the summer (Holmes and Rice Dyer and Rice 1999). Reliance on shallow roots to access summer precipitation is likely to vary along gradients of water availability (Ehleringer and Dawson 1992), and vegetation further inland, away from the zone of frequent fog inundation-such as California's Central Valley-is less likely to rely on shallow roots for their primary water acquisition during the summer drought season.…”

“…The exotic annual grasses have been shown to suppress native perennial grasses in inland habitats (e.g. Dyer and Rice 1999;Hamilton et al 1999). In contrast, native perennial bunchgrasses in coastal prairies have been shown to be strong competitors against exotic annual grasses (Corbin and D'Antonio 2004).…”