Factors limiting rosette recruitment in scarlet gilia, Ipomopsis aggregata:: seed and disturbance limitation

We performed a factorial combination of seed additions, surface soil disturbance, and protection from potential seed predation in experimental plots to elucidate the relative importance of each factor in the recruitment of scarlet gilia seedlings. We detected a significant interaction between seed addition and surface soil disturbance on seedling recruitment over 2 years of establishment. Plots that received both supplemental seed addition and disturbance established 10.5 times as many rosettes as control plots. We did not detect main or interactive effects of caging on seedling establishment. We explored the importance of density-dependent recruitment by investigating the number of rosettes per experimental plot through time. In addition, we used planned contrasts to compare the absolute and percent changes in control and treatment plot rosette densities between the study years. We found significant downward concave curvature to the seedling recruitment curve, suggesting negative density-dependent recruitment across the experimental range of rosette densities. We found a significant difference between the control plots and the seed addition-disturbance plots in terms of absolute changes in plot density from 1997 to 1998. Greater net mortality occurred in plots that received both seed addition and disturbance. However, we found no differences among treatments in the percent change in rosette density across the study years. Our study demonstrates that seedling recruitment in natural plant populations may be limited by the interaction of seed and microsite availability, and that seedling recruitment in scarlet gilia may be negatively density dependent. Our results suggest that the fecundity effects of particular plant-animal interactions (e.g., pollination, herbivory) may affect scarlet gilia population dynamics, particularly under conditions of high soil disturbance.