Effects of plant competition, seed predation, and nutrient limitation on seedling survivorship of spotted knapweed (Centaurea stoebe)

Abstract

We measured seed germination and seedling survivorship of spotted knapweed, Centaurea stoebe, in a series of laboratory and field experiments to evaluate the efficacy of seed limitation as a management focus. This work was initiated 6 years after introduction of several biological control agents. The soil seed bank of the site used in this study contained a mean density of 5,848 seeds/m 2 (ranging from 0 to 16,364 seeds/m 2 ), and 92% of the seeds isolated from soils were shriveled, discolored, and/or partially decayed. Additionally, none of the intact seeds germinated, suggesting that the viable seed bank at our field study site has been exhausted. Centaurea stoebe seeds were planted into pots under a range of soil nitrogen (N) availability, with half of the pots containing a single density of previously established seedlings of a native cool-season grass, slender wheatgrass (Elymus trachycaulus). A watering regime mimicking local precipitation was applied. Spotted knapweed exhibited large biomass responses to N addition, but the presence of grasses suppressed the ability to exploit this N. Surprisingly, low soil N conditions improved knapweed survivorship in the presence of grasses. Nevertheless, recruitment and biomass were still far below the levels reached in the absence of competition. To evaluate the effect of density on successful recruitment, Centaurea stoebe seed was introduced into a meadow at three densities matching reduced levels of seed production under the constraints of seed predators. These densities were sown with or without a seed mixture of native species, into an existing plant community lacking C. stoebe, and seedling recruitment was recorded over 2.5 years. Across all plots and densities sown (568-2,272 seeds m -2 year -1 ), seedling recruitment was less than 1%. The invasion potential of spotted knapweed was greatly diminished when realistic levels of plant competition and biological control limit seed production. We therefore conclude that a combination of seed limitation and shortage of 'safe sites' within undisturbed vegetation can limit densities of C. stoebe.

Biological control and precipitation effects on spotted knapweed (Centaurea stoebe): empirical and modeling results

“…In general, spotted knapweed cover is negatively correlated with background forb and grass cover (D. G. Knochel et al, unpublished manuscript). While the general trend is clear, the exact effects can be highly variable (Pokorny et al 2005, Maron and Marler 2008, Knochel et al 2010a). Our seeding experiments have shown a large range in seedling survivorship across sites (0-11.2%; A. P. Maines, D. G. Knochel, and T. R. Seastedt, unpublished manuscript).…”

“…Biological controls L. minutus and C. achates were released from 2001(Seastedt et al 2007). The site was grazed prior to 2006, but grazing by cattle has not occurred at the study area since 2006 (Knochel et al 2010a). Further description of the study site is reported in sources ( Table 1).…”

“…Values for precipitation are mean with SD in parentheses. Sources for estimates are: 1, Knochel et al (2010a); 2, A. P. Maines, unpublished manuscript; 3, Sheley et al (1998);4, Knochel et al (2010b); 5, Knochel and Seastedt (2010);6, NOAA (2012). F1 is calculated from values in the table.…”

“…The only control on population growth was conspecific density. Seed bank survival was determined from a germination test conducted on seed isolated from soil cores taken from five locations at the study site (Knochel et al 2010a). This value was used under all scenarios, since biological control should not affect seed bank survival.…”

“…Other parameter estimates came from a common garden experiment where plants in enclosures had biological controls excluded or added at densities observed in the field (Knochel et al 2010b) and from a field experiment where known densities of spotted knapweed seeds were sown and the resulting number of seedlings and rosettes were assessed (Knochel et al 2010a; A. P. Maines, D. G. Knochel, and T. R. Seastedt, unpublished manuscript). The yearly seed bank germination probability was the average of estimates reported by Knochel et al (2010a) and A. P. Maines, D. G. Knochel, and T. R. Seastedt (unpublished manuv www.esajournals.org script). The estimate for the adult survival probability, S3, was based on a 9-year lifespan (Sheley et al 1998).…”